Moonlight drives nocturnal vertical flight dynamics in black swifts

Many animals have evolved a migratory lifestyle as an adaptation to seasonality,^1,2 ranging from insects³ to fish,⁴ terrestrial and marine mammals,^5-7 and birds.⁸ Old World swifts have evolved an extraordinary aerial non-breeding life phase lasting for 6-10 months.^9-11 Swifts exploit the aerosphere in search of insects to meet the high energy demands of flight.¹² During this period they roost and likely also sleep in the open airspace. Nocturnal insectivores with restricted foraging time may use moonlight to increase energy intake.¹³ Using multisensor data loggers that record light for geolocation, acceleration for flight activity, and pressure for flight altitude, we investigated if Northern black swifts, Cypseloides niger borealis, breeding in North America, also lead an aerial lifestyle similar to their Old World relatives. Individual flight activity showed they are airborne >99% of the time, with only occasional landings during their 8-month non-breeding period. Unexpectedly, during periods around the full moon, they conducted regular nocturnal ascents to altitudes up to >4,000 m (mean 2,000 m). A lunar eclipse triggered a synchronized descent, showing a direct effect of moonlight on flight altitude. This previously unknown behavior of nocturnal ascents during moonlight nights could be either a response to predator avoidance or that moonlight provides a foraging opportunity. Observed elevated nocturnal flight activity during periods of moonlight compared to dark nights suggests swifts were hawking for prey. Our finding of this novel behavior provides new perspectives on nocturnal flight behavior during periods surrounding the full moon.