WHY DO GRAY-CATBIRDS DESTROY EGGS IN NESTS OF OTHER BIRDS - EXPERIMENTAL TESTS OF ALTERNATIVE HYPOTHESES

Although Gray Catbirds (Dumetella carolinensis) are known to destroy e ggs of other birds, the function of their egg-destroying behavior is u nknown. The behavior may (1) provide nutrients (consumption hypothesis ), (2) reduce competition for limited resources (competition hypothesi s), (3) reduce the chances of predation on catbird nests (predator-avo idance hypothesis), (4) prevent brood parasitism by the Brown-headed C owbird, Molothrus aler (antiparasitism hypothesis), or (5) serve in th e 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 catb ird responses to experimental nests and eggs. We found that catbirds ( i) break eggs in experimental nests throughout their nesting cycle, (i i) destroy eggs in heterospecific nests more frequently than in conspe cific nests, (iii) destroy cowbird eggs rather than conspecific eggs w hen both are presented in the same nest, (iv) peck eggs rather than yo ung when both are presented in the same nest, (v) consume broken eggs, (vi) avoid pecking eggs that are unpalatable, (vii) start pecking egg s soon after fledging. These results are most consistent with the cons umption and antiparasitism hypotheses and indicate that catbirds break eggs to consume their contents and to prevent brood parasitism of the ir own nests.