Changes in Nest Defense against a Brood Parasite over the Breeding Cycle

We measured the responses of nesting least flycatchers ( Empidonax minimus ) to a model of the brood‐parasitic brown‐headed cowbird ( Molothrus ater ). A greater number of vocalizations, approaches and threat displays were directed toward the cowbird model than toward a control model of a fox sparrow ( Passerella iliaca ), suggesting that the cowbird was recognized as a threat. Flycatchers were more likely to give threat displays during laying when the consequences of brood parasitism were greatest; however, the intensity of all other defensive behavior did not change over the nesting cycle. We suggest that least flycatchers remained aggressive throughout nesting because cowbirds were also predators on eggs and nestlings.