Journal
CURRENT BIOLOGY
Volume 29, Issue 16, Pages 2766-+Publisher
CELL PRESS
DOI: 10.1016/j.cub.2019.07.028
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Funding
- University of Vienna
- University of Veterinary Medicine, Vienna
- Austrian Science Fund (FWF) [P 31047-B29]
- Completion grant'' of the University of Vienna
- Max Planck Society
- Piccole Isole Project of the Italian National Institute for Environmental Protection and Research (ISPRA
- Istituto Superiore per la Protezione e la Ricerca Ambientale) [63]
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Each spring and fall, millions of normally diurnal birds switch to migrating at night. Most of these are small songbirds (passerine) migrating long distances that need to alternate their migratory flights with refueling stopovers [1, 2], which can account for up to 80% of the total migratory period [3]. After a long nocturnal flight, these birds face the contrasting needs to recover sleep and refill depleted energy stores, all while vulnerable to predation [4, 5]. Here, we investigated how garden warblers at a Mediterranean stopover site modulate their sleep behavior in relation to their metabolic state. At night, garden warblers in poor metabolic condition sleep more and exhibit less migratory restlessness than birds in good condition do. In addition, rather than sleeping with their head facing forward, birds in poor condition prefer to sleep with their head turned and tucked in their feathers. We further show that sleep with the head tucked is associated with lower respiratory and metabolic rates and reduced heat loss mediated by hiding the head-the body part with the highest heat dissipation-under the feathers. However, the benefit of conserving energy while sleeping with the head tucked was countered by reduced anti-predator vigilance. Birds presented with a sound simulating the approach of a predator responded more slowly when the head was tucked than when it was untucked. Consequently, our study demonstrates that through changing their sleep position and intensity, migrating songbirds can negotiate a previously unknown trade-off between sleep-mediated energy conservation and anti-predatory vigilance.
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