Costa's hummingbird has an extended sensitive phase of vocal learning

Vocal learning is an experience-dependent behaviour that requires exposure to appropriate vocalizations for learning to take place. The timing of this exposure potentially matters: early in life, the brain may have a sensitive phase in which it is plastic and can be altered by experience. Birds tutored during the sensitive phase should learn to sing the tutor song, while those tutored too early or too late should not. Hummingbirds have independently evolved vocal learning relative to parrots and passerines, but their sensitive phase has not been characterized. Here we tested whether Costa's hummingbirds, Calypte costae, have a sensitive phase. Sixteen males were raised in individual isolation and exposed to song under one of three different tutoring periods: 35-65 days posthatch, 75-105 days posthatch, or 115-145 days posthatch. Each bird received 20 h of tutoring with approximately 4000 song exposures over 30 days. As an adult, none of these birds produced isolate song (i.e. they all showed signs of learning), but none of the birds were a particularly good match to the tutor song either; many of the birds sang songs that were longer and more variable than the tutor song. There was no effect of tutoring period on the match between the experimental birds' songs to Costa's song. These results suggest that the sensitive phase begins before day 35 and continues until after age 115 days and that ~4000 songs over 20 h of tutoring is not sufficient for learning to be completed in Costa's hummingbird. Published by Elsevier Ltd on behalf of The Association for the Study of Animal Behaviour.