Sex-specific nestling growth in an obligate brood parasite: Common Cuckoo males grow larger than females

Growth is a key life history trait that is closely related to individual fitness. In altricial birds, growth is restricted to a relatively short period, and depends primarily on the amount or quality of food and hence on parental care. Obligate brood parasites do not care for their own offspring but impose this burden on other species (hosts). As many brood parasites exploit various host species, their progeny are expected to receive different levels of parental care. Parasite growth has thus often been explored in the context of host parenting abilities and only rarely with respect to its sex specificity. Here, we fill this gap in knowledge and explore sex differences in the growth of Common Cuckoo (Cuculus canorus) nestlings reared by 2 warbler hosts in the genus Acrocephalus. As adult Common Cuckoo males are 5-16% heavier than females, we assumed that nestlings would also differ in size and thus in growth performance. To test this assumption, we used a nonlinear mixed effects modeling approach to fit an overall logistic curve across all nestling masses and ages. We chose the logistic growth model over its alternatives because it is one of the most used models for birds and it is suitable for the growth of Common Cuckoo nestlings. We found that both sexes exhibited similar mass after hatching and grew at a similar rate. Nevertheless, males reached similar to 10% higher asymptotic mass than females, while fledging at a similar age as females. These findings imply that male Common Cuckoo nestlings may have higher needs than female nestlings; however, this still awaits proper testing.