Silence in the nest – nestling anti-predator strategies in a species under high predation pressure (#906)
Nest predation is a major source of reproductive failure in birds and thus it can exert selection on both parental and offspring strategies. Begging calls are known to be a powerful component of parent-offspring communication but on the other hand they increase predation risk by revealing the location of the nest. One of the apparent solutions may be a decrease of vocalization amplitude. In fact, species subject to higher nest predation have begging calls with lower amplitude than species less prone to predation. Theoretically, reduction of begging loudness, in the extreme form of anti-predator adaptation, might even lead to a silent begging. This, however, would contradict the principal role of begging calls in nestling provisioning, and so far silent begging has not been described in scientific literature. Here we demonstrate a sophisticated strategy of development of begging vocalization in a species under high nest predation. Blackcap nestlings spend most of their nesting period silent and develop begging calls just before they are able to fledge. The onset of begging vocalization matches the onset of endothermy, which in turn enables Blackcap chicks to leave the nest. We experimentally prove that begging calls function as a signal of increased needs of homeothermic nestlings. Playback of begging calls conducted in nests with silent nestlings resulted in significant increase of feeding rate and decrease of brooding. Development of begging calls only at the age of endothermy allows species under high nest predation to shorten the risky period of begging vocalization and frequent feeding to a minimum. The described strategy constitutes an evolutionary solution to high predation pressure as we have also experimentally demonstrated that begging calls significantly increase the level of predation in the studied species. This is the first study demonstrating silent begging as an evolutionary solution of nestling anti-predator strategies.