The Use of Nocturnal Flights for Barrier Crossing in a Diurnally Migrating Songbird: Implications for Our Understanding of Fly-and-Forage Migration Strategies

Abstract Background : The migration patterns of land birds can generally be divided into those species that migrate principally during the day and those that migrate during the night. Some species may show individual plasticity in the use of day or night flight, particularly when crossing large, open-water or desert barriers. However, individual plasticity in circadian patterns of migratory flights in diurnal songbirds that use a fly-and-forage migration strategy has never been investigated. Methods : We used high precision GPS tracking of a diurnal, migratory swallow Purple martin, ( Progne subis ), to determine whether individuals were flexible in their spring migration strategies to include some night flight, particularly at barrier crossing. Results : We found that individuals made large (sometimes >1000 km), open-water crossings of the Caribbean Sea and the Gulf of Mexico that included the use of night flight. Some birds-initiated barrier crossing flights at night, demonstrating that night flight is not only used to complete barrier crossings but may confer other advantages for diurnal birds. All birds also used some, shorter night flights when making overland flights not associated with barrier crossing. Birds were more likely to initiate water crossings with supportive northward winds, and preliminary data suggest that moonlight may influence nighttime migratory movements. Conclusions : Overall, our results demonstrate an unexpected high degree of individual plasticity in migration strategies on a circadian scale in a ‘diurnal’ songbird and suggest that barrier crossing at night may complement fly-and-forage migration strategies.