Ecological correlates and sex differences in early development of a generalist brood parasite

Parents inluence the phenotype of their offspring by determining the environment in which early development occurs. The many factors that affect growth in avian brood parasites provide an excellent context in which to examine how ecological variables and sex differences influence plasticity of early development. We used a model-selection approach to determine the most important variable(s) for explaining patterns in growth rate of the Brown-headed Cowbird (Molothrus ater). Using published growth-rate estimates across various host species, we found that the age-adjusted size of Brown-headed Cowbird chicks increases with increasing hatching synchrony between host and parasite chicks. We also quantified Brown-headed Cowbird growth rates in nests of Song Sparrows (Melospiza melodia) and Yellow Warblers (Dendroica petechia), two common host species at Mono Lake, California, to examine the role of variation in hatching synchrony in broods within host species. Statistical models to explain variation in Brown-headed Cowbird chicks' growth rates were constructed from ecological variables (host species, brood size, multiple parasitism, hatching synchrony between parasite and host chicks) and chick sex. The best model included only sex and there was a 99% chance that this was the best model, given the data set and models compared. Male Brown-headed Cowbird gained an average of 0.7 g day^-1 more than females and weighed 13% more at the same age. The only significant ecological variable, host-parasite hatching synchrony, was found to be sex-dependent, with males more likely than females to hatch earlier than their nest mates. We discuss the possible mechanisms underlying this sex effect and the importance of determining sex when studying nestling growth and competition.