Many North American bird species either accept or reject all Brown-headed Cowbird (Molothrus ater) eggs, but the Common Grackle (Quiscalus quiscula) is unusual in that it rejects a small proportion of experimentally introduced cowbird eggs. We tested whether individual Common Grackles and eight typical rejecter species were consistent in their responses by parasitizing nests a second time after initial cowbird eggs were ejected. We also assessed intraclutch variation in egg appearance in Common Grackles to determine whether it may influence phenotypic plasticity in rejection frequency. Rejections were recorded at 11.9% of 337 experimentally parasitized Common Grackle nests. Only 6 (33.3%) of 18 Common Grackles that ejected initial cowbird eggs also ejected a second experimentally added cowbird egg, however, which suggests that individuals that possess rejection behavior express it only 33.3% of the time. By contrast, the other eight rejecter species rejected between 91% and 100% of initial cowbird eggs (n = 302) and rejected 100% of second cowbird eggs (n = 125). Common Grackles are unique in three ways: they reject eggs at a low frequency, they have a high rate of intraclutch variation in egg appearance, and they exhibit phenotypic plasticity in response to repeated parasitism. It is difficult to ascertain the significance of these attributes, but combined with the fact that Common Grackles are rarely parasitized by cowbirds but may have been parasitized more frequently in the past, they suggest that egg rejection has no current adaptive value and may incur a cost such that Common Grackles may be losing rejection via drift or direct selection.
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Vol. 127 • No. 2