Nestling birds learn socially to eavesdrop on heterospecific alarm calls through acoustic association

Animals often eavesdrop on signals intended for others to gather
information about their environment. While adult animals have been shown
to learn to recognize unfamiliar heterospecific alarm calls through both
social and asocial learning, it remains unclear whether and how young
animals learn to recognize unfamiliar alarm calls. We show experimentally
that nestling Daurian redstarts Phoenicurus auroreus can socially learn to
recognize unfamiliar heterospecific alarm signals by associating them with
conspecific alarm calls. We trained nestlings by presenting two unfamiliar
sounds, one together with conspecific alarm calls (training) and one
without (control). Before training, nestlings showed similarly little
response to both novel sounds. After training, however, nestlings showed
clear anti-predator responses to the training sound, but not to the
control sound. These results show that nestling birds can socially learn
to associate novel sounds with known alarm calls, even without visual
confirmation of danger.