Term meaning the sleeping or resting behaviour of birds. Roosting is only performed when two special requirements are met: 1) the roost has to provide protection from predators and 2) body-temperature can be maintained for a longer time without burning all the reserves, which is particularly important in cold climates for small birds (large surface/volume ratio!). This can be done by fluffing the feathers (improves insulation), using a hole, or huddling close together.