Sleep is a necessity for all living beings, but most birds, this need must be satisfied while perching on a branch or any other place that they can grip. You might have fallen asleep while reading a book or holding your cellphone, and you know how hard it is to keep hold onto something while sleeping. So, how do birds actually do it without falling?
Well, Birds are blessed with distinct anatomical arrangements that prevent them from falling during sleep. Thanks to their powerful tendons, they are able to keep their upright pose and perch on trees, branches, and even on electric lines.
These animals possess a skeletal system that has adapted to their ways of life. For instance, flying alone requires various adaptations, such as making their bones stronger and lighter. Most of their bones are combined into one, with few of them are hollow, and others have internal struts to provide power. These special and unique adaptations can also be found on swimming birds and flightless birds.
So, if you ever wondered why a sleeping bird does not fall, the answer is that birds as well have adapted to the perching life.
The feet of birds are powered by tendons located on the back of their feet. In fact, they have two of them, the flexor halluciss longus and the flexor digitorum longus, that are linked to their deep flexor muscles in their legs. The former is responsible for the back toe, called the hallux, while the latter works the front three toes of the bird. Both tendons stretch over the ankle. Whenever a bird lands on a branch or an electric line, its ankle bends, forcing the muscles and toes to clench. They will remain locked as long as the ankle is bent.
The said functionality provides perching birds with a reliable grip that allows them to stay in an upright position even when sleeping. What is even more remarkable is that there is no muscular effort exerted by the bird in grasping onto the branch or electric line, everything is automatic. When the birds fly, its legs straighten, the tendons relax, and their toes free their grasp.
Other birds also use the same anatomical response for grasping their prey. As a preying bird closes in on its target, its legs are outstretched. Once it hits the victim, its ankles bend, automatically pulling the tendons and clenching the toes. Thus, providing a dependable grip on its prey. Its muscles and toes will stay locked on its dinner even while flying if it keeps its legs bent.
This incredible feature, however, is not present on all types of birds. For instance, those birds that do not fly, such as the emu, kiwi, or ostrich, do not possess a hind or first toe. Those birds that run as the curlew, courser, or plover also do not have a hind toe. Meanwhile, those birds with a shorter rear toe, such as the quail, duck, gull, or partridge, are not comfortable perching, which is why they slumber on the ground.
Nevertheless, those species have their own unique adaptations for them to sleep safely and adequately. For instance, some birds sleep with one of its eyes open. Certain birds, such as the common blackbird and the mallards, can make half of their brain alert (keeping the corresponding eye open) while the other half gets it breaks. For wild ducks or mallards, they keep their open eye towards the open area to sight potential danger and gear the closed eye towards its neighbors.
However, you might be curious why you do not see any dead bird perching on a branch, having died from old age, exposure to cold, or in its sleep. Well, they might be given this fantastic anatomical arrangement, but in death, the muscles relax, then birds just fall out of the branch or a line.
- Birds (Wikipedia)