Functional morphology as a barrier to the evolution of grasp-ejection in hosts of the Brown-headed Cowbird (Molothrus ater)

Acceptance of eggs of the Brown-headed Cowbird ( Molothrus ater (Boddaert, 1783)) by hosts is enigmatic because hosts usually raise fewer of their own young when parasitized. Ejection may not be adaptive for small hosts because they cannot eject cowbird eggs efficiently. Grasp-ejection apparently has a negligible cost but requires a bill of a minimum length that is not known. In this study, we examined the limits of grasp-ejection of the American Robin ( Turdus migratorius L., 1766) and the Gray Catbird ( Dumetella carolinensis (L., 1766)). We determined the largest object width that each species is capable of grasping (limit width) by observing individuals grasping models larger than cowbird eggs in 104 video-recorded ejections and one direct observation. We standardized the limit width to the tomial length of each species (limit ratio) and extrapolated to the width of a cowbird egg to predict the minimum tomial length required for grasp-ejection. Our results suggest that the minimum tomial length required to grasp-eject a cowbird egg is 15.9–16.1 mm and the probability of host-egg damage during grasp-ejection does not increase as the limit ratio increases. Bill length may prevent the evolution of grasp-ejection in cowbird hosts with bills shorter than 16 mm.