There are five species of parasitic cowbirds (Molothrus) and the Brown-headed Cowbird (M. ater) is the only widespread species in North America. The Brown-headed Cowbird is a host generalist and is typically found in ...There are five species of parasitic cowbirds (Molothrus) and the Brown-headed Cowbird (M. ater) is the only widespread species in North America. The Brown-headed Cowbird is a host generalist and is typically found in open habitats and forest edges. The cowbirds are of a more recent origin than many other brood parasites and perhaps as a result, cowbird adaptations for parasitism and their hosts’ counter-adaptations to thwart parasitism do not appear as sophisticated as those of other brood parasite-host systems. Because of its generalist nature, the cowbird has the potential to negatively impact endangered host species whose populations are limited due to anthropogenic habitat loss. As a consequence, the Brown-headed Cowbird is one of the few brood parasitic species that is the subject of control programs to limit its effects on such hosts.展开更多
The brood-parasitic brown-headed cowbird(Molothrus ater)has one of the shortest incubation periods of any bird.Brown-headed cowbird eggs,and those of other avian brood parasites,tend to be more spherical due to their ...The brood-parasitic brown-headed cowbird(Molothrus ater)has one of the shortest incubation periods of any bird.Brown-headed cowbird eggs,and those of other avian brood parasites,tend to be more spherical due to their greater relative width.The traditional explanation for this egg shape is that it,combined with the thicker eggshells,resists host puncture-ejection.However,very few North American hosts of the brownheaded cowbird actually engage in puncture-ejection and therefore wider eggs may instead provide greater contact with a host’s brood patch during incubation,especially in large host nests.We tested whether greater egg width increased mean temperature and reduced temperature variation in brown-headed cowbirds by inserting temperature probes into brown-headed cowbird and house sparrow(Passer domesticus)eggs and placing them into red-winged blackbird(Agelaius phoeniceus)nests.House sparrow eggs are similar in appearance and in length to cowbird eggs,but are not as wide.We found no signifcant relationship between brown-headed cowbird egg width and mean incubation temperature.However,brown-headed cowbird eggs experienced less temperature variation than house sparrow eggs,and within brown-headed cowbird eggs,more spherical eggs experienced less temperature variation when accounting for differences in width.These results suggest that brownheaded cowbirds may have short incubation periods in part because their eggs exhibit less temperature variation over the course of incubation.The brown-headed cowbird’s egg shape may contribute to its accelerated embryonic development rate relative to host eggs of similar size,which explains its ability to hatch in a variety of host nests.展开更多
Background:Red-winged Blackbirds(Agelaius phoeniceus),hereafter red-wings,are much less frequently parasitized by Brown-headed Cowbirds(Molothrus ater)in eastern North America than in central North America and had not...Background:Red-winged Blackbirds(Agelaius phoeniceus),hereafter red-wings,are much less frequently parasitized by Brown-headed Cowbirds(Molothrus ater)in eastern North America than in central North America and had not been recorded as hosts in our study area in southeastern Pennsylvania.Although hosts of Old World cuckoos(Cuculidae)often show geographic variation in egg rejection behavior,cowbird hosts typically exhibit uniform responses of all acceptance or all rejection of cowbird eggs.Thus,geographic variation in cowbird parasitism frequencies might reflect a different behavioral response to parasitism by hosts where only some populations reject parasitism.In this study,we tested whether egg rejection behavior may explain the lack of parasitism observed in our eastern red-wing population,which may provide insight into low parasitism levels across eastern North America.Methods:We parasitized red-wing nests with model cowbird eggs to determine their response to parasitism.Nests were tested across three nest stages and compared to control nests with no manipulations.Because rejection differed significantly by stage,we compared responses separately for each nest stage.We also monitored other songbird nests to identify parasitism frequencies on all potential hosts.Results:Red-wings showed significantly more rejections during the building stage,but not for the laying and incubation stages.Rejections during nest building involved mostly egg burials,which likely represent a continuation of the nest building process rather than true rejection of the cowbird egg.Excluding these responses,red-wings rejected 15%of cowbird eggs,which is similar to rejection levels from other studies and populations.The overall parasitism frequency on 11 species surveyed in our study area was only 7.4%.Conclusions:Egg rejection behavior does not explain the lack of parasitism on red-wings in our eastern population.Alternatively,we suggest that cowbird preference for other hosts and the low abundance of cowbirds in the east might explain the lack of parasitism.Future research should also explore cowbird and host density and the makeup of the host community to explain the low levels of parasitism on red-wings across eastern North America because egg rejection alone is unlikely to explain this broad geographic trend.展开更多
Avian brood parasitism is a model system for studies of coevolution and ecological interactions between parasites and their hosts. However, recent work may have led to misconceptions concerning the Brown-headed Cowbir...Avian brood parasitism is a model system for studies of coevolution and ecological interactions between parasites and their hosts. However, recent work may have led to misconceptions concerning the Brown-headed Cowbird (Molothrus ater), the most widely studied brood parasitic bird in the world, and its effects on host species. Potential misconceptions about this species that could affect management issues are as follows: cowbird populations are increasing; cowbirds are relatively new to North America; recently exposed hosts are defenseless against parasitism; cowbirds have caused widespread declines of songbirds; and cowbird control is always effective in increasing the size of endangered host populations. Potential coevolutionary misconceptions are that cowbirds are typically 'host tolerant'; cowbirds evict host nestmates; and the mafia effect is widespread. It is important to clarify these issues because such misconceptions could hinder our understanding of parasite-host interactions, and thus obscure the direction of basic research and of management efforts taken to limit cowbird impacts on endangered species. We discuss these issues and suggest future research directions to enhance our understanding of this fascinating species.展开更多
文摘There are five species of parasitic cowbirds (Molothrus) and the Brown-headed Cowbird (M. ater) is the only widespread species in North America. The Brown-headed Cowbird is a host generalist and is typically found in open habitats and forest edges. The cowbirds are of a more recent origin than many other brood parasites and perhaps as a result, cowbird adaptations for parasitism and their hosts’ counter-adaptations to thwart parasitism do not appear as sophisticated as those of other brood parasite-host systems. Because of its generalist nature, the cowbird has the potential to negatively impact endangered host species whose populations are limited due to anthropogenic habitat loss. As a consequence, the Brown-headed Cowbird is one of the few brood parasitic species that is the subject of control programs to limit its effects on such hosts.
文摘The brood-parasitic brown-headed cowbird(Molothrus ater)has one of the shortest incubation periods of any bird.Brown-headed cowbird eggs,and those of other avian brood parasites,tend to be more spherical due to their greater relative width.The traditional explanation for this egg shape is that it,combined with the thicker eggshells,resists host puncture-ejection.However,very few North American hosts of the brownheaded cowbird actually engage in puncture-ejection and therefore wider eggs may instead provide greater contact with a host’s brood patch during incubation,especially in large host nests.We tested whether greater egg width increased mean temperature and reduced temperature variation in brown-headed cowbirds by inserting temperature probes into brown-headed cowbird and house sparrow(Passer domesticus)eggs and placing them into red-winged blackbird(Agelaius phoeniceus)nests.House sparrow eggs are similar in appearance and in length to cowbird eggs,but are not as wide.We found no signifcant relationship between brown-headed cowbird egg width and mean incubation temperature.However,brown-headed cowbird eggs experienced less temperature variation than house sparrow eggs,and within brown-headed cowbird eggs,more spherical eggs experienced less temperature variation when accounting for differences in width.These results suggest that brownheaded cowbirds may have short incubation periods in part because their eggs exhibit less temperature variation over the course of incubation.The brown-headed cowbird’s egg shape may contribute to its accelerated embryonic development rate relative to host eggs of similar size,which explains its ability to hatch in a variety of host nests.
基金provided by a BEARS Grant and a Neag Undergraduate Research Grant from Kutztown University
文摘Background:Red-winged Blackbirds(Agelaius phoeniceus),hereafter red-wings,are much less frequently parasitized by Brown-headed Cowbirds(Molothrus ater)in eastern North America than in central North America and had not been recorded as hosts in our study area in southeastern Pennsylvania.Although hosts of Old World cuckoos(Cuculidae)often show geographic variation in egg rejection behavior,cowbird hosts typically exhibit uniform responses of all acceptance or all rejection of cowbird eggs.Thus,geographic variation in cowbird parasitism frequencies might reflect a different behavioral response to parasitism by hosts where only some populations reject parasitism.In this study,we tested whether egg rejection behavior may explain the lack of parasitism observed in our eastern red-wing population,which may provide insight into low parasitism levels across eastern North America.Methods:We parasitized red-wing nests with model cowbird eggs to determine their response to parasitism.Nests were tested across three nest stages and compared to control nests with no manipulations.Because rejection differed significantly by stage,we compared responses separately for each nest stage.We also monitored other songbird nests to identify parasitism frequencies on all potential hosts.Results:Red-wings showed significantly more rejections during the building stage,but not for the laying and incubation stages.Rejections during nest building involved mostly egg burials,which likely represent a continuation of the nest building process rather than true rejection of the cowbird egg.Excluding these responses,red-wings rejected 15%of cowbird eggs,which is similar to rejection levels from other studies and populations.The overall parasitism frequency on 11 species surveyed in our study area was only 7.4%.Conclusions:Egg rejection behavior does not explain the lack of parasitism on red-wings in our eastern population.Alternatively,we suggest that cowbird preference for other hosts and the low abundance of cowbirds in the east might explain the lack of parasitism.Future research should also explore cowbird and host density and the makeup of the host community to explain the low levels of parasitism on red-wings across eastern North America because egg rejection alone is unlikely to explain this broad geographic trend.
文摘Avian brood parasitism is a model system for studies of coevolution and ecological interactions between parasites and their hosts. However, recent work may have led to misconceptions concerning the Brown-headed Cowbird (Molothrus ater), the most widely studied brood parasitic bird in the world, and its effects on host species. Potential misconceptions about this species that could affect management issues are as follows: cowbird populations are increasing; cowbirds are relatively new to North America; recently exposed hosts are defenseless against parasitism; cowbirds have caused widespread declines of songbirds; and cowbird control is always effective in increasing the size of endangered host populations. Potential coevolutionary misconceptions are that cowbirds are typically 'host tolerant'; cowbirds evict host nestmates; and the mafia effect is widespread. It is important to clarify these issues because such misconceptions could hinder our understanding of parasite-host interactions, and thus obscure the direction of basic research and of management efforts taken to limit cowbird impacts on endangered species. We discuss these issues and suggest future research directions to enhance our understanding of this fascinating species.
