You might think that once birds finish raising their young, they nest, curl up and go to sleep in their warm bird houses. But the reality is, birds find places to roost at night just about anywhere. Some of these spots include tree cavities, snags and bushes, building crevices and crow's nests. Birds may also sleep in eaves, barns and bridges.
During the day, birds are active, but when darkness falls they become vulnerable to predators. For this reason, most birds roost in protected areas at night. Flocks of gregarious birds, such as blackbirds and red-winged blackbirds, sleep in trees or thick shrubbery where they are safe from predators. Those who choose to roost in holes, cracks or snags of trees do so because they provide protection from cold temperatures and wind.
In addition, some species of birds like geese and ducks sleep standing up at night. This helps them conserve body heat and fend off predators. They usually stand on one leg and tuck their beaks under their back feathers. They also fluff their feathers and bury their heads into them to retain warmth.
On very cold nights, it's even harder for birds to stay warm. To avoid hypothermia and frostbite, some species of birds roost together in large roost flocks to share their body heat. Other species, such as starlings, sleep individually on the ground or in the tops of tall trees. They will often perch close to the trunk of a tree because it holds heat longer than the foliage.