Brood parasites reduce the reproductive success of many bird species by laying eggs in their nests. Hosts that reject parasitic eggs (“rejecters”) avoid most costs of brood parasitism altogether by physically ejecting eggs from nests or abandoning parasitized nesting attempts. Species that accept parasitic eggs once these are laid (“accepters”) may reduce or eliminate costs by aggressively responding to brood parasites at their nests to prevent parasitism from taking place. Accordingly, accepters should recognize brood parasites and nest predators as different nest threats with different levels of aggression, whereas rejecters may not. We exposed active Eastern Phoebe (Sayornis phoebe, an accepter host) and American Robin (Turdus migratorius, a rejecter host) nests to models of a female brood parasitic Brown-headed Cowbird (Molothrus ater), an eastern chipmunk (Tamias striatus, nest predator), and a European Starling (Sturnus vulgaris, nonthreatening control) during the incubation stage. Phoebes alarm-called equally toward the nest predator and brood parasite models, but attacked the nest predator model more than the brood parasite model. Robins, in contrast, alarm-called toward and attacked all 3 models equally. Interpreting these results is challenging due to experimental design elements, specifically small sample sizes and restricting the experiment to the incubation stage. Nonetheless, our experiment contributes to the paucity of comparative studies on accepter versus rejecter nest defense behavior in response to both nest parasites versus predators, and adds a new tested accepter species to the literature.

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