This garden warbler (Sylvia borin) looks sharp and well rested in this photo. The question is what risks it might have taken to get that way.
Most migratory songbirds must make stops during their long-distance journeys to recharge their batteries. Now research has shown that they sleep differently depending on how tired they are – sometimes putting sleep ahead of security.
An international team led by Leonida Fusani from the University of Vienna, Austria, studied garden warblers at a stopover site in the Mediterranean.
They found that when birds are low on fat reserves, they tuck their heads under their feathers for a deep sleep, even though this position slows their reaction to the sound of potential trouble.
Sleeping with the head tucked is associated with lower respiratory and metabolic rates. By hiding the head, the birds lose less heat.
“We did not expect to find such a strong difference between the two sleeping postures in terms of metabolic rate – the amount of energy required to fuel the bird’s physiological functions,” Ferretti says.
“Although there was good reason to think that birds reduce heat loss by tucking their heads in their feathers, we were surprised to see that they actually reduce their alertness when sleeping in this position.”
The study’s findings are published in the journal Current Biology.
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