Open-ended vocal learning in Costa's hummingbird

Here we examine whether vocal learning in Costa's hummingbird, Calypte costae, is open-ended. Open-ended learning is the ability of a vocal learning animal to memorize and learn to incorporate new song material into its vocal repertoire after reaching sexual maturity. Open-ended vocal learners are able to learn as adults because they have either a sensitive phase that never closes or a seasonal reopening of the sensitive phase. In prior experiments, we raised 18 individually housed male Costa's hummingbirds in isolation chambers from fledging (day 21 posthatch) until they were approximately 1 year old. During that time, they were tutored and learned song that was individually specific and stable. Here we report what happened when we moved cohorts of eight (in 2017) and six (in 2018) of these ∼1-year-old birds to communal housing in two outdoor aviaries, placing them in physical, visual and acoustic contact with other adult Costa's hummingbirds that sang songs to which each individual had never previously been exposed. The remaining four 1-year-old birds (in 2018) were instead kept in isolation for their second year as a control, then first exposed to each other at 2 years of age. Within 2 months, all of the 1-year-old birds rapidly changed their songs to produce novel songs that were unique to each aviary. The control birds that remained in isolation for a second year did not change their songs. These second-year birds then changed their songs when they were moved to the aviaries and exposed to novel song for the first time at 2 years of age. Although additional experiments are important (e.g. tutoring adults raised in the wild), our results show that Costa's hummingbirds have open-ended vocal learning through at least their third year.