All birds roost – that is they have a period of inactivity analogous to sleep in human beings. Some birds do it alone, others with mobs of compatriots, some change roosting habits with the season. Birds are amazing creatures having evolutionary and adaptive techniques and skills to help them get the rest they need, stay safe and warm. Here are just a few facts on avian roosting:
- Birds have both “quiet and active-sleep” phases. Quiet sleep is dozing and birds open their eyes and peek around. Mated males, smaller flocks, those in less-protected areas, breeding males with conspicuous plumage all peek more. Peeking increases during breeding season and with species who engage in promiscuous sex. Sleep with one eye opened allows them to rest one-half of their brain at a time. During active-sleep slumber, the eyes are shut.
- They have evolved the ability to drop body temperatures during sleep to conserve energy. Red-tailed Hawks, when deprived of food, drop 5-7°. Hummingbirds, swifts and poorwills enter a state of torpor in which body temps may drop as much as 50°. “Shallow torpor” for short periods is thought to be more widespread among birds than previously believed. Migrating passerines at stopover sites lower body temps by 8-10°.
- To prevent falling, feet and talons work with a pully system with their tendons enabling them to “lock down” until the legs are straightened again. This same grasp enables raptors to clamp down on their prey.
- Some sleep on the wing: wintering Chimney Swifts and frigatebirds who actually close both eyes into active sleep.
- Communal vs solitary roosting is closely related to communal and solitary nesters. Roosting is for protection, warmth and perhaps companionship. Flocks of millions – starlings and blackbirds – swoop down in seconds to their night retreats. Some change roosting habits with the season. Red-winged Blackbirds roost alone when breeding. Similar species gather together primarily for warmth and protection like nuthatches and creepers roosting in cavities. Older birds seem to sleep at the center of a large group.
- Roosting in a variety of places: cavities, tree branches, cliffs, tops of cacti, islands (seabirds), wires, cattails and nests (Verdin and Bushtits).
Cool fact: The record-holding communal rooster was the now-extinct Passenger Pigeon, who not only roosted, but nested in gigantic colonies containing billions of birds covering square miles.
Contributed by Lisa Grubbs