Brood parasitism by the brown-headed cowbird and loss of riparian habitat may be the primary causes neotropical migratory songbirds, Rio Grande, riparian, southwestern willow flycatcher. The Authors .. where they developed a commensal relationship with ultimate problem facing migratory songbirds in the South-. The Brown-headed Cowbird (Molothrus ater) is a brood parasite, meaning that it lays its eggs in nests of other Cowbird eggs require a shorter incubation period than most other songbirds and thus usually hatch first. A Compound Problem. Brood parasitism by Brown-headed Cowbirds (Molothrus ater) typically decreases Many migrant songbird species specific to riparian communities during the .. of cowbird parasitism, across North America in relation to habitat fragmentation. .. We predicted differences in disease resistance among the blackbird species.
For small songbirds, the importance of nest-site selection is well recognized reviewed by Limawhich can influence predation patterns observed at natural nests Schmidt and Whelan a ; Latif et al. The extent to which small songbirds can influence predation risk following nest initiation is less certain.
Parental and nestling activity e. Birds can further reduce predation risk by defending their nests, either actively Blancher and Robertson ; Hogstad or passively Halupka Nevertheless, studies do provide evidence for effective nest defense even by small songbirds initially reviewed by Martin ; see also Pietz and Granforswith intensity and efficacy dependent on food availability Duncan Rastogi et al.
By definition, nest predation involves predators, but determining the extent to which parents are involved can help narrow the range of mechanisms and thus causal factors underlying a pattern of interest. Patterns could arise exclusively from variation in predator ecology, namely their abundance or behavior Thompson Parents can adaptively respond to these patterns when selecting nest sites, in which case parents can influence observed patterns Schmidt and Whelan a ; Latif et al.
Alternatively, post-initiation parental behavior i.
How cowbirds take advantage of other birds
If predation patterns are driven exclusively by predator ecology, information regarding alternative prey for predators Schmidt and Ostfeld a or predator-habitat relationships Chalfoun et al.
The negative effects of cowbirds on nest survival in addition to fledging brood size in less forested landscapes suggest that cowbirds may be a primary cause of forest fragmentation effects on songbird productivity in the Midwest.
Our results underscore the dynamic nature of demographic parameters, which should be accounted for in predictive models of wildlife responses to future environmental conditions. Introduction Many species of Neotropical migrant songbirds have experienced significant long-term population declines . Identifying causes of the declines is challenging because the life-cycles of migrant songbirds can involve multiple habitat types across vast spatial scales . Nevertheless, conservation biologists have made major advances in our understanding of factors that limit migrant bird populations reviewed in .
On the breeding grounds in eastern North America, habitat loss and fragmentation have decreased songbird productivity by reducing species occurrence and pairing success in small patches  and decreasing rates of nest survival .
Songbird productivity is furthered hampered by brood parasitism from Brown-headed Cowbirds Molothrus ater; hereafter cowbirdwhich exhibit increased abundances that result in increased rates of brood parasitism with decreasing regional forest cover in the midwestern United States .
In addition to the reduced host productivity occurring as a direct result of parasitism nests with cowbirds may experience greater predation rates because cowbird begging attracts predators .
General Bird & Nest Info
Recent studies of video-monitored nests have also shown that cowbirds are frequent nest predators  and that nest predation by cowbirds increases with decreasing regional forest cover which further implicates cowbirds as important drivers of declines in productivity associated with forest fragmentation.
Like many other passerines, cowbirds have exhibited long-term declines in population abundances  ; the North American Breeding Bird Survey BBS indicates a survey-wide 0.
A female cowbird quietly searches for female birds of other species that are actively laying eggs. The foster parents then unknowingly raise the young cowbirds, usually at the expense of their own offspring.
Effects of parents and Brown-headed Cowbirds (Molothrus ater) on nest predation risk for a songbird
Cowbird eggs require a shorter incubation period than most other songbirds and thus usually hatch first. Cowbird nestlings also grow large very quickly. These advantages allow them to command the most food from their foster parents, usually resulting in reduced nesting success of the host species. It is unknown whether they developed their breeding strategy because they had to move frequently to keep up with the bison herds, or whether they were able to follow the herds because their breeding strategy gave them the freedom to do so.
Expansion of agricultural areas and removal of forest cover have greatly benefited this species by providing more overall habitat and by giving cowbirds access to new host species that have not developed defensive strategies against nest parasitism. While it is clear that cowbirds have benefited from forest fragmentation, their role in population-level declines of many forest birds is less certain.
A Compound Problem The cowbird does not depend exclusively on a single host species; it has been known to parasitize over different species of North American birds and therefore spreads its impact across many populations.