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Birds Creative at Getting Sleep and Staying Warm

November 02, 2009 - With the arrival of cool weather, I expect activity at your feeders has increased. I like to monitor when the chickadees, goldfinches and other feeder birds leave the feeder in the afternoon. The birds are going off to their roosts, and the timing of their departure for their roosts, relative to sundown, is remarkably constant from day to day.

At their nighttime roosts, most birds spend most of the night sleeping. Birds spend about half of their lives sleeping but the amount of sleep they get depends greatly on the latitude and the season of the year. During the early part of the Maine summer, birds may sleep less than nine hours when the night is so short. In the winter, Maine birds sleep more than 15 hours a night.

Birds living in the vicinity of the equator experience near equal days and nights each day, so spend 12 hours a day sleeping throughout the year.

Some birds spend the night in communal roosts. European starlings and American crows are good local examples. In more southern parts of North America, huge roosting flocks of common grackles and red-winged blackbirds are common.

Migration poses a real challenge for birds in terms of getting adequate sleep. Most songbird migration occurs at night and then the birds must forage during the following day to restock their fat reserves. Not a lot of time for sleeping.

A recent study of migrating Swainson's thrushes revealed a behavior that helps these birds fight sleep deprivation. The birds take minute-long naps through the day. During these short siestas, a thrush will puff its feathers out and squint their eyes. A migrating Swainson's thrush will spend up to 15 percent of its day sleeping in these very short bouts.

Many birds choose solitary roosts. Red-tailed hawks, bald eagles, black-capped chickadees and purple finches are good examples.

Particularly during the winter, the choice of roost site can mean the difference between life and death. Many of our northern finches (common redpolls, pine siskins, purple finches) roost in evergreen trees. Such trees offer protection from the chilling effect of winter winds. The birds can also absorb a little of the infrared radiation emitted by the trees. In the brutal economy of a Maine winter, the little bit of heat absorbed from the environment can be critical for a wintering bird.

Woodpeckers, nuthatches and chickadees will spend the night in cavities, often the same cavities used for nesting during the breeding season. House finches will seek out the eaves of houses for a relatively warm roost site.

Some birds sleep on the ground. Members of the grouse family are typical of this sleep behavior. Northern bobwhite arrange themselves in tight circles on the ground with the heads facing outward. If a predator disturbs the roost, the birds explode in all directions, certainly giving the predator a dramatic shock.

The posture of a sleeping bird can be important in conserving heat during a long winter night. The unfeathered bill is a potential source of heat loss. Many sleeping birds will turn their head under their scapular feathers while they sleep. Mourning doves turn their heads down, nestling them in their dense breast feathers. Some birds sleep while standing on one leg, reducing by half the heat loss across the unfeathered legs.

Aquatic birds may sleep while floating on the water. The common eider, abundant along the Maine coast in the winter, snoozes while bobbing on the ocean surface at night. The down feathers of the eiders, renowned for their warmth, keep the eiders warm.

Some birds are able to sleep while on the wing. Some species of swift spend months in the air without landing. Albatrosses often spend months at sea. Sleeping on the surface of the ocean is possible but exposes the birds to fish and whale predators. Similarly, Sooty terns are reported to stay airborne from the end of one nesting season to the beginning of the next breeding season. Ornithologists believe these birds spend time sleeping in flight.

Have you ever wondered why birds do not fall off their perches while they are sleeping? Songbirds have a flexor tendon in the lower part of each leg. When they perch, the flexor tendon tightens, causing the toes to curl around the perch. The tendon essentially locks, keeping the toes firmly around the perch during sleep. The same type of tendon is useful in raptors when they are carrying prey. Once a fish is in the talons of an osprey, that fish has no chance.

HERB WILSON / BIRDING, Portland Press Herald, October 30, 2009

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