Video Recording Reveals the Method of Ejection of Brown-Headed Cowbird Eggs and No Cost in American Robins and Gray Catbirds

Abstract

Abstract. Despite the importance of knowing the method and cost of ejection in understanding the persistence of brood parasitism, anecdotal records of witnessed ejections of real Brownheaded Cowbird (Molothrus ater) eggs exist for only eight of ∼30 ejecter species. The probability of a host damaging its own egg while ejecting a parasite's egg is thought to be lower for hosts that grasp-eject, but grasp-ejection is an option only for hosts with appropriate bills. For hosts incapable of grasp-ejection, the cost of puncture-ejection may render acceptance adaptive. We video-recorded 12 ejections of real cowbird eggs by American Robins (Turdus migratorius) and 17 by Gray Catbirds (Dumetella carolinensis). With no damage to their own eggs, robins graspejected all cowbird eggs, whereas catbirds grasp-ejected 14 eggs and puncture-ejected three eggs. Our study revealed that a few species use a mixture of ejection methods and even large species may puncture-eject with little cost.