Meeting Abstract
Two species of cowbirds are found within North America; the brown-headed cowbird (Molothrus ater) and the bronzed cowbird (Molothrus aeneus). The males of these species use multimodal courtship displays to attract females. However, the courtship display of the bronzed cowbird is a highly energetic display that is substantially costlier than the display of the male brown-head cowbird. Behavioral measurements reveal that the male bronzed cowbird performs ground and aerial hover displays for an average total duration of 5.4 seconds. While in the aerial hover display the male bronzed cowbird beats his wings an average of 9.8 beats/sec (n = 8) for an average of 5.4 (n = 8) seconds and performs this display an average of 0.6 meters (2 feet) off the ground. On the ground, before or after performing an air-display, the male bronzed cowbird will do a combination of behaviors entailing either lightly beating its wings, displaying its ruff, producing a single call, and/or displaying its full wingspan for an average total duration of 10.1 (n = 8) seconds. By contrast, the male brown-headed cowbird performs only ground displays for an average total duration of 1.6 (n = 8) seconds and beats its wings an average of 2 beats/sec (n = 8) for approximately 1.6 (n = 8) seconds. We are currently using scanning electron microscopy of wing features, quantitative PCR in pectoralis muscles and androgen hormone assays to examine the physiological and anatomical mechanisms that may underlie the evolution of this striking difference in courtship displays between two closely related cowbirds.
