Authors: Nathan W. Cooper, Bryant C. Dossman, Lucas E. Berrigan, J. Morgan Brown, Alicia R. Brunner, Helen E. Chmura, Dominic A. Cormier, Camille Bégin-Marchand, Amanda D. Rodewald, Philip D. Taylor, Christopher M. Tonra, Junior A. Tremblay and Peter P. Marra
Publication: Movement Ecology
Publication Link: https://doi.org/10.1186/s40462-023-00382-5
Keywords: Biological timing, Celestial cues, Circadian rhythm, Civil dusk, Navigation, Nocturnal departure timing, Migration phenology, Orientation, Sunset
Background Each spring and fall billions of songbirds depart on nocturnal migrations across the globe. Theory
suggests that songbirds should depart on migration shortly after sunset to maximize their potential for nightly flight
duration or to time departure with the emergence of celestial cues needed for orientation and navigation. Although
captive studies have found that songbirds depart during a narrow window of time after sunset, observational studies
have found that wild birds depart later and more asynchronously relative to sunset than predicted.
Methods We used coded radio tags and automated radio-telemetry to estimate the time that nearly 400 individuals
from nine songbird species departed their breeding or wintering grounds across North America. We also assessed
whether each species was most likely beginning long-distance migratory flights at departure or instead first making
non-migratory regional flights. We then explored variation in nocturnal departure time by post-departure movement
type, species, age, sex, and season.
Results We found that 90% of individuals from species that were likely initiating long-distance migratory flights
departed within 69 min of civil dusk, regardless of species, season, age, or sex. By contrast, species that likely
first made non-migratory regional movements away from the migratory destination departed later and more
asynchronously throughout the night. Regardless of post-departure movement type, 98% of individuals departed
after civil dusk but otherwise showed no preference in relation to twilight phase.
Conclusions Although the presence of celestial orientation cues at civil dusk may set a starting point for departure
each night, the fact that species likely beginning long-distance migration departed earlier and more synchronously
relative to civil dusk than those first making non-migratory regional movements is consistent with the hypothesis
that departing promptly after civil dusk functions to maximize the potential for nightly flight duration and distance.
By studying the onset of migration, our study provides baseline information about departure decisions that may
enhance our understanding of departure timing throughout migration.