Brown-headed cowbirds select nests to parasitize based on individual host attributes rather than nest type

Choosing nests of appropriate hosts is crucial to the success of obligate avian brood parasites because they rely on these hosts to care for their young. Female brood parasites may choose nests by imprinting on the habitat in which they were born, parasitizing nests randomly within their natal habitat, parasitizing a specific nest type or imprinting and preferring a specific host species. We took advantage of our system of nestboxes to conduct the first study on how female brown-headed cowbirds, Molothrus ater , decide which nests to parasitize by testing the nest site and host preference hypotheses. First, we tested cowbird host selection using a paired nestbox experiment with the prothonotary warbler, Protonotaria citrea . One nestbox had an entrance large enough for warblers and cowbirds to enter, while the second box had an entrance that only allowed warblers to enter. Cowbirds only parasitized nestboxes containing warblers and when warblers used nestboxes with small entrances, the cowbirds attempted to lay in those boxes instead of laying in the paired boxes with larger entrances that contained dummy eggs, indicating a preference for the host rather than the nest site. Second, we monitored cowbird host choice within the cavity-nesting community to ascertain whether cowbirds choose hosts based on nest type and use nestboxes at an equal rate (the nest site hypothesis) or whether they select specific hosts within the nestboxes (the host preference hypothesis). Cowbirds never parasitized inactive nestboxes and parasitized prothonotary warblers significantly more frequently than northern house wrens, Troglodytes aedon , and tree swallows, Tachycineta bicolor . Our results support the host preference hypothesis that brown-headed cowbirds choose nests to parasitize based on the individual attributes of hosts, although they may use additional cues such as the nest structure within the cavities and appearance of host eggs. • We tested whether cowbirds choose hosts based on nest type or species. • Cowbirds showed a preference for a specific host rather than a nest type. • Cowbirds showed a clear preference for prothonotary warbler hosts. • Cowbirds only parasitized active host nests.