Egg discrimination underlies the evolution of a host's ability to reject a brood parasite's egg, but some hosts do not reject if it is too costly or if the parasitic egg mimics the host egg. We examined cues used by the Yellow Warbler (Setophaga petechia), a host of the Brown-headed Cowbird (Molothrus ater), in its decision to reject (bury or desert) experimentally parasitized clutches. Yellow Warblers that returned to nests after a model egg was added spent significantly more time peering at and probing their nest contents and shuffling than before eggs were added. Rejection probability increased with the proportion of time Yellow Warblers probed their nest contents after settling on their clutch. Accepters and rejecters, however, peered at or shuffled their eggs the same amount of time and visited the nest and probed its contents (before settling) the same number of times. Burial occurred at 29 of 144 nests (20.1%) experimentally parasitized with model cowbird eggs, of which 14 nests survived long enough for replacement clutches to be parasitized. Only one of the 14 (7.1%) females that initially buried parasitized clutches buried again when the replacement clutch was parasitized, whereas the others accepted their parasitized replacement clutch. Repeated parasitism revealed that an individual's response to parasitic eggs is plastic and that it may reject or forgo rejection after recognizing a parasitism event.
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Vol. 129 • No. 1