Experimental temperature manipulations alter songbird autumnal nocturnal migratory restlessness

Migrating birds may respond to a variety of environmental cues in order to time migration. During the migration season nocturnally migrating songbirds may migrate or stop-over at their current location, and when migrating they may vary the rate or distance of migration on any given night. It has long been known that a variety of weather-related factors including wind speed and direction, and temperature, are correlated with migration in free-living birds, however these variables are often correlated with each other. In this study we experimentally manipulated temperature to determine if it would directly modulate nocturnal migratory restlessness in songbirds. We experimentally manipulated temperature between 4, 14, and 24°C and monitored nocturnal migratory restlessness during autumn in white-throated sparrows (Zonotrichia albicollis). White-throated sparrows are relatively shortdistance migrants with a prolonged autumnal migration, and we thus predicted they might be sensitive to weatherrelated cues when deciding whether to migrate or stopover. At warm temperatures (24°C) none of the birds exhibited migratory restlessness. The probability of exhibiting migratory restlessness, and the intensity of this restlessness (number of infra-red beam breaks) increased at cooler (14°C, 4°C) temperatures. These data support the hypothesis that one of the many factors that birds use when making behavioural decisions during migration is temperature, and that birds can respond to temperature directly independently of other weather-related cues.