Site selection and repeatability in Brown-Headed Cowbird (Molothrus ater) parasitism of Eastern Phoebe (Sayornis phoebe) nests

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 fro m laying eggs preferentially and repeatedly at safer sites. Observations on patterns of Brown-headed Cowbird (Molothrus ater) parasitism on Eastern Ph oebes (Sayornis phoebe) across 2 years suggested that parasitism occurred a t above chance levels during the first rather than the second nesting attem pts and at nests located under eaves rather than bridges. Previously parasi tized nests were more likely to be parasitized again in the subsequent bree ding season. Sites under eaves and bridges did not differ in whether Brown- headed Cowbirds could be detected in the proximity of the nest. However, ne sts from first nesting attempts and nests under eaves were less likely to b e 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.