The influence of fledgling location on adult provisioning: a test of the blackmail hypothesis.

One theory to explain the existence of conspicuous solicitation is that it is a way for young to 'blackmail' carers into provisioning them, by threatening their own destruction. Fledgling birds offer a unique opportunity to investigate the 'blackmail theory', as their mobility enables them to influence the predation risk they face. We investigated a novel solicitation behaviour in fledgling pied babblers (Turdoides bicolor), where fledglings use their location to influence provisioning rates. We show that fledglings face a trade-off: the ground is a much more profitable location in terms of provisioning rate from adult carers, but they are at greater risk from predators owing to their limited flying ability and slow response to alarm calls. Young babbler fledglings move to the ground when hungry, signalling their state, and this stimulates adults to increase their provisioning rates. Once satiated, fledglings return to the safety of cover. By experimentally increasing terrestrial predation risk, we found that adults increased their provisioning rate to terrestrial but not arboreal fledglings. Thus, by moving to a riskier location, fledglings revealed their need and were able to manipulate adults to achieve higher provisioning rates. These results provide support for the 'blackmail theory'.