Birds can sleep more efficiently by sleeping with both hemispheres simultaneously (bihemispheric slow wave sleep) when in safe conditions, but will increase the use of USWS if they are in a potentially dangerous environment.
During flight, birds maintain visual vigilance by utilizing USWS and by keeping one eye open. When migrating, birds may undergo unihemispheric slow-wave sleep in order to simultaneously sleep while visually navigating flight. This can allow certain species of birds to sleep without hindering migration and to prevent making frequent stops along the way. Certain bird species are more likely to utilize USWS during soaring flight, such as the Albatross, but it is also possible for birds to undergo USWS in flapping flight as well.
Swainson’s thrushes travel up to 3,000 miles every autumn from northern Canada and Alaska to winter in South America and in spring make the long trek back. The birds fly at night for hours at a time without sleep. The thrushes will utilize USWS while they continue to fly. To help make up for sleep lost during long night flights, studies suggest these migratory birds take hundreds of power naps during the day, while at roost, each lasting an average of 9 seconds.
Studies have shown that ducks tend to take short naps while they fly by shutting off half of their brains. One eye closes and half of the brain shuts down while the other half keeps the bird’s basic functions going. The flock rotates the birds that are on the outside (fully awake) and those that are on the inside (“sleeping”). This keeps everyone fresh and rested, and it also helps protect the flock by putting the most alert birds on the outside.
On land, this half-asleep adaptation is important to survival whether a bird is solitary or in a group. In flocks of birds at roost, those at the outer edge of the flock often have one eye open. The eye connected to the sleeping half of the brain closes; the eye of the wakeful hemisphere remains open and vigilant. Birds in the middle of the flock are resting the entire brain, sleeping with both eyes closed.
References and further reading:
Livescience.com; Wikipedia.com; about.com; birdnote.org; and Cornell Lab’s All About Birds.