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1
Zanette, Liana, Daniel T. Haydon, James N. M. Smith, Mary J. Taitt, and Michael Clinchy. "Reassessing the Cowbird Threat." Auk 124, no. 1 (January 1, 2007): 210–23. http://dx.doi.org/10.1093/auk/124.1.210.
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Abstract The demographic significance of parasitism by Brown-headed Cowbirds (Molothrus ater; hereafter “cowbirds”) has been debated for years, because manipulative studies are few and mathematical models of cowbird-host systems have not led to tests of their predictions. We combine results from a cowbird- removal experiment (Smith et al. 2002, 2003) with a stochastic simulation model that we developed, to reassess the effect of cowbirds on the annual reproductive success (ARS) and nesting success in Song Sparrows (Melospiza melodia). Our model followed the breeding success of individual Song Sparrow nests and extrapolated to output variables including ARS, nest survival, and parasitism rates. We parameterized the model with field data from cowbird control sites (i.e., cowbirds not manipulated) and found that output variables matched those observed. We used the reduced parasitism rates observed on cowbird-removal sites, reran the model, and compared output with observed values. On removal sites, ARS was greater than predicted by the model, which indicates that the model failed to account for some biological phenomenon that occurred when cowbirds were in the system. To assess what this phenomenon might be, we conducted further analyses that indicated that cowbirds may facilitate nest predation by “conventional” predators. Using elasticity analyses, we show that cowbirds have their largest effect on ARS through egg removal and that conventional predators are more important than cowbirds in directly affecting nest survival rates. We report that to assess the effect of cowbirds on nest survival, researchers must avoid using the common technique of comparing survival rates of parasitized and unparasitized nests within populations. Réévaluation de la Menace du Vacher
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Smith, James N. M., Mary J. Taitt, Liana Zanette, and Isla H. Myers-Smith. "How do Brown-Headed Cowbirds (Molothrus Ater) CAUSE NEST FAILURES IN SONG SPARROWS (MELOSPIZA MELODIA)? A REMOVAL EXPERIMENT." Auk 120, no. 3 (July 1, 2003): 772–83. http://dx.doi.org/10.1093/auk/120.3.772.
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Abstract A removal experiment was conducted to measure how much and by what mechanisms brood parasitic Brown-headed Cowbirds (Molothrus ater) cause nest failures in a commonly used host, the Song Sparrow (Melospiza melodia). When numbers of female cowbirds were reduced experimentally, nest failures fell from 65.0% (n = 663 nests) to 49.9% (n = 331). Cowbird reduction reduced the frequency of nest failure to one-third of control levels in Song Sparrows during the last 80 days of the sparrow's breeding season, the period when most parasitic laying took place. Cowbird reduction decreased nest failures strongly at the egg stage, and weakly at the nestling stage. Daily nest-failure rates were independent of whether or not a nest was parasitized by cowbirds. Two hypotheses were tested to explain how cowbirds cause host nests to fail: first, egg removal by female cowbirds lowers clutch size below a threshold where the host deserts; second, cowbirds cause host nests to fail by destroying entire clutches or broods. In support of the first hypothesis, desertion following parasitism and egg removal was less frequent when cowbird numbers were reduced (8.9% of n = 158 nests) than for unmanipulated controls (16.5% of n = 424 nests). In support of the second hypothesis, there were fewer cases where young were killed in the nest, or found dead near it, after cowbird numbers were reduced (2.5% of 158 nests) than in controls (4.7% of 424 control nests). In contrast, proportions of nests that failed after the disappearance of all eggs, young, or both, and after unparasitized clutches were deserted, increased when cowbird numbers were reduced. Although our study supports both hypotheses, cowbird-induced desertion had a greater effect on nest failure rates than did cowbird predation. Our study suggests that cowbird removal programs are likely to benefit commonly used and endangered hosts by reducing rates of nest failure.
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De Mársico, María C., Mariela G. Gantchoff, and Juan C. Reboreda. "Host–parasite coevolution beyond the nestling stage? Mimicry of host fledglings by the specialist screaming cowbird." Proceedings of the Royal Society B: Biological Sciences 279, no. 1742 (May 30, 2012): 3401–8. http://dx.doi.org/10.1098/rspb.2012.0612.
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Egg mimicry by obligate avian brood parasites and host rejection of non-mimetic eggs are well-known textbook examples of host–parasite coevolution. By contrast, reciprocal adaptations and counteradaptations beyond the egg stage in brood parasites and their hosts have received less attention. The screaming cowbird ( Molothrus rufoaxillaris ) is a specialist obligate brood parasite whose fledglings look identical to those of its primary host, the baywing ( Agelaioides badius ). Such a resemblance has been proposed as an adaptation in response to host discrimination against odd-looking young, but evidence supporting this idea is scarce. Here, we examined this hypothesis by comparing the survival rates of young screaming cowbirds and non-mimetic shiny cowbirds ( Molothrus bonariensis ) cross-fostered to baywing nests and quantifying the similarity in plumage colour and begging calls between host and cowbird fledglings. Shiny cowbirds suffered higher post-fledging mortality rates (83%) than screaming cowbirds (0%) owing to host rejection. Visual modelling revealed that screaming cowbirds, but not shiny cowbirds, were indistinguishable from host young in plumage colour. Similarly, screaming cowbirds matched baywings' begging calls more closely than shiny cowbirds. Our results strongly support the occurrence of host fledgling mimicry in screaming cowbirds and suggest a role of visual and vocal cues in fledgling discrimination by baywings.
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Louder, Matthew I. M., Wendy M. Schelsky, Amber N. Albores, and Jeffrey P. Hoover. "A generalist brood parasite modifies use of a host in response to reproductive success." Proceedings of the Royal Society B: Biological Sciences 282, no. 1814 (September 7, 2015): 20151615. http://dx.doi.org/10.1098/rspb.2015.1615.
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Avian obligate brood parasites, which rely solely on hosts to raise their young, should choose the highest quality hosts to maximize reproductive output. Brown-headed cowbirds ( Molothrus ater ) are extreme host generalists, yet female cowbirds could use information based on past reproductive outcomes to make egg-laying decisions thus minimizing fitness costs associated with parasitizing low-quality hosts. We use a long-term (21 years) nest-box study of a single host, the prothonotary warbler ( Protonotaria citrea ), to show that local cowbird reproductive success, but not host reproductive success, was positively correlated with the probability of parasitism the following year. Experimental manipulations of cowbird success corroborated that female cowbirds make future decisions about which hosts to use based on information pertaining to past cowbird success, both within and between years. The within-year pattern, in particular, points to local cowbird females selecting hosts based on past reproductive outcomes. This, coupled with high site fidelity of female cowbirds between years, points to information use, rather than cowbird natal returns alone, increasing parasitism rates on highly productive sites between years.
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Ellison, Kevin, Spencer G. Sealy, and Hope R. McGaha. "Color Variation Among Nestling Brown-Headed Cowbirds (Molothrus Ater) Does Not Reflect Differential Success With Hosts in Texas." Auk 124, no. 2 (April 1, 2007): 526–36. http://dx.doi.org/10.1093/auk/124.2.526.
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Abstract As brood parasites, nestling Brown-headed Cowbirds (Molothrus ater) may exhibit characters that improve their fledging success when reared alongside host young. The coloration of mouthparts of nestlings can influence adult care and, thus, the polymorphism of yellow or white rictal flanges among nestling Brown-headed Cowbirds may reflect differential success with different hosts on the basis of flange color. Moreover, because Brown-headed Cowbirds in the southern United States co-occur with Bronzed Cowbirds (M. aeneus), whose young have white flanges, cowbird nestlings' flange colors may reflect a means for reducing interspecific competition through partitioning of host species on the basis of nestling flange color. To determine whether flange color influences cowbird fledging success with hosts of either color, we recorded the flange colors of cowbirds and their hosts at a site in Texas. We also tested whether flange color was influenced by nestling sex. Most hosts of Brown-headed Cowbirds had young with yellow flanges (81%, n = 16 spp.), yet Brown-headed Cowbirds with white flanges were more common (61%, n = 107). Bronzed Cowbirds parasitized primarily species whose young had white flanges (86%, n = 348 eggs). Despite the differential use of hosts with regard to flange color, the frequencies of each were similar among nestling and juvenile Brown-headed Cowbirds. Likewise, the frequencies of each color did not differ significantly between the sexes. Therefore, we suggest that a flange color matching that of nestmates is not strongly selected for by hosts. La Variación del Color entre Polluelos de Molothrus ater no Refleja Éxito Diferencial al Ser Criados por Especies Hospederas en Texas
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Winnicki, S. K., B. M. Strausberger, N. D. Antonson, D. E. Burhans, J. Lock, A. M. Kilpatrick, and M. E. Hauber. "Developmental asynchrony and host species identity predict variability in nestling growth of an obligate brood parasite: a test of the “growth-tuning” hypothesis." Canadian Journal of Zoology 99, no. 3 (March 2021): 213–20. http://dx.doi.org/10.1139/cjz-2020-0147.
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Generalist obligate brood parasites are excellent models for studies of developmental plasticity, as they experience a range of social and environmental variation when raised by one of their many hosts. Parasitic Brown-headed Cowbirds (Molothrus ater (Boddaert, 1783)) exhibit host-specific growth rates, yet Cowbird growth rates are not predicted by hosts’ incubation or brooding periods. We tested the novel “growth-tuning” hypothesis which predicts that total asynchrony between Cowbirds’ and hosts’ nesting periods results in faster parasitic growth in nests where host young fledge earlier than Cowbirds. We tested this prediction using previously published and newly added nestling mass data across diverse host species. Total nesting period asynchrony (summed across incubation and brooding stages) predicted Cowbird growth; 8-day-old Cowbirds were heavier in host nests with relatively shorter nesting periods. We further explored the drivers of variation in growth using mass measurements of Cowbirds in Song Sparrow (Melospiza melodia (A. Wilson, 1810)) and Red-winged Blackbird (Agelaius phoeniceus (Linnaeus, 1766)) nests. Our top models included host species (Cowbirds grew faster in Sparrow nests), numbers of nestmates (slowest when raised alone), and sex (males grew faster). These results confirm that multiple social and environmental factors predict directional patterns of developmental plasticity in avian generalist brood parasites.
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Teather, Kevin, and Shannon Cronin. "The response of yellow warblers to brown-headed cowbird models in Prince Edward Island." Canadian Journal of Zoology 78, no. 5 (May 1, 2000): 883–85. http://dx.doi.org/10.1139/z99-261.
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The negative impact of brown-headed cowbird parasitism (Molothrus ater) on the reproductive success of many songbirds has led to the evolution of host defensive behaviours that may reduce the chance of being parasitized. Populations of songbirds that have only recently experienced cowbird parasitism, however, may have had insufficient time to develop such behaviours. We examined the response of nesting yellow warblers (Dendroica petechia), a host species, to cowbird models on Prince Edward Island, where cowbirds have been present for less than 50 years. Yellow warblers gave more alarm calls and remained closer to their nests when presented with a cowbird model than when presented with a model of a white-throated sparrow (Zonotrichia albicollis), a nonthreatening local species. Our results suggest that yellow warblers recognize brown-headed cowbirds as greater threats to their nests and respond in a way that is similar to other populations which have been exposed to cowbirds for a much longer period of time.
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Peer, Brian D., Lyndon R. Hawkins, Edwin P. Steinke, Patricia Blair Bollinger, and Eric K. Bollinger. "Eastern Bluebirds Eject Brown-Headed Cowbird Eggs." Condor 108, no. 3 (August 1, 2006): 741–45. http://dx.doi.org/10.1093/condor/108.3.741.
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Abstract The relationship between the Brown-headed Cowbird (Molothrus ater) and its cavity-nesting hosts has received little attention because of the assumption that cowbirds rarely parasitize these hosts. We tested the Eastern Bluebird (Sialia sialis), a host that is sometimes heavily parasitized by cowbirds, for egg ejection behavior. Bluebirds ejected 65% of experimentally added cowbird eggs (n = 20), but ejected no experimentally added conspecific eggs (n = 66). This suggests that cowbird parasitism, not conspecific brood parasitism, is the selective pressure responsible for egg ejection in this species. This level of rejection may be conservative because bluebirds nest in dark cavities, which may make cowbird eggs difficult to detect by bluebirds.
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Pribil, Stanislav, and Jaroslav Picman. "Parasitism of House Wren nests by Brown-headed Cowbirds: why is it so rare?" Canadian Journal of Zoology 75, no. 2 (February 1, 1997): 302–7. http://dx.doi.org/10.1139/z97-038.
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We tested five hypotheses that may explain why House Wren (Troglodytes aedon) nests are rarely parasitized by Brown-headed Cowbirds (Molothrus ater). House Wrens may prevent parasitism in five ways: (1) by choosing to nest in cavities with small entrances (inaccessible-entrance hypothesis), (2) by restricting the size of the entrance with nest material (nest-structure hypothesis), (3) by puncturing and ejecting parasitic eggs (puncture–ejection hypothesis), (4) by burying the parasitized clutch under a new nest (egg-burial hypothesis), or (5) by abandoning the parasitized nest altogether (nest-desertion hypothesis). We tested these hypotheses in field experiments and found that (i) female cowbirds cannot enter circular entrances smaller than 38 mm in diameter, (ii) wrens prefer cavities with small entrances (inaccessible to cowbirds) to those with large entrances (accessible to cowbirds), (iii) when forced to breed in cavities with large entrances, wrens do not reduce the entrance size with nest material, (iv) despite the unusual strength of cowbird eggs, wrens are physically capable of puncture–ejecting them, (v) wrens do not puncture–eject cowbird eggs from their own nests, (vi) wrens do not abandon parasitized nests or bury the parasitized clutches under new nests. These results are consistent with the inaccessible-entrance hypothesis. We propose that additional nesting adaptations, as well as active cowbird avoidance of House Wrens, may contribute to the low frequency of cowbird parasitism.
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Peer, Brian D., Kevin S. Ellison, and Spencer G. Sealy. "Intermediate Frequencies of Egg Ejection by Northern Mockingbirds (Mimus polyglottos) Sympatric with Two Cowbird Species." Auk 119, no. 3 (July 1, 2002): 855–58. http://dx.doi.org/10.1093/auk/119.3.855.
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Abstract We experimentally parasitized Northern Mockingbird (Mimus polyglottos) nests to determine whether that species is more likely to eject immaculate eggs of the Bronzed Cowbird (Molothrus aeneus) or the spotted eggs of the Brown-headed Cowbird (M. ater); the latter species lays eggs that more closely resemble mockingbird eggs. Mockingbirds ejected 69% of model Bronzed Cowbird eggs, indicating that contrary to previous evidence Northern Mockingbirds eject Bronzed Cowbirds eggs at a high frequency. Mockingbirds also ejected 60% of model and real Brown-headed Cowbird eggs. Bronzed Cowbird eggs were ejected faster than Brown-headed Cowbird eggs (1.3 and 2.1 days, respectively). Ejection by mockingbirds may account for the lack of observed parasitism at our study sites in southern Texas, but it is more likely that cowbirds did not parasitize mockingbirds.
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Walsh, John J., Ty A. Tuff, Alexander Cruz, and Jameson F. Chace. "Differential Parasitism Between Two Suitable Cowbird Hosts." Open Ornithology Journal 8, no. 1 (July 31, 2015): 32–38. http://dx.doi.org/10.2174/1874453201508010032.
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Host choice by the brood parasitic Brown-headed Cowbird (Molothrus ater) is an evolved response to host suitability, resulting in patterns of differential parasitism rates among species within a community. In the ponderosa pine (Pinus ponderosa) forests of the Colorado Front Range, we recorded that Western Wood-pewee (Contopus sordidulus) is infrequently parasitized (1%, n = 259 nests) by the Brown-headed Cowbird, whereas the Plumbeous Vireo (Vireo plumbeus) is heavily parasitized (51%, n = 292). To account for differences in parasitism rates on these species we experimentally parasitized pewee nests with cowbird eggs, and we compared host aggression towards cowbird models, host nest attentiveness, nest placement, and egg-laying dates in these species. Pewees accepted cowbirds eggs and reacted more aggressively towards the cowbird model than the control model, were more attentive at their nest sites than vireos, and placed their nests higher and closer to the trunk than vireos. Egg-laying dates for vireos and cowbirds overlapped more than the egg-laying dates for pewees and cowbirds. We suggest that temporal asynchrony in host availability, coupled with differences in nest placement and behavior at the nest, help to account for the observed differences in parasitism rates between these two species.
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Astie, Andrea A., and Juan C. Reboreda. "Creamy-Bellied Thrush Defenses Against Shiny Cowbird Brood Parasitism." Condor 107, no. 4 (November 1, 2005): 788–96. http://dx.doi.org/10.1093/condor/107.4.788.
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AbstractWe studied Creamy-bellied Thrush (Turdus amaurochalinus) defenses against brood parasitism by Shiny Cowbirds (Molothrus bonariensis). Shiny Cowbirds decrease the reproductive success of Creamy-bellied Thrushes, and having historical habitats and ranges that overlap, we expected that thrushes possess antiparasitic defenses. We analyzed nest attendance during prelaying, laying and incubation; responses to the presentation of a model of a female cowbird or a control species close to the nest; nest abandonment associated with parasitism; and responses to experimental parasitism with white or spotted cowbird eggs (with or without the simultaneous presentation of a female cowbird model). Nest attendance was 58%–68% during prelaying and 83%–90% during laying and incubation. Thrushes had a shorter latency in returning near the nest and visited nests more frequently when we presented the cowbird model than the control model. The frequency of abandonment of parasitized nests was low and was not temporally associated with parasitism. Thrushes ejected white eggs more frequently than spotted eggs when parasitism was associated with the presentation of the cowbird model, but there were no differences when the model was absent. Our results indicate that Creamy-bellied Thrushes recognize cowbirds as a threat and eject white but not spotted cowbird eggs. We postulate that the low impact of cowbird parasitism on thrush hatching success and chick survival and the likelihood of recognition errors when parasite eggs resemble host eggs may have prevented the evolution of egg ejection in this host.
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Mason, Paul. "Brood Parasitism in a Host Generalist, the Shiny Cowbird: I. The Quality of Different Species as Hosts." Auk 103, no. 1 (January 1, 1986): 52–60. http://dx.doi.org/10.1093/auk/103.1.52.
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Abstract The Shiny Cowbird (Molothrus bonariensis) of South America, Panama, and the West Indies is an obligate brood parasite known to have used 176 species of birds as hosts. This study documents wide variability in the quality of real and potential hosts in terms of response to eggs, nestling diet, and nest survivorship. The eggs of the parasite are either spotted or immaculate in eastern Argentina and neighboring parts of Uruguay and Brazil. Most species accept both morphs of cowbird eggs, two reject both morphs, and one (Chalk-browed Mockinbird, Mimus saturninus) rejects immaculate eggs but accepts spotted ones. No species, via its rejection behavior, protects the Shiny Cowbird from competition with a potential competitor, the sympatric Screaming Cowbird (M. rufoaxillaris). Cross-fostering experiments and natural-history observations indicate that nestling cowbirds require a diet composed of animal protein. Because most passerines provide their nestlings with such food, host selection is little restricted by diet. Species-specific nest survivorship, adjusted to appropriate values of Shiny Cowbird life-history variables, varied by over an order of magnitude. Shiny Cowbirds peck host eggs. This density-dependent source of mortality lowers the survivorship of nests of preferred hosts and creates natural selection for greater generalization. Host quality is sensitive to the natural-history attributes of each host species and to the behavior of cowbirds at nests.
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Peer, Brian D., Scott K. Robinson, and James R. Herkert. "Egg Rejection by Cowbird Hosts in Grasslands." Auk 117, no. 4 (October 1, 2000): 892–901. http://dx.doi.org/10.1093/auk/117.4.892.
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Abstract We tested Field Sparrows (Spizella pusilla), Vesper Sparrows (Pooecetes gramineus), Lark Sparrows (Chondestes grammacus), Grasshopper Sparrows (Ammodramus savannarum), Dickcissels (Spiza americana), Eastern Meadowlarks (Sturnella magna), and Western Meadowlarks (S. neglecta) to determine whether the low level (<10%) of observed parasitism by Brown-headed Cowbirds (Molothrus ater) on these grassland hosts is a result of egg rejection. Western Meadowlarks rejected 78% of artificial and real cowbird eggs, Eastern Meadowlarks rejected 36% of artificial cowbird eggs, and Dickcissels rejected 11% of artificial cowbird eggs. None of the other hosts regularly rejected cowbird eggs. Thus, egg rejection may account for some, but not all, of the low level of observed parasitism on grassland hosts in the Midwest. Meadowlarks were also tested with nonmimetic eggs, and the remaining hosts were tested with undersized mimetic and nonmimetic eggs when possible. All hosts, with the exception of the Field Sparrow, demonstrated some level of rejection of the nonmimetic eggs. These results suggest that some grassland hosts, which apparently have been in contact with cowbirds the longest, have evolved some form of rejection behavior that might have selected for mimetic eggs in cowbirds. The intermediate levels of rejection by both species of meadowlarks also may indicate that rejection is increasing in these populations.
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Confer, John L., Jeffery L. Larkin, and Paul E. Allen. "Effects of Vegetation, Interspecific Competition, and Brood Parasitism on Golden-Winged Warbler (Vermivora Chrysoptera) Nesting Success." Auk 120, no. 1 (January 1, 2003): 138–44. http://dx.doi.org/10.1093/auk/120.1.138.
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Abstract The recent decline of Golden-winged Warblers (Vermivora chrysoptera) correlates with the loss of suitable nesting habitat, range expansion by Blue-winged Warblers (V. pinus), and eastward expansion of Brown-headed Cowbirds (Molothrus ater). Multivariate statistics were used to examine effects of those factors on Golden-winged Warbler reproduction in north central New York. Herb and shrub cover were positively correlated with clutch size. Blue-winged Warbler proximity was negatively correlated with Golden-winged Warbler clutch size. Tree cover and perhaps herb cover, after adjusting for brood size, correlated with a reduced number of Golden-winged Warbler fledglings. Herbaceous cover correlated with a greater number of cowbird eggs in Golden-winged Warbler nests. Cowbird parasitism correlated with a reduction in the number of Golden-winged Warbler eggs incubated and proportion of incubated eggs that hatched. However, cowbird parasitism, after adjusting for brood size, did not significantly affect nestling success rate. Cowbirds parasitized 30% of Golden-winged Warbler nests, which reduced the number of Golden-winged Warblers fledged by ∼17%. Average herb and tree cover values were 69 and 22 in Golden-winged Warbler territories and 60 and 23 in Blue-winged Warbler territories, respectively, with herb cover significantly greater for Golden-winged Warblers. Territories in the earliest stages of succession used by Golden-winged Warblers supported larger clutches and a reduction in the strong, negative effect of Blue-winged Warbler proximity and an increase in the negative effect correlated with cowbirds, if cowbirds were locally abundant.
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Hahn, D. Caldwell, Roger D. Price, and Peter C. Osenton. "Use of Lice to Identify Cowbird Hosts." Auk 117, no. 4 (October 1, 2000): 943–51. http://dx.doi.org/10.1093/auk/117.4.943.
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Abstract Brood-parasitic nestlings have a unique opportunity to encounter host-specific lice (Phthiraptera). Lice are permanent ectoparasites found strictly on the body of the host, and they are transferred almost exclusively by bodily contact during copulation and care of young. We investigated whether Brown-headed Cowbird (Molothrus ater) nestlings become infested with lice from their host parents and carry these after fledging, in effect bearing ectoparasite indicators of the species that raised them. Examining lice on cowbirds to identify foster parents would be less costly than determining parasitism patterns in the conventional way by finding many host nests. The 244 cowbird fledglings that we examined carried 11 species and 6 genera of lice, almost the entire spectrum of louse genera known from passerines. We also examined 320 songbirds from 30 species of hosts. As a group, the diversity of louse species on hosts was comparable to that on fledgling cowbirds: 13 species and 7 genera. In contrast, most individual host species yielded only one or two louse species, significantly fewer than on cowbird fledglings. Of 44 fledgling cowbirds with lice, 11 were linked with probable avian foster parents, and 18 other fledglings were linked with one of two possible foster parents. We conclude that cowbird fledglings carry away host lice and that our technique provides a partial assessment of parasitism patterns. The incomplete state of louse taxonomy requires that users of the technique obtain a reference collection of lice from host species in addition to the sample collection from cowbird fledglings. Lice from cowbird fledglings can be identified by a taxonomist and linked to particular host species, and the principal difficulty is the scarcity of skilled louse taxonomists. We also found an unusually rich louse fauna on 219 adult cowbirds, which supports the interpretation that lack of opportunity due to physical isolation has been the fundamental factor in the host specificity of lice in certain avian orders.
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Granfors, Diane A., Pamela J. Pietz, and Lisa A. Joyal. "Frequency of Egg and Nestling Destruction by Female Brown-headed Cowbirds at Grassland Nests." Auk 118, no. 3 (July 1, 2001): 765–69. http://dx.doi.org/10.1093/auk/118.3.765.
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Abstract Researchers have suggested that Brown-headed Cowbirds (Molothrus ater) destroy nest contents of potential hosts to induce renesting and thus enhance future opportunities for parasitism. Although cowbird destruction of passerine nests has been witnessed and surmised, few data are available on frequency of those events. We used miniature video-cameras at nests of grassland passerines and documented partial or complete destruction of eggs or nestlings by cowbirds at 7 of 132 nests monitored with cameras. At least three of the seven cases appeared to be attempts to totally destroy the nest contents; those cowbirds did not appear to be motivated by food or an intent to parasitize the nest. Three cases probably were associated with parasitism, but two involved egg removal late in incubation and the third was unusually destructive. Cowbirds were responsible for 24% of egg losses and 5% of nestling losses caused by predators. The importance of cowbirds as an agent of egg and nestling loss undoubtedly varies among sites and years, but it should not be overlooked.
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Kozlovic, Daniel R. "Parasitism by Brown-headed Cowbirds and productivity of House Finch hosts." Canadian Journal of Zoology 76, no. 9 (September 1, 1998): 1714–21. http://dx.doi.org/10.1139/z98-108.
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The effect of parasitism by Brown-headed Cowbirds (Molothrus ater) on the reproductive success of a recently established population of House Finches (Carpodacus mexicanus) was studied at St. Catharines, Ontario, during 1983-1985. House Finches began to colonize Ontario in 1972 and breeding was first observed there in 1978. Cowbirds laid in 40.2% of House Finch nests and parasitism was most prevalent during the peak of House Finch nesting. Clutches initiated early in the season were free of cowbird eggs. Most (76%) House Finch nests were parasitized within 2 days after the first House Finch egg was laid. Parasitized House Finch nests contained 1-3 cowbird eggs and the mean number of cowbird eggs per parasitized nest was 1.30. Frequency of multiple parasitism was 24.2% and the number of cowbird eggs per nest was not significantly different from a truncated Poisson distribution. Cowbird parasitism depressed House Finch clutch size and number of hatchlings and fledglings, but the proportion of surviving eggs in parasitized nests that yielded hatchlings and fledglings was not influenced by parasitism. Overall growth of House Finch nestlings did not differ significantly between parasitized and unparasitized nests. Thus, House Finches were able to successfully rear most of their young irrespective of parasitism. The decrease in reproductive output of House Finches was mainly due to egg removal by cowbirds, which effectively reduced the clutch size of parasitized nests by about 1 House Finch egg.
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Ellison, K., S. G. Sealy, and H. L. Gibbs. "Genetic elucidation of host use by individual sympatric bronzed cowbirds (Molothrus aeneus) and brown-headed cowbirds (M. ater)." Canadian Journal of Zoology 84, no. 9 (September 2006): 1269–80. http://dx.doi.org/10.1139/z06-091.
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Species of avian brood parasites that use one or several species of hosts are called host specialists and generalists, respectively. To determine host use of individual bronzed cowbirds, Molothrus aeneus (Wagler, 1829), and brown-headed cowbirds, Molothrus ater (Boddaert, 1783), we assigned maternity to eggs using microsatellite DNA markers. We measured patterns of host use by individual sympatric cowbirds. This allowed us to determine whether these species competed for host nests and the number of females laying at nests that were already parasitized by conspecifics. We monitored 1447 nests of 42 potential host species and found that each species of cowbird used primarily four host species, with minimal overlap in the species used, yet at least some individuals acted as generalists. Individual cowbirds tended to avoid laying again at nests each had already parasitized, and multiple parasitism was frequently due to same-day laying by more than one female (19% and 44% of 27 and 39 cases for brown-headed and bronzed cowbirds, respectively). Our results suggest that both cowbirds can differentially parasitize host species; however, host use does not appear to be refined, as many eggs were laid in already-parasitized nests when unparasitized nests of other suitable host species were available.
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Burhans, Dirk E., Bill M. Strausberger, and Michael D. Carey. "Regional Variation in Response of Field Sparrows to the Threat of Brown-headed Cowbird Parasitism." Auk 118, no. 3 (July 1, 2001): 776–80. http://dx.doi.org/10.1093/auk/118.3.776.
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Abstract We conducted aggression experiments using model cowbirds on nesting Field Sparrows (Spizella pusilla) in heavily, moderately, and rarely parasitized populations. We also documented Field Sparrow morning nest arrival times during the laying period, because Field Sparrows appear to desert nests in response to encounters with laying female cowbirds. Field Sparrows responded most aggressively to cowbird models and arrived the earliest in Illinois, where they were most heavily parasitized. Field Sparrows responded the least to models in Pennsylvania, where they are almost never parasitized. Our results suggest that those host behaviors result from some aspect of host–cowbird interactions, but the extent to which such behaviors are genetic or learned needs further study.
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Mermoz, Myriam E., and Gustavo J. Fernández. "Breeding Success of a Specialist Brood Parasite, the Screaming Cowbird, Parasitizing an Alternative Host." Condor 105, no. 1 (February 1, 2003): 63–72. http://dx.doi.org/10.1093/condor/105.1.63.
Full textAbstract:
Abstract The Screaming Cowbird (Molothrus rufoaxillaris) is a specialized brood parasite that primarily parasitizes the Bay-winged Cowbird, (Agelaioides badius; Screaming Cowbirds parasitize 80–100% of this species' nests). In contrast, the Shiny Cowbird (M. bonariensis) parasitizes more than 200 hosts. According to the differential reproductive success hypothesis, we expect that Screaming Cowbirds would have a lower reproductive success than Shiny Cowbirds when parasitizing other hosts. We assessed the breeding success of the Screaming Cowbird using an alternative host, the Brown-and-yellow Marshbird (Pseudoleistes virescens). This species is a common host of the Shiny Cowbird with 60–70% of nests parasitized, and is also regularly parasitized by the Screaming Cowbird but with lower frequency (6–20% of the nests). We compared the breeding success of Screaming and Shiny Cowbirds parasitizing this host species. No differences were found in the number of fledglings produced per egg laid between cowbird species. About 8–10% of cowbird eggs produced fledglings. The daily survival rate of Screaming Cowbird eggs was higher than daily survival rates for Shiny Cowbird eggs, but no differences were detected in the nestling daily survival rates. Moreover, we could not detect any difference in the hatching success (nestlings per egg), fledging success (fledglings per nestling), or growth rates of the two parasitic cowbird chicks. Furthermore, breeding success and growth rates of Screaming Cowbird chicks were similar to those previously reported while parasitizing the Bay-winged Cowbird. Our results are not consistent with the differential reproductive success hypothesis proposed as an explanation for the specialized parasitism of Screaming Cowbirds. Éxito Reproductivo del Parásito de Cría Especialista, Molothrus rufoaxillaris, Parasitando un Hospedador Alternativo Resumen. Molothrus rufoaxillaris es un parásito de cría especialista que parasita principalmente a Agelaioides badius (80–100% de los nidos son parasitados). Contrariamente, Molothrus bonariensis parasita más de 200 especies. Basándonos en la hipótesis del éxito reproductivo diferencial, esperamos que M. rufoaxillaris tenga un menor éxito reproductivo que M. bonariensis al parasitar a otros hospedadores. En este trabajo evaluamos el éxito reproductivo de M. rufoaxillaris parasitando un hospedador alternativo, Pseudoleistes virescens. Esta especie es un hospedador común de M. bonariensis con 60–70% de los nidos parasitados, y es también regularmente parasitado por M. rufoaxillaris pero con menor frecuencia (6–20% de los nidos). Comparamos el éxito reproductivo de M. rufoaxillaris y M. bonariensis parasitando esta especie. No encontramos diferencias en el número de volantones producidos por huevo puesto entre ambos parásitos. Alrededor del 8–10% de los huevos puestos por los Molothrus produjeron volantones. La tasa de supervivencia diaria de los huevos de M. rufoaxillaris fue más alta que la de huevos de M. bonariensis. Tampoco encontramos diferencias en el éxito de eclosión (pichón por huevo), éxito de emplumamiento (volantón por pichón), ni en las tasas de crecimiento de los pichones de Molothrus. Además, el éxito reproductivo de M. rufoaxillaris y las tasas de crecimiento de sus pichones fueron similares a las previamente descritas cuando parasita a A. badius. En consecuencia, nuestros resultados no avalan la hipótesis del éxito reproductivo diferencial para explicar la especialización en el parasitismo de M. rufoaxillaris.
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Carmody, Lisa C., Alexander Cruz, and Jameson F. Chace. "Brood Parasitism Defense Behaviors Along an Altitudinal Gradient in the American Robin (Turdus Migratorius)." Open Ornithology Journal 9, no. 1 (November 21, 2016): 39–49. http://dx.doi.org/10.2174/1874453201609010039.
Full textAbstract:
Some host species accept eggs from brood parasites over parts of their range and reject them in other areas representing an “evolutionary lag” in the development of rejection behavior or the loss of an adapative behavior when the selection pressure of brood parasitism is removed. Hosts may deter brood parasitism through egg rejection and aggressive nest defense behavior specifically targetting female brood parasites during the egg incubation period. In areas where parasitism frequencies are spatially and temporally variable, anti-parasite behaviors may decline as costs outweigh the benefits. Along the Colorado Front Range, American robins (Turdus migratorius) breed from low elevations where the brood parasitic Brown-headed Cowbird (Molothrus ater) is abundant to near timberline (3700 m) where cowbirds are uncommon. We tested the hypothesis that egg rejection and nest defense behaviors decline with reduced probability of parasitism. We found that robins accepted 100% of immaculate (robin-like) experimental eggs at both low and high elevations, but were more likely to reject spotted (cowbird-like) experimental eggs at low elevations than high elevations. Response to egg size was more variable than to egg color. When presented with a mount of a cowbird and Song Sparrow (Melospiza melodia) near the nest, robins responded more aggressively to cowbird models than to sparrows (control), and nest defense behavior towards cowbirds was longer and more aggressive at the lower elevation sites where cowbirds are common. These results suggest that egg rejection and nest-site aggression are costly adaptations to cowbird parasitism, and these behaviors decline when the threat of parasitism is reduced.
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Fauth, Peter T. "Reproductive Success of Wood Thrushes in Forest Fragments in Northern Indiana." Auk 117, no. 1 (January 1, 2000): 194–204. http://dx.doi.org/10.1093/auk/117.1.194.
Full textAbstract:
AbstractI monitored 278 Wood Thrush (Hylocichla mustelina) nests in 14 forest fragments (range 7 to 500 ha) in northern Indiana to explore patterns of brood parasitism by Brown-headed Cowbirds (Molothrus ater), nest predation, and reproductive success. Density of thrushes was negatively related to area of forest fragments. Cowbirds were common throughout the landscape, but I found no relationship between their abundance and forest area. Overall, 90% of the thrush nests were parasitized by cowbirds (x̄ = 2.4 cowbird eggs per nest). The number of cowbird eggs per thrush nest was not related to forest area, abundance of host species, or distance to a forest edge but was positively related to thrush abundance. Nest predation rates averaged 58% and were lower than those reported in much of neighboring Illinois but also were unrelated to forest area and distance to a forest edge. The combination of cowbird parasitism and nest predation resulted in relatively low reproductive success (x̄ = 0.6 thrush fledglings per nesting attempt). I estimated that Wood Thrushes in northern Indiana made an average of three nesting attempts per breeding season (based on 17 color-marked females) and had relatively low seasonal fecundity (x̄ = 0.9 female fledglings per adult female per season). Nonetheless, considerable annual variation in seasonal fecundity suggested that some sites exceeded the source-sink threshold in some years. Regardless, the overall landscape appeared to be part of a regional sink for Wood Thrushes, although its negative influence on regional demography was not as severe as elsewhere in the midwestern United States. The poor demographic balance in much of the agriculturally dominated Midwest suggests that conservation efforts for Wood Thrushes and other Nearctic-Neotropical migrants should be directed at preserving and enhancing possible source habitats in regions where parasitism by cowbirds and nest predation are reduced.
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Mermoz, Myriam E., and Juan C. Reboreda. "Reproductive Success of Shiny Cowbird (Molothrus Bonariensis) Parasitizing the Larger Brown-and-Yellow Marshbird (Pseudoleistes Virescens) in Argentina." Auk 120, no. 4 (October 1, 2003): 1128–39. http://dx.doi.org/10.1093/auk/120.4.1128.
Full textAbstract:
Abstract Shiny Cowbirds (Molothrus bonariensis) often parasitize larger hosts. It has been proposed that larger hosts are preferred by that parasite because they provide higher reproductive success, but available data are quite variable. We studied the reproductive success of Shiny Cowbirds in nests of Brown-and-yellow Marshbirds (Pseudoleistes virescens), a larger and often multiply parasitized host. To estimate the extent of interspecific competition, we compared the hatching success of parasite eggs in nests with and without reduction of the clutch size of the host as a result of egg punctures inflicted by the parasite, and the survival and growth of parasite chicks reared with and without host chicks. To estimate the extent of intraspecific competition, we compared Shiny Cowbird egg losses, hatching success, and chick survival in singly versus multiply parasitized nests. Reproductive success of Shiny Cowbirds was 8% with depredation causing 80% of losses. Clutch reduction due to egg punctures were higher in multiply than in singly parasitized nests, but it did not improve hatching success of parasite eggs. Neither survival nor growth of parasite chicks was affected by the presence of host chicks. Shiny Cowbird hatching success and chick survival did not differ between singly and multiply parasitized nests. Parasite chicks were smaller than same-age Brown-and-yellow Marshbird chicks. However, because parasite chicks hatched one or two days before host chicks and had a higher growth rate, they were the larger chicks in the nest. Overall Shiny Cowbird reproductive success in Brown-and-yellow Marshbird nests was apparently higher than that reported in other smaller or similar-sized hosts. We think that host life-history traits like large clutch size, a longer incubation period, and slower growth rate of chicks are responsible for the high reproductive success of Shiny Cowbirds with Brown-and-yellow Marshbirds.
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Strausberger, Bill M., and Dirk E. Burhans. "Nest Desertion by Field Sparrows and its Possible Influence on the Evolution of Cowbird Behavior." Auk 118, no. 3 (July 1, 2001): 770–76. http://dx.doi.org/10.1093/auk/118.3.770.
Full textAbstract:
Abstract In this study, Field Sparrows (Spizella pusilla) deserted 46% of nests, parasitized by Brown-headed Cowbirds (Molothrus ater) and only 1% of unparasitized nests suggesting that desertion functions primarily as an antiparasite defense. Sparrows did not desert nests following various clutch manipulations that are often associated with parasitism, indicating that desertion was not in response to the presence of cowbird eggs. Sparrows often deserted nests following encounters with real or mounted cowbirds, suggesting that nest desertion is a response to adult cowbirds. Sparrows deserted nests only in stages most vulnerable to the effects of parasitism. That finding is consistent with the possibility that desertion is a parasite-specific response. Sparrows arrived at nests earlier in the day at our Illinois site, where parasitism was greater, than in Missouri. Our findings confirm that host vigilance can prevent successful parasitism, and we provide the first direct evidence that encounters with cowbirds may cause hosts to desert nests. Our findings may help explain why cowbirds parasitize nests extremely early in the morning and lay rapidly. We suggest that consideration be given to host response following interactions with adult brood parasites because those interactions may have implications for both the ecology and evolution of both the parasite and host.
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Jensen, William E., and Jack F. Cully. "Geographic Variation in Brown-Headed Cowbird (Molothrus Ater) Parasitism on Dickcissels (Spiza Americana) in Great Plains Tallgrass Prairie." Auk 122, no. 2 (April 1, 2005): 648–60. http://dx.doi.org/10.1093/auk/122.2.648.
Full textAbstract:
Abstract The incidence of brood parasitism by Brown-headed Cowbirds (Molothrus ater; hereafter “cowbirds“) within host species typically reflects the continental pattern in cowbird abundance across North America, where parasitism is heaviest in the Great Plains. However, we found considerable variation in cowbird parasitism on Dickcissel (Spiza americana) nests within a subregion of the Great Plains (the Flint Hills), where the highest levels of cowbird parasitism on grassland bird nests had been previously reported. Local parasitism frequencies on Dickcissel nests varied latitudinally across the Flint Hills, ranging from 0% to 92% of nests parasitized. Interestingly, we found no obvious patterns in habitat or host attributes that were associated with this steep geographic gradient in brood parasitism. Cowbird parasitism on Dickcissel nests was not correlated with the vertical density of local prairie vegetation, mean nest distance to edge, proportion of forest to grassland habitat surrounding study sites (≤5 to 10 km), geographic variation in host abundance, or Dickcissel density or nest initiation dates. Parasitism frequencies and intensities (number of cowbird eggs per parasitized nest) on Dickissel nests were only significantly related to variation in local female cowbird density. Dickcissel clutch size and apparent fledging success were negatively correlated with local cowbird parasitism levels. Geographic patterns in cowbird abundance within and among regions should be considered when establishing conservation areas for grassland birds or other cowbird hosts of concern.
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Goguen, C. B., D. R. Curson, and N. E. Mathews. "Costs of multiple parasitism for an avian brood parasite, the Brown-headed Cowbird (Molothrus ater)." Canadian Journal of Zoology 89, no. 12 (December 2011): 1237–48. http://dx.doi.org/10.1139/z11-104.
Full textAbstract:
The Brown-headed Cowbird ( Molothrus ater (Boddaert, 1783)) is a generalist brood parasite that often lays into nests that contain conspecific eggs. Although it has often been assumed that this multiple parasitism reduces Cowbird survival, this has rarely been evaluated. We measured Cowbird survival in nests of the Blue-gray Gnatcatcher ( Polioptila caerulea (L., 1766)), Plumbeous Vireo ( Vireo plumbeus Coues, 1866), and Western Tanager ( Piranga ludoviciana (A. Wilson, 1811)) in New Mexico, USA. Our objectives were to measure the costs of intraspecific competition on Cowbird survival in multiply-parasitized nests, evaluate if these costs were related to host size, and to compare the costs of multiple parasitism relative to other mortality sources that occur over the entire nesting cycle. Intraspecific competition reduced Cowbird survival during the nestling period in nests of all three hosts, and was of particular importance in nests of the two smaller hosts. When all sources of egg mortality were considered, however, the costs of multiple parasitism were small compared with the large effects of predation and nest desertion. Given that multiple parasitism reduces Cowbird egg survival, it is unclear why Cowbirds multiply-parasitize. Possible explanations depend on an improved understanding of fecundity and level of host nest selectivity by female Cowbirds.
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Igl, Lawrence D., and Douglas H. Johnson. "Brown-headed Cowbird, Molothrus ater, Parasitism and Abundance in the Northern Great Plains." Canadian Field-Naturalist 121, no. 3 (July 1, 2007): 239. http://dx.doi.org/10.22621/cfn.v121i3.471.
Full textAbstract:
The Brown-headed Cowbird (Molothrus ater) reaches its highest abundance in the northern Great Plains, but much of our understanding of cowbird ecology and host-parasite interactions comes from areas outside of this region. We examine cowbird brood parasitism and densities during two studies of breeding birds in the northern Great Plains during 1990–2006. We found 2649 active nests of 75 species, including 746 nonpasserine nests and 1902 passerine nests. Overall, <1% of nonpasserine nests and 25% of passerine nests were parasitized by Brown-headed Cowbirds. Although the overall frequency of cowbird parasitism in passerine nests in these two studies is considered moderate, the frequency of multiple parasitism among parasitized nests was heavy (nearly 50%). The mean number of cowbird eggs per parasitized passerine nest was 1.9 ± 1.2 (SD; range = 1–8 cowbird eggs). The parasitism rates were 9.5% for passerines that typically nest in habitats characterized by woody vegetation, 16.4% for grassland-nesting passerines, 4.7% for passerines known to consistently eject cowbird eggs, and 28.2% for passerines that usually accept cowbird eggs. The Red-winged Blackbird (Agelaius phoeniceus) was the most commonly parasitized species (43.1% parasitism, 49.6% multiple parasitism, 71.2% of all cases of parasitism). Passerine nests found within areas of higher female cowbird abundance experienced higher frequencies of cowbird parasitism than those found in areas of lower female cowbird abundance. Densities of female cowbirds were positively related to densities and richness of other birds in the breeding bird community.
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Burhans, Dirk E., Frank R. Thompson, and John Faaborg. "Costs of Parasitism Incurred by Two Songbird Species and Their Quality as Cowbird Hosts." Condor 102, no. 2 (May 1, 2000): 364–73. http://dx.doi.org/10.1093/condor/102.2.364.
Full textAbstract:
Abstract We measured the costs of Brown-headed Cowbird (Molothrus ater) parasitism incurred by Field Sparrows (Spizella pusilla) and Indigo Buntings (Passerina cyanea). We predicted that the frequent occurrence of nest desertion as a response to cowbird parasitism in Field Sparrows would be reflected by a higher cost of parasitism for that species. We also compared growth and survival of cowbird nestlings between hosts, predicting that they would do poorly at Field Sparrow nests because the latter appear to be avoided by cowbirds. Both species experienced reduced body mass gain in parasitized broods, but only Indigo Bunting suffered reduced tarsus growth. Both species experienced reductions in clutch size, hatching success, and nestling survival due to parasitism, but these losses did not differ among the two hosts. Multiple parasitism did not affect hatching success or nestling survival more than single parasitism for Indigo Buntings. Once accepted, cowbird offspring fared equally well in nests of both species, but almost half of all cowbird eggs laid in Field Sparrow nests were lost through nest abandonment. As parasitism costs to both species appear to be substantial, the rarity of nest desertion in Indigo Buntings may be due to other factors. Infrequent parasitism of Field Sparrows is consistent with host avoidance by cowbirds but other explanations should be explored.
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Hilario-Pérez, Alexis D., and Ashley P. G. Dowling. "Nasal mites from specimens of the brown-headed cowbird (Icteridae: Molothrus ater) from Texas and Arkansas, U.S.A." Acarologia 58, no. 2 (February 15, 2018): 296–301. http://dx.doi.org/10.24349/acarologia/20184242.
Full textAbstract:
Avian nasal mites are obligate parasites that spend their entire life in the respiratory system of birds. In North America, four families in three orders parasitize birds: Rhinonyssidae (Mesostigmata), Ereynetidae (Prostigmata), Cytoditidae (Astigmata) and Turbinoptidae (Astigmata). Mechanisms of nasal mite transmission among hosts is still unclear and brood parasites like the Brown-headed Cowbird (Molothrus ater Boddaert, 1783), which utilize multiple bird species as hosts, are interesting for the study of parasites such as nasal mites. We examined 126 cowbirds for nasal mites and found 84 individuals (66.6 %) to be infested. The most common nasal mite species recovered was Ptilonyssus icteridius Strandmann and Furman, 1956. The presence of P. richmondenae George, 1961, in Brown-headed Cowbird represents a new host association, and a potential new species of Ptilonyssus was also recovered. The ereynetid species Boydaia quiscali Clark, 1960 was found in two Brown-headed Cowbirds, both times a co-infestation with species of Ptilonyssus.
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Astié, Andrea A., and Juan C. Reboreda. "Costs of Egg Punctures and Parasitism by Shiny Cowbirds (Molothrus Bonariensis) at Creamy-Bellied Thrush (Turdus Amaurochalinus) Nests." Auk 123, no. 1 (January 1, 2006): 23–32. http://dx.doi.org/10.1093/auk/123.1.23.
Full textAbstract:
AbstractMost studies on cowbird parasitism have focused on its effects on parasitized nests, whereas few have considered the costs at nests that cowbirds visit but at which they do not lay eggs. Shiny Cowbirds (Molothrus bonariensis) peck and puncture host eggs both in nests where they lay eggs and in unparasitized nests. We analyzed the effect of egg punctures in unparasitized and parasitized nests of a large host, the Creamy-bellied Thrush (Turdus amaurochalinus; hereafter “thrush”), as well as the costs of Shiny Cowbird eggs and chicks in this host’s nests. We determined thrush egg survival, hatching success, and chick survival in successful nests, and nest survival during the egg and nestling stages. Frequency of parasitism was 60%, and its intensity 1.6 ± 0.1 eggs nest−1. Number of host eggs punctured was positively associated with intensity of parasitism. The host’s eggs were frequently punctured in parasitized nests (71%) and in unparasitized nests (42%). Egg punctures reduced the number of eggs at hatching in 23% and 49% of unparasitized and parasitized nests, respectively. Nests with egg punctures had a lower survival rate than nests without them, but nest survival was not associated with parasitism. Presence of a Shiny Cowbird egg was associated with a decrease in the hatching success of host eggs, but presence of a Shiny Cowbird chick did not have any detrimental effect on either the survival and growth rate of host chicks in successful nests or the survival of the whole nest. Our results show that egg punctures were the primary determinant of thrush reproductive success. Consequently, comparison of unparasitized and parasitized nests gives an incomplete estimation of the effects of Shiny Cowbirds on host reproductive success, because the cost of egg punctures is also important in nests where there is no Shiny Cowbird egg laying.Costos de la Perforación de Huevos y el Parasitismo por Molothrus bonariensis en Nidos de Turdus amaurochalinus
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Gloag, Ros, Vanina D. Fiorini, Juan C. Reboreda, and Alex Kacelnik. "Brood parasite eggs enhance egg survivorship in a multiply parasitized host." Proceedings of the Royal Society B: Biological Sciences 279, no. 1734 (December 7, 2011): 1831–39. http://dx.doi.org/10.1098/rspb.2011.2047.
Full textAbstract:
Despite the costs to avian parents of rearing brood parasitic offspring, many species do not reject foreign eggs from their nests. We show that where multiple parasitism occurs, rejection itself can be costly, by increasing the risk of host egg loss during subsequent parasite attacks. Chalk-browed mockingbirds ( Mimus saturninus ) are heavily parasitized by shiny cowbirds ( Molothrus bonariensis ), which also puncture eggs in host nests. Mockingbirds struggle to prevent cowbirds puncturing and laying, but seldom remove cowbird eggs once laid. We filmed cowbird visits to nests with manipulated clutch compositions and found that mockingbird eggs were more likely to escape puncture the more cowbird eggs accompanied them in the clutch. A Monte Carlo simulation of this ‘dilution effect’, comparing virtual hosts that systematically either reject or accept parasite eggs, shows that acceptors enjoy higher egg survivorship than rejecters in host populations where multiple parasitism occurs. For mockingbirds or other hosts in which host nestlings fare well in parasitized broods, this benefit might be sufficient to offset the fitness cost of rearing parasite chicks, making egg acceptance evolutionarily stable. Thus, counterintuitively, high intensities of parasitism might decrease or even reverse selection pressure for host defence via egg rejection.
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Sealy, Spencer G., and Diane L. Neudorf. "Reactions of Four Passerine Species To Threats of Predation and Cowbird Parasitism: Enemy Recognition or Generalized Responses?" Behaviour 123, no. 1-2 (1992): 84–105. http://dx.doi.org/10.1163/156853992x00138.
Full textAbstract:
AbstractFour host species of the parasitic brown-headed cowbird (Molothrus ater) were exposed to taxidermic mounts of a female cowbird, fox sparrow (Passerella iliaca), and common grackle (Quiscalus quiscula) at their nests during their egg-laying or nestling stage. Red-winged blackbirds (Agelaius phoeniceus), a species that accepts cowbird eggs laid in their nests, responded more aggressively to cowbird models early in their nesting cycle, indicating that they recognized the unique threat the cowbird posed. Gray catbirds (Dumetella carolinensis), northern orioles (Icterus galbula), and cedar waxwings (Bombycilla cedrorum) can remove cowbird eggs from their nests and for the most part they responded similarly to cowbird models and the "nonthreatening control," i.e. a fox sparrow. Cedar waxwings were nonaggressive to all the models and may rely on concealment to protect their nests from enemies. Removal of cowbird eggs by puncture ejection is more risky than grasp ejection. Despite this, orioles and waxwings (puncture ejectors) were not significantly more aggressive to cowbird models at egg laying than catbirds (grasp ejectors). Responses of the three rejector species toward the cowbird model did not change over the nesting cycle, indicating further that they do not recognize cowbirds as a unique threat. Rejector species may not recognize cowbirds because they have little experience with them. With the exception of waxwings, all of the hosts recognized the grackle as an enemy and increased their levels of defence from the laying to nestling stages. Three of the host-species did not simply respond in a generalized manner to any intruder at their nests but indeed recognized specific enemies. Considerable interspecific variability exists amongst the four species in defensive behaviours, which may reflect their different nesting habitats.
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Hosoi, S. Aki, Stephen I. Rothstein, and Adrian L. O'Loghlen. "Sexual Preferences of Female Brown-Headed Cowbirds (Molothrus Ater) for Perched Song Repertoires." Auk 122, no. 1 (January 1, 2005): 82–93. http://dx.doi.org/10.1093/auk/122.1.82.
Full textAbstract:
Abstract The importance of repertoire size to mate choice by female songbirds has been widely debated but has not been addressed in Brown-headed Cowbirds (Molothrus ater; hereafter ”cowbirds”), despite extensive work on this species' songs. Male cowbirds cycle through repertoires of 2–8 perched song (PS) types in rapid sequences immediately prior to and during copulation, which suggests that repertoire size or content is important in mating contexts. Because male mating success in one well-studied cowbird population is correlated with large PS repertoires, we predicted that repertoire size is a cue in female mate choice. To test that prediction, we played three repetitions of conspecific songs in rapid succession to estradiol-primed female cowbirds (M. a. obscurus), to simulate the rapid song sequences that accompany naturally occurring matings. Females were presented with three different PS types, three repetitions of the same type, and heterospecific control songs of the same duration as the conspecific playbacks. Females gave longer copulation solicitation displays to three different PS types than to three repetitions of a single PS type, regardless of whether the playback songs were local or foreign. This is one of the few experimental studies on songbirds to show greater female responsiveness to a song repertoire presented so as to mimic singing in a natural mating context, and the first to present such a result for cowbirds. We conclude that presentation of multiple PS types is more sexually stimulating to females than a single type, because females prefer larger repertoires or singing patterns with a higher frequency of song-type switching. Preferencias Sexuales de Hembras de Molothrus ater por Repertorios de Canto de Percha
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McLaren, Celia M., Todd J. Underwood, and Spencer G. Sealy. "Conflicting Temporal Changes in the Frequency and Intensity of Cowbird Parasitism on Four Common Hosts in Ontario over 130 Years." Condor 108, no. 1 (February 1, 2006): 238–42. http://dx.doi.org/10.1093/condor/108.1.238.
Full textAbstract:
AbstractWe tested the hypothesis proposed by Friedmann (1963) that multiple parasitism (nests with more than one parasitic egg) by Brown-headed Cowbirds (Molothrus ater) is a recent phenomenon associated with increased densities of cowbirds. We used Ontario Nest Records Scheme data to quantify frequencies of parasitism and multiple parasitism on four common host species over the last 130 years. Frequency of multiple parasitism significantly increased over all decades only in Chipping Sparrows (Spizella passerina). We also analyzed data separately before and after 1970 because at about this time cowbird abundance began to stabilize and then decline. Pre-1970, multiple parasitism significantly increased on Chipping Sparrows and Song Sparrows (Melospiza melodia), but neither had significant trends post-1970. There were no significant trends in multiple parasitism on Red-eyed Vireos (Vireo olivaceus) or Northern Cardinals (Cardinalis cardinalis). For all time periods analyzed, there were no significant trends in overall frequency of parasitism on any species. Our results provide limited support for the hypothesis that temporal variation in parasitism patterns has followed changes in cowbird density over the past century.
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Florisvaldo Batisteli, Augusto, and Marco Aurelio Aurélio Pizo. "The location of thrush nests on buildings affects the chance of cowbird parasitism." Ecosistemas 31, no. 1 (April 26, 2022): 2196. http://dx.doi.org/10.7818/ecos.2196.
Full textAbstract:
Nest site placement is a critical choice among passerines, being an important factor that affects their breeding success. In urban areas, human buildings offer suitable nesting sites usually less exposed to predators and brood parasitic birds and readily available to be reused for several breeding seasons. However, the extent to which the features of nest placement sites in buildings contribute to reduce nest detectability by predators and brood parasites is still unknown. Here, we tested whether the features of the nesting site (i.e., lateral concealment, distance to the building ceiling, and height above ground) affect the chance of brood parasitism by Shiny Cowbirds (Molothrus bonariensis) in Pale-breasted Thrushes (Turdus leucomelas) nests placed on buildings in a Brazilian suburban area from 2013 to 2019. Cowbird parasitism increased throughout the study years, and nests closer to ceilings, supposedly better concealed, were more likely to be parasitized. Laying date, height above ground, and lateral concealment were not related to the probability of cowbird parasitism. We suggest that less concealed nests enhance vicinity monitoring by parents, allowing a faster agonistic response to the presence of cowbirds near the nest. Our results indicate that nest site location in cities may have consequences for the breeding success of cowbird hosts.
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Florisvaldo Batisteli, Augusto, and Marco Aurelio Aurélio Pizo. "The location of thrush nests on buildings affects the chance of cowbird parasitism." Ecosistemas 31, no. 1 (April 26, 2022): 2196. http://dx.doi.org/10.7818/ecos.2196.
Full textAbstract:
Nest site placement is a critical choice among passerines, being an important factor that affects their breeding success. In urban areas, human buildings offer suitable nesting sites usually less exposed to predators and brood parasitic birds and readily available to be reused for several breeding seasons. However, the extent to which the features of nest placement sites in buildings contribute to reduce nest detectability by predators and brood parasites is still unknown. Here, we tested whether the features of the nesting site (i.e., lateral concealment, distance to the building ceiling, and height above ground) affect the chance of brood parasitism by Shiny Cowbirds (Molothrus bonariensis) in Pale-breasted Thrushes (Turdus leucomelas) nests placed on buildings in a Brazilian suburban area from 2013 to 2019. Cowbird parasitism increased throughout the study years, and nests closer to ceilings, supposedly better concealed, were more likely to be parasitized. Laying date, height above ground, and lateral concealment were not related to the probability of cowbird parasitism. We suggest that less concealed nests enhance vicinity monitoring by parents, allowing a faster agonistic response to the presence of cowbirds near the nest. Our results indicate that nest site location in cities may have consequences for the breeding success of cowbird hosts.
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Hauber, Mark E. "Egg-Capping is a Cost Paid by Hosts of Interspecific Brood Parasites." Auk 120, no. 3 (July 1, 2003): 860–65. http://dx.doi.org/10.1093/auk/120.3.860.
Full textAbstract:
Abstract Egg-capping is the slipping of a hatched eggshell fragment over an intact shell. It is a rare phenomenon among avian species, probably because most eggs in the same clutch hatch synchronously, are similar in size, and parents typically remove debris—including shell fragments—from nests. Hatching asynchrony and egg-size differences are typically more pronounced in the context of interspecific brood parasitism, making conditions for egg-capping more likely. Indeed, egg-capping of host eggs occurred in 33% of clutches of Eastern Phoebes (Sayornis phoebe) parasitized by Brown-headed Cowbirds (Molothrus ater), always by the earlier hatched cowbird's eggshell, whereas it was not detected in nonparasitized clutches near Ithaca, New York. To determine experimentally if host parents rejected their own eggs capped by the parasite's shell, cowbird shell fragments were introduced into nonparasitized phoebe clutches either as slipped over intact phoebe eggs (caps) or simply placed into the nests (noncaps). The latency for cowbird egg-fragments to disappear was greater for caps (1–13 days) because all noncaps were removed in <1 day. Whether phoebe eggs hatched was also related to the outcome of the capping treatments: eggs that remained capped for >1 day had a greater probability of hatching failure (0.6) than noncapped eggs (0). Those data suggest that egg-capping by shell-fragments of earlier hatched cowbird eggs reduces phoebes' reproductive success and may represent a fitness cost to hosts in other parasitized species as well.
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Massoni, Viviana, and Juan Carlos Reboreda. "A Neglected Cost of Brood Parasitism: Egg Punctures by Shiny Cowbirds During Inspection of Potential Host Nests." Condor 104, no. 2 (May 1, 2002): 407–12. http://dx.doi.org/10.1093/condor/104.2.407.
Full textAbstract:
Abstract Parasitized hosts of the Shiny Cowbird (Molothrus bonariensis) suffer several costs, and among the most important is the loss of eggs through egg punctures inflicted by the parasite. Unparasitized nests also have eggs damaged by cowbirds, but researchers usually ignore these losses. To quantify this cost we compared three groups of nests of the Yellow-winged Blackbird (Agelaius thilius): parasitized and unparasitized nests from an area used by Shiny Cowbirds, and unparasitized nests from an area not used by the parasite. Because cowbirds puncture eggs as soon as the first host eggs are laid, we calculated the clutch size only for those nests found during construction. Unparasitized nests in the area used by cowbirds had lower egg survival rate and hatching success and higher probability of nest desertion than unparasitized nests in the cowbird-free area. Our results indicate that one must consider egg punctures at unparasitized nests to avoid underestimating the impact of parasitism. Un Costo Ignorado del Parasitismo de Cría: Perforación de Huevos por Molothrus bonariensis durante la Inspección de Potenciales Nidos de Hospedadores Resumen. Los hospedadores de Molothrus bonariensis sufren varios costos, entre los que se destaca la pérdida de huevos debida a perforaciónes hechas por el parásito. Los nidos no parasitados también tienen huevos perforados por M. bonariensis, pero estas pérdidas son normalmente ignoradas. Para cuantificar este costo comparamos tres grupos de nidos de Agelaius thilius: nidos parasitados y no parasitados de un área usada por M. bonariensis, y nidos no parasitados de un área no utilizada por el parásito. Como los parásitos perforan los huevos del hospedador tan pronto como éstos son puestos, el tamaño de puesta se calculó utilizando únicamente los nidos encontrados durante su construcción. Los nidos no parasitados del área visitada por los parásitos tuvieron menor supervivencia de sus huevos durante la incubación, menor éxito de eclosión y mayor probabilidad de ser abandonados que los nidos no parasitados del área libre de parásitos. Los resultados obtenidos indican que deben considerarse las perforaciones de huevos en nidos no parasitados para evitar subestimar el impacto del parasitismo.
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Sealy, Spencer G. "Egg laying in inappropriate nests by the Brown-headed Cowbird (Molothrus ater): acts of parasitism or emergency egg dumping?" Canadian Field-Naturalist 129, no. 1 (June 5, 2015): 60. http://dx.doi.org/10.22621/cfn.v129i1.1668.
Full textAbstract:
The generalist, brood-parasitic Brown-headed Cowbird (Molothrus ater) has been found to parasitize the nests of about 220 species, mostly passerine birds. Among the thousands of documented cases of parasitism are rare records of egg laying in nests in which the cowbird stands no chance of success, because its diet or developmental strategy are incompatible with those of the “host” species. Forty-four nests of 16 such inappropriate host species are reviewed: 23 nests of nine precocial species (waterbirds and shorebirds) plus 21 nests of seven altricial species (a raptor, doves, cuckoos, a hummingbird, and a woodpecker). Two hypotheses explain inappropriate egg laying. In the “normal laying” hypothesis, Brown-headed Cowbirds may lay dozens of eggs in nests they encounter, including the occasional inappropriate nest. In the “emergency laying” hypothesis, females, on discovering that a selected nest has failed, must lay or “dump” her eggs elsewhere, in nests of inappropriate hosts or already-parasitized nests of regular hosts. Support for either hypothesis will require electronic surveillance of movements of nest-searching and laying Brown-headed Cowbirds to generate fine-scale spatial data that confirm whether parasitism on inappropriate nests occurs at the usual laying time for pre-selected nests (around sunrise) or later in the day if the chosen nest has failed and emergency laying is required.
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Strausberger, Bill M., and Mary V. Ashley. "Breeding Biology of Brood Parasitic Brown-Headed Cowbirds (Molothrus Ater) Characterized by Parent-Offspring and Sibling-Group Reconstruction." Auk 120, no. 2 (April 1, 2003): 433–45. http://dx.doi.org/10.1093/auk/120.2.433.
Full textAbstract:
Abstract We characterized several equivocal aspects of the breeding biology of the brood parasitic Brown-headed Cowbird (Molothrus ater) at a study site in northeastern Illinois. A total of 175 offspring and a partial sample of parents were sampled and genotyped at six microsatellite loci. A combination of sibling-group and parentage assignment enabled us to characterize mating behavior and area used for reproduction by breeding adults, even for those not sampled. We assigned a mean of 4.7 (range 1–13) and 4.4 (range 1–16) offspring to 33 female and 32 male parents, respectively. Adults typically reproduced with a “primary partner” but up to three partners were common. Offspring females and males were spread over 9 and 12 ha, respectively. Half of the polygynous males produced offspring with an additional mate that parasitized nests near to or within areas overlapping those of their primary partner. That suggests that mate choice takes place at females' egg-laying areas as opposed to more remote social areas. Multiple females frequently parasitized a single host nest and areas used for reproduction overlapped extensively for individuals of either sex. High frequencies of parasitism and superparasitism indicate a high cowbird density relative to that of hosts. Frequent promiscuity by cowbirds at our site contrasts with other studies reporting monogamy and may be due to higher densities of breeding cowbirds at our site.
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Hauber, Mark E. "Site selection and repeatability in Brown-Headed Cowbird (Molothrus ater) parasitism of Eastern Phoebe (Sayornis phoebe) nests." Canadian Journal of Zoology 79, no. 8 (August 1, 2001): 1518–23. http://dx.doi.org/10.1139/z01-091.
Full textAbstract:
The reproductive success of obligate brood parasitic birds depends on their ability to seek out heterospecific nests. Some nests are more suitable for parasitism than others and, for example, parasitic females may benefit from laying eggs preferentially and repeatedly at safer sites. Observations on patterns of Brown-headed Cowbird (Molothrus ater) parasitism on Eastern Phoebes (Sayornis phoebe) across 2 years suggested that parasitism occurred at above chance levels during the first rather than the second nesting attempts and at nests located under eaves rather than bridges. Previously parasitized nests were more likely to be parasitized again in the subsequent breeding season. Sites under eaves and bridges did not differ in whether Brown-headed Cowbirds could be detected in the proximity of the nest. However, nests from first nesting attempts and nests under eaves were less likely to be lost as a result of structural failure of the Eastern Phoebe's mud nest. These data suggest that site discrimination by Brown-headed Cowbirds leads to adaptive patterns of parasitism among available Eastern Phoebe nests.
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Ward, David, and James N. M. Smith. "Brown-Headed Cowbird Parasitism Results in a Sink Population in Warbling Vireos." Auk 117, no. 2 (April 1, 2000): 337–44. http://dx.doi.org/10.1093/auk/117.2.337.
Full textAbstract:
Abstract The Warbling Vireo (Vireo gilvus) is one of the most heavily parasitized host species of the Brown-headed Cowbird (Molothrus ater), suffering up to 80% parasitism in some areas. Warbling Vireo nests that are parasitized by cowbirds typically produce no vireo young. To make predictions about the consequences of brood parasitism on local host populations, we investigated factors that allow such high parasitism to occur. The major factors leading to high levels of brood parasitism on Warbling Vireos in the Okanagan Valley, British Columbia, are habitat overlap with cowbirds and the lack of response of vireos to cowbird eggs. Warbling Vireos responded more strongly to a cowbird dummy placed near the nest than to a dummy of a “neutral” species (junco) or to a common nest predator (crow). Preliminary simulation models based on population parameters obtained from three summers of fieldwork suggest that Warbling Vireos are in danger of extirpation from the Okanagan Valley if little movement of birds occurs between areas with different levels of brood parasitism. The models also indicate the sensitivity of this species to variation in adult and juvenile survival rates and number of successful broods produced per season. Our results emphasize the need for a metapopulation analysis that compares the population ecology of Warbling Vireos at high elevations (and probably with low parasitism) with that at low elevations (high parasitism) and that ascertains whether sufficient dispersal occurs between these populations to prevent extinction at lower elevations. Sufficient dispersal is indicated by apparently stable vireo populations in the Okanagan Valley and by Breeding Bird Survey data that show an overall increasing trend for this species in British Columbia.
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Kilpatrick, A. Marm. "Variation in growth of Brown-headed Cowbird (Molothrus ater) nestlings and energetic impacts on their host parents." Canadian Journal of Zoology 80, no. 1 (January 1, 2002): 145–53. http://dx.doi.org/10.1139/z01-217.
Full textAbstract:
I tested several hypotheses about the plasticity of avian growth by comparing growth of Brown-headed Cowbird (Molothrus ater) nestlings in 20 different host species. Growth of cowbird nestlings was not strongly correlated with host adult mass and nearly all hosts were able to provision cowbirds with enough food to grow at nearly the highest observed rate. Cowbird growth was positively correlated with site latitude (which negatively covaried with maximum temperature) and negatively correlated with the nestling period of the host species. The metabolizable energy expenditure of cowbird and host nestlings was estimated as an approximate measure of the food provisioned by parents. As host adult mass varied from 6 to 113 g, one cowbird nestling was equivalent to 3.40.56 host nestlings in terms of peak daily energy intake. Cowbird nestlings impose a substantial energetic demand on smaller host parents that may reduce their future survival or fecundity. The quantity of energy delivered to parasitized nests demonstrates that parents are often willing to provision nests at a much higher rate than for an average clutch of their own young.
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Weatherhead, Patrick J., and Gordon F. Bennett. "Ecology of parasitism of Brown-headed Cowbirds by haematozoa." Canadian Journal of Zoology 70, no. 1 (January 1, 1992): 1–7. http://dx.doi.org/10.1139/z92-001.
Full textAbstract:
We quantified haematozoa infections in 964 Brown-headed Cowbirds (Molothrus ater) sampled over three summers in eastern Ontario. Our objective was to use the cowbird–haematozoa system to assess practical and theoretical aspects of testing Hamilton and Zuk's parasite hypothesis of sexual selection. We found that most individuals resampled within and between years were consistently scored as parasitized or unparasitized, although some individuals changed status, including going from parasitized to unparasitized. Many hatching-year birds were parasitized, which suggests substantial early exposure to parasites. Significant variation within and between years in general parasite prevalence, specific parasite prevalence, and intensity of infections indicated that population estimates of parasitism would be highly dependent on when birds were sampled and on the age and sex composition of the sample. Analysis of body condition and within-season recapture patterns indicated that parasitism did not have a negative effect on the health of cowbirds. Parasitized birds were also recaptured at the same rate as unparasitized birds between years, indicating that there were no viability effects due to parasitism. Our results suggest a number of pitfalls, some of which are potentially critical, to using haematozoa infections in passerines to test the Hamilton and Zuk hypothesis.
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White, David J., J. Arthur, H. B. Davies, and M. F. Guigueno. "Cognition and reproductive success in cowbirds." Learning & Behavior 50, no. 1 (December 16, 2021): 178–88. http://dx.doi.org/10.3758/s13420-021-00506-0.
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AbstractUnderstanding the relationships between cognitive abilities and fitness is integral to an evolutionary study of brain and behavior. However, these relationships are often difficult to measure and detect. Here we draw upon an opportunistic sample of brown-headed cowbird (Molothrus ater) subjects that had two separate research experiences: First, they engaged in a large series of cognitive tests in David Sherry’s Lab in the Advanced Facility for Avian Research (AFAR) at Western University, then subsequently moved to the Field Avian Research Megalab (FARM) at Wilfrid Laurier University where they lived in large breeding flocks in aviaries with other wild-caught cowbirds. Thus, we had extensive measures of cognitive abilities, breeding behavior, and reproductive success for these birds. We report here, for the fist time, the surprisingly strong connections we found among these different measures. Female cowbirds’ spatial cognitive abilities correlated positively with how intensely they were courted by males, and with their overall egg production. Males’ spatial cognition correlated positively with their ability to engage in singing contests (“countersinging”) with other males. In addition, a separate non-spatial cognitive ability correlated positively with the attractiveness of the songs they sung. In sum, these results suggest the cognitive skills assessed in the lab were strongly connected to breeding behavior and reproductive success. Moreover, since certain cognitive abilities related to different aspects of breeding success, it suggests that cognitive modules may have specialized adaptive value, but also that these specialized skills may interact and influence fitness in surprising ways.
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Ford, Thomas B., Donald E. Winslow, Donald R. Whitehead, Matthew A. Koukol, and M. du Plessis. "Reproductive Success of Forest-Dependent Songbirds Near an Agricultural Corridor in South-Central Indiana." Auk 118, no. 4 (October 1, 2001): 864–73. http://dx.doi.org/10.1093/auk/118.4.864.
Full textAbstract:
Abstract Potential source populations of forest-breeding Neotropical migrant birds may be threatened by anthropogenic changes that increase brood parasitism by Brown-headed Cowbirds (Molothrus ater) and nest predation in heavily forested breeding areas. In south-central Indiana, corridors of agriculture and rural development, ranging from <50 m to several thousand meters in width, penetrate interior portions of the heavily forested landscape. These corridors provide habitat for cowbirds and nest predators. We monitored breeding success of six species of Neotropical migrants and one resident species near an agricultural corridor and in interior forest. We found that nest survival was lower near the agricultural corridor for most of the species in the nestling stage, but no consistent difference in nest survival was detected during the egg stage. Levels of cowbird parasitism were generally elevated near the agricultural corridor. Estimates of the number of fledglings per nesting attempt indicated that seasonal productivity was lower near the agricultural corridor for six of the seven species. Status of populations of birds in south-central Indiana as sources in the Midwest may be compromised by extensive intrusion of agricultural corridors within the contiguous, heavily forested landscape.
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Scott, David M. "The time of day of egg laying by the Brown-headed Cowbird and other icterines." Canadian Journal of Zoology 69, no. 8 (August 1, 1991): 2093–99. http://dx.doi.org/10.1139/z91-292.
Full textAbstract:
I present data on the time of day of oviposition for 16 species of icterines to evaluate the idea that laying exceptionally early in the morning by the brood parasite, the Brown-headed Cowbird (Molothrus ater), is a specialization for parasitism. This cowbird, the earliest known layer among icterines, typically lays before sunrise (sunrise −9.14 ± 2.52 (SE) min, n = 36). Another brood parasite, the Shiny Cowbird (Molothrus bonariensis), also sometimes lays before sunrise. The best-known nonparasitic icterines, the Red-winged Blackbird (Agelaius phoeniceus) and the Common Grackle (Quisacalus quiscula), lay shortly after sunrise. Fragmentary records show that several other icterines lay within an hour or so after sunrise. Data are inadequate to conclude that early laying by Brown-headed Cowbirds has arisen as an adaptation for parasitism. The Brown-headed Cowbird at London, Ontario, lays earlier than most local passerines, as is shown by new data on oviposition by seven host species arranged in order of increasing lateness of oviposition: Agelaius phoeniceus, Dendroica petechia, Melospiza melodia, Cardinalis cardinalis, Vireo olivaceus, Dumetella carolinensis, and Turdus migratorius.
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O'Connor, C. M. "KINDERGARTEN FOR COWBIRDS." Journal of Experimental Biology 216, no. 15 (July 10, 2013): v. http://dx.doi.org/10.1242/jeb.081620.
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Budnik, Joel M., Dirk E. Burhans, Mark R. Ryan, and Frank R. Thompson III. "Nest Desertion and Apparent Nest Protection Behavior by Bell's Vireos in Response to Cowbird Parasitism." Condor 103, no. 3 (August 1, 2001): 639–43. http://dx.doi.org/10.1093/condor/103.3.639.
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AbstractBell's Vireos (Vireo bellii) deserted 51% of nests parasitized by Brown-headed Cowbirds (Molothrus ater) in central Missouri from 1996–1998. Deserting vireo pairs fledged more host young within a season than pairs that accepted cowbird eggs when only successful nests were considered; parasitized acceptor nests never fledged any vireo young. Vireo pairs that deserted did not always desert parasitized nests. We observed five encounters between female cowbirds and Bell's Vireos at four nests. In one observation the female vireo used nest-protection behavior, which resulted in the cowbird egg appearing beneath the nest; this nest was not deserted. We found a total of eight nests where cowbird eggs remained on the ground below nests, five of which were abandoned. Nest desertion appears to benefit Bell's Vireos by allowing for unparasitized renests, but the stimuli eliciting nest desertion by Bell's Vireos remain in need of further study.Deserción de Nidos y Aparente Comportamiento Protector de Vireo bellii en Respuesta al Parasitismo de Molothrus aterResumen. Individuos de Vireo bellii abandonaron el 51% de los nidos que fueron parasitados por Molothrus ater en el centro de Missouri entre 1996–1998. Las parejas desertoras de V. bellii criaron más juveniles propios por estación que parejas que aceptaron huevos de M. ater cuando solamente se consideraron nidos exitosos; los nidos parasitados aceptados nunca produjeron ningún juvenil de V. bellii. Las parejas desertoras de Vireo no siempre abandonaron nidos parasitados. Observamos cinco encuentros entre hembras de M. ater y de V. bellii en cuatro nidos. En una observación, la hembra de Vireo realizó comportamientos de protección del nido que resultaron en la aparición del huevo de M. ater debajo del nido; este nido no fue abandonado. Encontramos un total de ocho nidos con huevos de M. ater tirados en el suelo debajo de los nidos, cinco de los cuales fueron abandonados. La deserción de nidos parece beneficiar a V. bellii al permitirle re-establecer nidos no parasitados, pero el estímulo que impulsa la deserción de nidos en V. bellii necesita aún ser estudiado.