Behavioral Responses of Brown-Headed Cowbirds to Nests and Eggs

Abstract

Captive 1st-year female brown-headed cowbirds respond to empty nests and to nests containing eggs by crouching and sitting in the nest cup. In sitting, the female cowbird assumes a posture similar to that of an incubating passerine. Of 30 females tested, 22 (73%) sat in the nest and 6 additional birds crouched in the nest at least one time. Mean and median durations of sitting periods of responding individuals were approximately 33 and 16 sec., respectively; maximum duration was 1552 sec. Responding females sat with a mean frequency of 1.3 times per hr. Tests of responses of cowbirds to empty nests suggest that sitting behavior is elicited primarily, if not entirely, by the visual stimulus of the nest, the eggs being a minor, if not insignificant, aspect of the stimulus situation. Injections of prolactin, progesterone, and an estrogen (estradiol benzoate) had no appreciable effect on the response of female cowbirds to nests and eggs, In the absence of convincing evidence that hormonal treatment can induce brown-headed cowbirds to perform nest-building, incubation, or other behaviors that are not part of the normal repertoire of the species, it was concluded that the physiological basis for the loss of these behaviors in this species involves an insensitivity of neural tissues to hormones which mediate these behaviors in non-parasitic species.