Abstract
The parameters of egg acceptance in Red-winged Blackbirds (Agelaius phoeniceus) and Yellow-headed Blackbirds (Xanthocephalus xanthocephalus) were investigated by introducing into their nests various eggs and objects. With the exception of miniature eggs, both species accepted all eggs, real and artificial, and rejected all nonegg-shaped objects. Egg shape and size were necessary components of egg acceptance whereas color and markings were not. The rejection of nonegg-shaped objects as large as Brown-headed Cowbird (Molothrus ater) eggs demonstrated that inability to remove cowbird eggs was not responsible for blackbird acceptance responses. Nests parasitized before the onset of egg laying did not increase the probability of either the egg being rejected or the nest being inactive.
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