Roosting Birds

The loud language of the birds bewilders and abuses the ears of all who get too close, however for the birds it is a relaxing lullaby, reminding them that when together they can sleep safe and sound. You wonder how they can sleep at all with such overwhelming sound, but as you look closer you can see that at least one third are sitting in slumber-land, able to defy even the most annoying of bird calls. The high metabolic rates of birds during the active part of the day is supplemented by rest at other times when they are roosting. Sleeping birds often use a type of sleep known as vigilant sleep, where periods of rest are interspersed with quick eye-opening "peeks", allowing them to be sensitive to disturbances and enable rapid escape from threats. Some birds have also demonstrated the capacity to fall into slow-wave sleep one hemisphere of the brain at a time. The birds tend to exercise this ability depending upon its position relative to the outside of the flock. This may allow the eye opposite the sleeping hemisphere to remain vigilant for predators by viewing the outer margins of the flock. Communal roosting is common because it lowers the loss of body heat and decreases the risks associated with predators. Many sleeping birds bend their heads over their backs and tuck their bills in their back feathers, although others place their beaks among their breast feathers. Many birds rest on one leg, while some may pull up their legs into their feathers, especially in cold weather. Perching birds have a tendon locking mechanism that helps them hold on to the perch when they are asleep.