Influence of shape on egg discrimination in American robins and gray catbirds

The eggs of some obligate brood parasites are more spherical than the eggs of their non-parasitic relatives and hosts, which contributes to the increased strength of their shells. We examined whether egg shape, including the more spherical shape of brown-headed cowbird eggs (Molothrus ater), influenced egg discrimination in American robins (Turdus migratorius) and gray catbirds (Dumetella carolinensis). We added a series of artificial objects to robin and catbird nests that varied in shape from a control host egg, a rounded, cowbird-like egg, to odd-shaped objects. Real cowbird eggs were significantly more spherical than catbird and robin eggs, which confirmed a potential cue for egg recognition. Object shape significantly influenced the probability of rejection and time to rejection in both robins and catbirds. However, rounded eggs and spheres were rejected infrequently and at frequencies similar to control eggs. Therefore, the shape of a brood parasite's egg does not appear to influence egg discrimination in these two rejecters. Robins and catbirds rejected significantly more odd-shaped objects than egg-shaped objects and odd-shaped objects were rejected significantly sooner than egg-shaped objects. The rejection of odd-shaped objects likely represents an expression of nest-sanitation behaviour where debris is removed from the nest. By comparison with other studies of accepters of cowbird eggs, robins and catbirds appear to reject higher proportions of odd-shaped objects, which suggests they may have more refined abilities to discriminate against foreign objects in their nests.