Blinded by the light? Circadian partitioning of migratory flights in a nightjar species

Abstract

Circadian migration patterns for most migratory birds can be partitioned into day and night flights, but can be flexible at barrier crossings. Whether obligate nocturnal species opportunely use some daylight during migration has not been previously investigated. We tested the night constraint hypothesis using autumn Global Positioning System (GPS) tracking of a migratory nightjar species (Eastern Whip-poor-will, Antrostomus vociferus), and found that a minimum of 89% of travel occurred in darkness. All seven GPS-tracked individuals avoided crossing the Gulf of Mexico, which would have required some daytime flight. Future research should investigate the mechanisms and implications of light restrictions on the migratory movements of nocturnal species.