Variation in growth of Brown-headed Cowbird (Molothrus ater) 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 20 different host species. Growth of cowbird nestlings was not strongly correlated with host adult mass and nearly all hosts were able to provision cowbirds with enough food to grow at nearly the highest observed rate. Cowbird growth was positively correlated with site latitude (which negatively covaried with maximum temperature) and negatively correlated with the nestling period of the host species. The metabolizable energy expenditure of cowbird and host nestlings was estimated as an approximate measure of the food provisioned by parents. As host adult mass varied from 6 to 113 g, one cowbird nestling was equivalent to 3.4-0.56 host nestlings in terms of peak daily energy intake. Cowbird nestlings impose a substantial energetic demand on smaller host parents that may reduce their future survival or fecundity. The quantity of energy delivered to parasitized nests demonstrates that parents are often willing to provision nests at a much higher rate than for an average clutch of their own young.