For any animal not at the top of the food chain, sleep involves increased risk from predators. Some migratory songbirds deal with this by sleeping lightly, ready to respond to danger. However, when they get really worn down they abandon this approach, tucking their head into their feathers to sleep. They’re more in danger in this pose, but sometimes you just don’t care.

Flying across continents and seas to take advantage of the seasons is extraordinarily stressful. Some remarkable birds sleep on the wing. Garden warblers, on the other hand, fall into the far more common category of birds that need to rest and feed on the way from their African wintering grounds to northern Europe. They must keep these lay-overs short; whether or not the early bird gets the worm, it enjoys the best breeding opportunities.

Dr Leonida Fusani of the University of Vienna observed some warblers at a stopover at Ponza, a Mediterranean island, and noticed some sleep with their heads out, while others tuck them into their feathers. Fusani studied the differences between these two positions and found they are far from a minor preference quirk.

In Current Biology Fusani and co-authors report that “sleep with the head tucked is associated with lower respiratory and metabolic rates and reduced heat loss.” In other words, it’s a better night's sleep – if you survive. The downside is that “birds presented with a sound simulating the approach of a predator responded more slowly when the head was tucked.”

Birds that arrived with ample fat reserves preferred to keep their head out during night-time rests, while those who limped into the site in poor condition tucked their head in to preserve what little they had. Well-fed birds also slept more than their hungry counterparts during daylight, but less at night.

"We discovered that migratory birds trade off safety for lower energy expenditure," Fusani said in a statement. "If they sleep with their head tucked in the scapular feathers, they enter a sort of deeper sleep that is associated with lower energy consumption but exposes them to a higher predation risk. Consequently, birds in good condition sacrifice some energy to sleep more safely with the head untucked, whereas birds in poor condition sacrifice vigilance to save energy while sleeping unsafely tucked in."

The authors were surprised at how much difference sleeping positions made to metabolic rates.

Whether the findings are of any significance for human sleep remains to be seen, but the work could benefit those seeking to assist migratory birds, particularly endangered species, on their route. Not only is it important to make food readily available at stopovers, but there should be sanctuary sites where weary travelers can tuck heads into feathers in safety.