Bird report: Sleep strategies of migrating birds

By Robyn Perkins
LSLBO Bander-in-Charge

September brings the beginning of the Lesser Slave Lake Bird Observatory’s owl migration monitoring program, and with it sleep loss for our banders as they juggle a program that runs nightly (owls) and another that begins at sunrise (songbirds). Concern over our sleep got me wondering how birds sleep with even busier schedules. Unfortunately, sleep patterns of wild birds are difficult to study, especially for those actively migrating and for small songbirds that cannot carry technology which could solve this mystery.

In 2016, researchers put such technology on a large bird, the great frigatebird. Frigatebirds live on southern coasts and fly over the ocean non-stop for weeks searching for food. How do they sleep when they do not land?

Similar to dolphins, frigatebirds perform uni-hemispheric slow wave sleep. This means that they are able to sleep with one eye open and power down half of their brain so the other half may stay alert. Even more impressive, frigatebirds were also recorded sleeping as we do and reaching brief REM states without impairing their flights!

But these feats may only provide the bare minimum amount of sleep required to stay alive. While on the wing, frigatebirds sleep for an average of 42 minutes a day compared to the 12 hours a day they usually sleep while resting on land.

Although songbirds may be more difficult to study due to their small sizes, we do have some clues. To migrate, many songbirds that are normally awake during the day flip their sleeping habits to be awake overnight.
Swainson’s thrush, which can be found locally, use three strategies to compensate for long nocturnal flights. First, they can rest half of their brain at a time similar to frigatebirds. Second, they take several micro-naps throughout the day averaging just nine seconds long. Last, they get drowsy like we do and can partially shut both eyes to remain somewhat alert while they relax.

Health and hunger seem to be key determinants to how much a songbird can sleep while migrating. A 2019 study found that healthy, well-fed birds were able to sleep more, since they did not need to forage as much as hungry birds. Sleeping posture also differed, with thin birds minimizing heat loss by tucking their head into their ‘shoulder’ to conserve energy. Although cute, this prevents them from easily seeing approaching predators. Well-fed birds are able to sit upright and sleep with exposed heads to avoid predation since they are less concerned about heat loss.
Although this week’s poor weather has guaranteed more sleep for our banders, very little has been seen migrating through the station and our nets have been shut for the most part. With our limited net hours we have only managed to band 117 birds, bringing our fall season total to 2,834 from 53 species.

The great frigatebird – it sleeps with one eye open while flying.
Swainson’s thrush – a micro-napper, nine seconds at a time.

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