Male Northern Orioles Eject Cowbird Eggs: Implications for the Evolution of Rejection Behavior

We experimentally parasitized 16 nests of the Northern Oriole (Icterus galbula) to determine the method of rejection of Brown-headed Cowbird (Molothrus ater) eggs, i.e., removal by spiking or by breaking the egg into pieces, and whether males remove cowbird eggs. Cowbird eggs were ejected from all 16 nests. We observed egg ejection at 14 of the parasitized nests. At six nests, cowbird eggs were ejected by breakage. Egg breakage did not result in more damage to Northern Oriole eggs than removal by spiking. For every cowbird egg ejected 0.38 Northern Oriole eggs disappeared or were damaged. Males removed the cowbird egg at two nests, females removed the cowbird egg at 11 nests, and at one nest both the male and female participated in the removal of the cowbird egg. At 13 nests, ejection was carried out by the bird that first inspected the nest after experimental parasitism. Damage to Northern Oriole eggs occurred proportionately more often during male ejections, which suggests males are less experienced egg ejecters, but this requires further testing. Because males can eject cowbird eggs the evolution of the rejecter trait may occur at an even faster rate than previously modeled.