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.