Obligate avian brood parasites lay their eggs in the nest of other species and impose reproductive costs upon their hosts. In response, many hosts have evolved a variety of defenses to avoid or reduce the costs associated with parasitism, including the rejection of parasitic eggs. Here, we review the different sensory and contextual cues involved in egg rejection by the American robin (Turdus migratorius), one of the handful of rejecter host species of the brood parasitic brown-headed cowbird (Molothrus ater) in North America. Robins are an open-cup nesting species whose eggs dramatically differ in appearance from the cowbirds' eggs; therefore, we expect that visual cues would be the primary sensory modality used when rejecting foreign eggs. The results of our comprehensive review confirm that visual cues, including eggshell color and maculation, are strong predictors of robins' egg rejection responses. Egg size, egg shape, and other contextual traits (including clutch composition, maternal status, and sympatry with the parasite) also affect egg rejection in this host species. Future research into this species' egg rejection mechanisms would benefit from comparisons to other strongly and moderately egg rejecter host species of brown-headed cowbirds, as well as from seeking to understand further why so many hosts of this particular parasitic lineage accept foreign eggs. We also discuss the implications of this review for other egg rejecter and non-rejecter hosts of diverse avian brood parasites.

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Twenty-two years of brood parasitism data do not support the mafia hypothesis in an accepter host of the Brown-headed Cowbird (Molothrus ater).