Sleeping unsafely tucked in to conserve energy in nocturnal migratory songbirds

Even penguins with modified feathers that do not cover their head sleep this way. Interestingly, fossils suggest that birds inherited this behavior from their feathered dinosaur ancestors. Several studies suggest that by hiding the poorly insulated head and bill in their feathers, birds reduce heat loss and thereby conserve energy. However, as conserving energy is seemingly always an adaptive strategy, it is paradoxical that birds sometimes sleep with their head untucked facing forward. In a recent study, a group of scientists coordinated by Andrea Ferretti and Leonida Fusani from the Department of Cognitive Biology (University of Vienna) and the Konrad Lorenz Institute of Ethology (University of Veterinary Medicine, Vienna) have shown that in migratory birds the choice between sleeping tucked and untucked reflects a compromise between opposing needs: conserve energy vs. avoid predation.

Warblers observed during bird migration
Nocturnally migrating songbirds that cross the Mediterranean to reach continental Europe often stop on islands close to the coast to rest before continuing their journey. Through detailed observations of Garden Warblers that had recently arrived on one of these island stopover sites, the researchers found that the sleeping position of the warblers depended on their physiological condition: birds that had used up most of their fat reserves preferred to sleep with their head tucked in, whereas fat birds preferred to sleep untucked. Measurements with a respirometry system showed that when in the tucked position birds reduced their energy consumption. Thus, choosing to sleep tucked in helps energetically depleted birds to save precious energy while in route. But why then wouldn’t all birds do so?

When the scientists tested sleeping birds with the noise of crushed leaves, which might signal the approach of a predator, the birds sleeping tucked in reacted more slowly than those sleeping untucked. The reduced metabolic rate and the slower reaction time suggest that tucked birds were sleeping deeper. Thus, migratory warblers face a dilemma during their migratory stopover: if they sleep tucked in they save energy, but increase their risk of being predated. These findings reveal new perspectives on the functions of avian sleep postures, as well as the ecological and physiological challenges birds face during migration.

The article “Sleeping unsafely tucked in to conserve energy in a nocturnal migratory songbird” by Andrea Ferretti, Niels C. Rattenborg, Thomas Ruf, Scott R. McWilliams, Massimiliano Cardinale and Leonida Fusani: Current Biology