Called an “incubation call,” these songs contain a special note that serves as a kind of familial password:
When they lay their eggs, they wait nine days, and then park themselves by the eggs and start singing. They will sing and sing the same tune every four minutes, over and over for a week, and the chicks inside those eggs not only hear the tune, they commit it to memory.
Interestingly, this song serves as protection against cuckoos (who lay their eggs in other birds’ nests). Because those chicks never end up learning the song.
A really interesting read. And made me remember the fantastic word “shibboleth.”