EGG-EATING BY FEMALE BROWN-HEADED COWBIRDS

We estimated the proportions of eggs and nestlings, removed from hosts' nests, that were eaten by female Brown-headed Cowbirds (Molothrus ater) from (1) a compilation of published and unpublished observations (n = 36) of egg or nestling removal and egg-eating by female cowbirds and (2) the proportion of removed eggs that were found uneaten below parasitized nests (n = 69) of Northern Cardinals (Cardinalis cardinalis). Female cowbirds were seen removing eggs only before midday or in late afternoon and early evening. Eggs were removed from both parasitized and unparasitized nests and at any nesting stage from laying to late incubation. Although eggs, contents and shell, were frequently eaten, removed nestlings were not observed to be eaten. However, as many as 40% of removed eggs were.not eaten, despite the apparent energetic benefits of egg-eating. We offer six explanations to account for the failure of cowbirds to eat many eggs, and suggest experiments to elucidate this puzzling behavior of cowbirds. We conclude that the use of eggs as food may not be the primary cause of egg removal by Brown-headed Cowbirds.