KILPATRICK, A.M.: Variation in growth of Brown-Headed Cowbird nestlings and energetic impacts on their host parents.

I tested several hypotheses about the plasticity of avian growth by comparing growth of Brown-Headed Cowbird (Molothrus ater) nestlings in twenty different host species. Cowbird growth was positively correlated with site latitude (which negatively co-varied with maximum temperature) and negatively correlated with the nestling period of the host species. Cowbird growth was only marginally influenced by host adult mass and the data suggested that nearly all hosts were able to provision cowbirds with enough food to grow near the highest observed rate. Host nestling period, temperature and host mass combined to explain over seventy percent of the variation in cowbird growth. The metabolizable energy expenditure of cowbird and host nestlings was also estimated as an approximate measure of the food provisioned by parents. One cowbird nestling was equivalent to 0.6 to 3.9 host nestlings in terms of peak energy intake, depending on the size of the host species. Cowbird nestlings impose a substantial energetic demand on smaller host parents which may reduce their future survival or fecundity. The quantity of energy provisioned to parasitized nests demonstrates that parents are often willing to provision nests at a much higher rate than they do for an average clutch of their own young.