How much of bird song is learned, and why is that so rare?

Bird song is part inherited and part learned, and the learned part is what makes birds special. Raise a songbird in isolation and it still sings, but its 'isolate song' resembles a wild bird's song while being distinctly different and lacking its complexity. So some of the blueprint is built in, and the rest has to be taught.

The proof comes from experiments. A chaffinch raised by canaries was found to have canary-like elements in its song, showing a strong role for tutors. Deafen a songbird and it usually produces highly abnormal song, so hearing is part of the process.

This ability, called vocal learning, is rare. Only eight animal groups have been shown to have it: humans, bats, whales and dolphins, seals, elephants, and three bird groups: songbirds, parrots and hummingbirds. It is not the same as simply remembering sounds, which all vertebrates tested can do. Vocal learners can imitate and produce new sounds.

Even among birds, learners differ. Closed-ended learners such as zebra finches learn in a limited period and then repeat one fixed song for life, while open-ended learners such as canaries and parrots keep learning new songs.

The big question is why so few animals do this. Two hypotheses: sexual selection for greater song variability might drive it, or predators might strongly select against it, since predators learn to ignore constant calls but are less likely to ignore variable ones.