Behavior of Warbling Vireos Ejecting Real and Artificial Cowbird Eggs

Abstract

Abstract We videotaped nine ejections of real (n = 5) and artificial (n = 4) Brown-headed Cowbird (Molothrus ater) eggs by Warbling Vireos (Vireo gilvus). All eggs were ejected within 6 min. There were no significant differences in time used for any ejection behavior by egg type, although artificial eggs were probed longer before ejection. Eight vireos ejected the cowbird egg using visual cues only because none sat on its nest before ejection. One male ejected the cowbird egg after sitting on the nest for a few minutes; consequently, both visual and tactile cues were available for its decision to eject the cowbird egg. Most vireos identified the cowbird egg by sight and, in most cases, rapid ejection of the cowbird egg precluded the possibility of using tactile cues. Grasp-ejection was the only ejection method confirmed for real and artificial eggs. Two male vireos ejected cowbird eggs at two nests, which is the first documentation of successful ejection by male Warbling Vireos, and the third cowbird ...