Drivers of variation in egg size in a cooperative breeder with a redirected helping system

Females are expected to balance the benefits of current reproductive investment against the costs of that investment for future reproduction. Egg size may be subject to this trade-off, the outcome of which may depend on the intrinsic characteristics of the laying female or the environmental conditions that she encounters, such as weather and food supply. In addition, a female’s social environment may affect egg investment: in some cooperatively breeding species, females adjust egg investment according to the availability of help at the nest. In this study, we used long-term data and a field experiment to investigate the factors influencing egg size in the long-tailed tit Aegithalos caudatus, a cooperative breeder with a redirected helping system and relatively variable egg size. We show that females laid eggs of a consistent size within and across clutches and that skeletally larger females laid larger eggs. However, we found no evidence that environmental conditions or social environment influenced egg investment. Therefore, egg size appears largely to be an intrinsic characteristic of individual females. We discuss the importance of the predictability of future conditions for females when making investment decisions during egg-laying and stress the need for further studies of pre-laying investment in a wider range of cooperative breeding systems.