Songbirds initiate migratory flights synchronously relative to civil dusk

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; Peter P Marra

doi:10.1186/s40462-023-00382-5

Songbirds initiate migratory flights synchronously relative to civil dusk

Mov Ecol. 2023 May 1;11(1):24. doi: 10.1186/s40462-023-00382-5.

Authors

Nathan W Cooper¹, Bryant C Dossman^{2

3}, Lucas E Berrigan^{4

5}, J Morgan Brown^{4

6}, Alicia R Brunner^{3

7}, Helen E Chmura⁸, Dominic A Cormier⁴, Camille Bégin-Marchand⁹, Amanda D Rodewald³, Philip D Taylor⁴, Christopher M Tonra⁷, Junior A Tremblay⁹, Peter P Marra²

Affiliations

¹ Migratory Bird Center, Smithsonian's National Zoo and Conservation Biology Institute, 3001 Connecticut Ave. NW - MRC 5503, Washington, DC, 20008, USA. coopern@si.edu.
² Department of Biology and McCourt School of Public Policy, Georgetown University, 37th and O Streets NW, Washington, DC, 20057, USA.
³ Cornell Lab of Ornithology, Department of Natural Resources and the Environment, Cornell University, 159 Sapsucker Woods Rd, Ithaca, NY, 14850, USA.
⁴ Department of Biology, Acadia University, 33 Westwood Avenue, Wolfville, NS, B4P 2R6, Canada.
⁵ Motus Wildlife Tracking System, N0E 1M0, Birds, Port Rowan, ON, Canada.
⁶ Institute for Biodiversity and Ecosystem Dynamics, University of Amsterdam, 904 Science Park, 1098XH, Amsterdam, The Netherlands.
⁷ School of Environment and Natural Resources, The Ohio State University, 2021 Coffey Rd, 43210, Columbus, OH, USA.
⁸ Rocky Mountain Research Station, USDA Forest Service, 800 East Beckwith Avenue, 59801, Missoula, MT, USA.
⁹ Wildlife Research Division, Environment and Climate Change Canada, 1550 Av. D'Estimauville, G1J 0C3, Québec, QC, Canada.

Abstract

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.

Keywords: Biological timing; Celestial cues; Circadian rhythm; Civil dusk; Migration phenology; Navigation; Nocturnal departure timing; Orientation; Sunset.