Each spring and fall, millions of birds switch to migrating at night. Most of these are small songbirds migrating long distances that need to alternate their migratory flights with refueling stopovers, which can account for up to 80% of the total migratory period. After a long nocturnal flight, these birds face the contrasting needs to recover sleep and refill depleted energy stores, all while vulnerable to predation. According to a new study published in the journal Current Biology, migrating songbirds that are low on fat reserves will tuck their heads under their feathers for a deep snooze, and they do so despite the fact that this more restful sleeping position slows their reaction to the noise of potential predators.
“We discovered that migratory birds trade off safety for lower energy expenditure,” said Dr. Leonida Fusani, a researcher with the University of Vienna and the University of Veterinary Medicine, Austria.
“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.”
Dr. Fusani and colleagues wondered how small migratory songbirds cope with sleep deprivation.
To find out, the team investigated sleep patterns of migrating garden warblers (Sylvia borin) on the island of Ponza, a major stopover site in the Mediterranean.
At night, garden warblers in poor metabolic condition slept more and exhibited less migratory restlessness than birds in good condition.
Rather than sleeping with their head facing forward, those birds in poor condition slept with their head turned and tucked in their feathers.
The scientists went on to show that sleeping with the head tucked is associated with lower respiratory and metabolic rates. By hiding the head, the birds lose less heat.
But the benefit of conserving energy while sleeping with the head tucked is countered by reduced vigilance.
When the researchers presented those birds with the sound of crunching leaves, they were slower to respond than birds whose heads faced forward.
“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,” Dr. Ferretti said.
“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.”
“Our findings suggest that migratory songbirds may benefit from stopover habitats that not only provide sufficient food, but are also conducive to undisturbed sleep,” the study authors said.
“We will continue to study the physiological basis of birds’ decision making during migration.”
Andrea Ferretti et al. 2019. Sleeping Unsafely Tucked in to Conserve Energy in a Nocturnal Migratory Songbird. Current Biology 29 (16): 2766-2772; doi: 10.1016/j.cub.2019.07.028