An experimental test of preferences for nest contents in an obligate brood parasite, Molothrus ater

We conducted a series of experiments designed to examine the stimuli to which female brown-headed cowbirds (Molothrus ater) attend when selecting a nest for parasitism. We gave hand-reared juvenile and wild-caught adult females opportunities to assess nests containing different numbers and types of mock eggs. We measured female interest in nests by recording the number of times they approached nests (experiments 1 and 2), and also, we maintained 5 groups of adult and 2 groups of juvenile females in outdoor aviaries where we could measure patterns of laying in nests (experiment 3). Results from both types of experiments were similar. Females of both age classes preferred nests with eggs over nests without eggs, nests containing 3 eggs to nests containing 1 egg, small eggs to medium sized eggs, and medium eggs to large eggs. In addition, there was an age-related effect in how females responded to a nest that already contained a cowbird egg. Adult females avoided nests containing cowbird eggs, whereas juveniles preferred these parasitized nests. Taken together, the results provide evidence of the adaptive nature of decision processes involved in host nest selection.