Individual songbirds call and answer each other during natural nocturnal migration

Nocturnal songbird migration has long been pictured as solitary, yet migrants call in flight and the airspace they cross is dense with the calls of others. Flight calls are almost always recorded from the ground, from unseen birds of unknown identity, age, and sex, so what a single bird does vocally across a night has been unknown. We radio-tagged Swainson’s thrushes ( Catharus ustulatus ) with microphone transmitters and followed them by vehicle through entire nocturnal migratory flights, recording the calls of known individuals over 34 h aloft by eight birds. Detected calling rate varied more than tenfold, from silence to 46 calls per hour, and was more clustered in time than random in six of the eight birds; one immature gave 149 of its 173 calls in the 16 min before it descended at dawn. Immatures were detected at 23.3 calls per hour against 6.2 for adults, a fourfold difference that eight birds cannot resolve and that we report as an effect size. No perched bird called during 38 h of night recording, tying the behavior to the flight state rather than to darkness. A broadcast Swainson’s thrush call, but not white noise matched in amplitude and frequency band, was answered within seconds, by gray-cheeked thrushes and veeries as well as by conspecifics. Flight-calling is therefore uneven across individuals and clustered within a flight, and vocal responses to it can cross species boundaries, which bears on how ground-based call counts are read.