Males feed their mates more and take more risks for nestlings with larger female-built nests: an experimental study in the Nuthatch Sitta europaea

Nest size has been suggested to be a sexually selected signal, allowing individuals to obtain reliable information about partner quality and thereby optimize paternal investment in reproduction. Studies concerning the potential role of nests as signals are scarce for avian species in which the female is the only builder. We have aimed at understanding whether males' reproductive investment (e.g., provisioning rates and risk taking) change in relation to experimental manipulation of nest size in the Nuthatch, Sitta europaea, as would be predicted if nest size is a sexually selected signal reflecting female's quality. To that end, we have experimentally manipulated nest size by approximately doubling its size and leaving other nests as controls. Experimental manipulation led males to increase their incubation feeding rates, and females from the experimental group showed higher levels of glutathione (tGSH), an important endogenous antioxidant whose synthesis may be enhanced indirectly through nutrition. Although male provisioning rates during the nestling stage did not differ between experimental groups, males responded to nest size manipulation by visiting nests sooner after human disturbance (an index of risk taking) as compared to control nests. Our study suggests that nest size constitutes a signal of female quality which elicits differential allocation of male resources both to females themselves and to their broods. Higher risk taking in favor of offspring by mates of strongly signaling females may denote an improvement in offspring survival chances in the field.