Information warfare in avian families: sex-specific begging responses to need and social environment in canary nestlings

Begging behaviour is a crucial component of parent-offspring communication. It is used by offspring to trigger the transfer of parental resources while at the same time allowing parents to access cryptic information about their offspring's need. But in a brood of more than one, offspring may gain indirect fitness benefits from responding to the need of its (related) siblings, not to withdraw all resources, especially if these are needier and would possibly contest more vigorously. Thus each offspring is thought to adjust its begging behaviour to its own intrinsic need as well as to its social environment, which is also shaped by the parents potentially having control over the distribution of resources. Here, we experimentally satiated the heaviest nestling within canary broods, Serinus canaria, in order to analyse: (1) whether nestlings honestly communicate their intrinsic hunger level; (2) whether nestlings adjust their begging behaviour to the need of their siblings; and (3) how parents respond to these begging strategies. Only female nestlings responded honestly, begging significantly less when satiated. Male nestlings, by contrast, did not alter their begging according to the level of satiation. Nestlings only weakly responded to the need of their siblings, and again only female nestlings did so. Thus female nestlings appear to be more sensitive to both intrinsic need and changes in their social environment, potentially owing to their lower competitiveness. Parents preferentially fed needier nestlings irrespective of the observed sex differences in begging strategies. They appear to control food distribution according to (cryptic) signals of need, which is important to take into account when studying the adaptive significance of any begging behaviour and strategy.