Why do Gray Catbirds destroy eggs in nests of other birds? Experimental tests of alternative hypotheses

Abstract

Although Gray Catbirds (Dumetella carolinensis) are known to destroy eggs of other birds, the function of their egg-destroying behavior is unknown. The behavior may (1) provide nutrients (consumption hypothesis), (2) reduce competition for limited resources (competition hypothesis), (3) reduce the chances of predation on catbird nests (predator-avoidance hypothesis), (4) prevent brood parasitism by the Brown-headed Cowbird, Molothrus ater (antiparasitism hypothesis), or (5) serve in the acquisition of mates (sexual selection hypothesis). The goal of our study was to test predictions of these five hypotheses in a series of field and laboratory experiments. In the experiments, we examined catbird responses to experimental nests and eggs. We found that catbirds (i) break eggs in experimental nests throughout their nesting cycle, (ii) destroy eggs in heterospecific nests more frequently than in conspecific nests, (iii) destroy cowbird eggs rather than conspecific eggs when both are presented in the same nest, (iv) peck eggs rather than young when both are presented in the same nest, (v) consume broken eggs, (vi) avoid pecking eggs that are unpalatable, (vii) start pecking eggs soon after fledging. These results are most consistent with the consumption and antiparasitism hypotheses and indicate that catbirds break eggs to consume their contents and to prevent brood parasitism of their own nests.