Daytime naps in night-migrating birds: behavioural adaptation to seasonal sleep deprivation in the Swainson's thrush, Catharus ustulatus

Many typically diurnal songbird species are routinely subjected to severe sleep deprivation during their seasonal, nocturnal migrations, yet they seem to suffer few of the negative consequences so obvious in sleep-deprived mammals. Studying the Swainson's thrush, a Nearctic–Neotropical long-distance migrant, we investigated seasonal changes in behaviour as possible adaptations to naturally occurring sleep loss during the migratory season. We found that captive Swainson's thrushes substantially change their daytime behaviour in response to significant loss of night-time sleep during migration, engaging in numerous episodes of daytime sleep, unilateral eye closure and drowsiness. Our observations suggest that brief episodes of sleep and sleep-like behaviour during the day, including lateralized naps in the form of unihemispheric sleep, enable migratory birds to compensate for extended periods of nocturnal sleep loss. Although episodes of daytime sleep and unilateral eye closure rarely exceed 30 s, these brief episodes, considering the short sleep cycle of birds, appear sufficiently long to provide recuperative opportunities without stretching a migrant's time budget. Importantly, the temporal structure of the daytime rest and sleep behaviour would not preclude migrants from foraging and replenishing fat deposits during a large portion of the day.