A bird chooses a roosting site, a place to sleep and rest, with care. The roosting site can mean the difference between life and death on a cold winter night.
You know from watching your bird feeders that birds quit coming to feed about a half hour before dusk. In the dim twilight, birds find a roost site and hunker down for the evening. Of course, nocturnal birds like owls have a topsy-turvy schedule so go to roost around dawn.
Particularly in the winter, the roost site should provide some protection from the elements. Heat is easily lost to convection, the movement of a cold fluid over a solid structure. Meteorologists are always warning us about wind-chill; convective heat loss provides the explanation of wind-chill.
The obvious way to minimize wind-chill is to get out of the wind. Many birds choose a roost site in a conifer where the needles reduce the wind. Roosting close to the trunk has triple benefits. The area next to the trunk will experience the least wind because of the dense foliage extending outward from the trunk. A central roost in a conifer maximizes the safety of a roosting bird from owls or other nocturnal predators. Finally, the trunk of a tree emits a modest amount of infrared radiation (sensible heat) that can provide just enough heat to get a chickadee through the night.
Some birds roost in cavities. Woodpeckers maintain a roost cavity, often separate from the nesting cavity, were they spend the night in relatively cozy comfort. Ruffed Grouse and Common Redpolls take advantage of the remarkable insulating properties of snow by roosting beneath the snow surface. Having a grouse fly up from the snow as you walk by on a nighttime snowshoe hike is a heart-stopping experience. To get into the snow, the grouse flies into a snow bank to create an instant den for the night.
Some birds roost alone. Black-capped Chickadees provide a local example. Others huddle at a roosting site. The energetic advantage is clear. By huddling with other birds, some of the heat lost to the cold can be absorbed by a neighboring bird in the huddle rather than lost to the atmosphere.
My favorite anecdote of huddling in birds comes from an English garden. After a particularly cold night, a homeowner checked the contents of a small nest box measure 4x4x5 inches. What a surprise it must have been to find 61 Common Wrens (similar to our Winter Wren) huddled together to snugly pass the night.
I have received lots of questions this winter about the massive American Crow roosts that are often in heavily populated areas. I wrote a column about crow winter roosts last year. You can read it at: http://bit.ly/1cIYz0f
American Crow roosts vary in size from several hundred birds to two million. Crows are one-upped by the Red-billed Quelea, an Old World sparrow related to the House Sparrow. Quelea roosts reach the tens of millions of birds. Alas, the most social of roosting birds is not longer with us. Roosts of the extinct Passenger Pigeon numbered in the billions and covered many square miles.
Communal roosts may involve several species. Several species of herons and egrets may roost together.
For any roosting bird, you may wonder if they ever fall off their perch at night. Birds have special flexor tendons that cause the toes (usually three pointing forward and one pointed back) to lock into place when the legs are bent. The weight of the roosting bird keeps the legs bent and the flexor tendons firmly locked through the night. When the bird awakes in the bird, it straightens its legs and the flexor tendons relax, freeing the toes from the perch. The same flexor tendons in birds of prey prevent prey from escaping by locking the talons into the unfortunate prey item.