Shiny cowbird Molothrus bonariensis egg size and chick growth vary between two hosts that differ markedly in body size

In birds, egg size aff ects chick growth and survival and it is an important component of reproductive success. Th e shiny cowbird Molothrus bonariensis is an extreme generalist brood parasite that uses hosts with a wide range of body masses. Survival of cowbird chicks decreases with host body mass, as competition for food with nestmates is more intense in large than in small hosts. We studied variation in shiny cowbird egg size and chick growth in two hosts that diff er markedly in body size: the chalk-browed mockingbird Mimus saturninus (70 – 75 g), and the house wren Troglodytes aedon (12 – 13 g). We analyzed: 1) if females parasitizing mockingbirds lay larger eggs than those parasitizing wrens, and 2) the association between egg size and chick growth. We experimentally controlled for time of parasitism and number of host chicks and evaluated growth rate of male and female parasite chicks. Shiny cowbirds parasitizing mockingbird nests laid larger eggs than those parasitizing wren nests. Chick body mass after hatching was positively associated with egg size until chicks were fi ve days of age, but there was no association between egg size and growth rate, or asymptotic mass. Th ere were no sexual diff erences in egg size or body mass at the time of hatching, but growth rate was higher in males than in females leading to sexual dimorphism in asymptotic mass. Diff erences in egg size between hosts and the eff ect of egg size on body mass after hatching support the hypothesis that diff erent females are specialized in the use of hosts that diff er in body mass.