Why birds stay perched

WHY don't sleeping birds fall off their perch?

This has to do with the tendons - think cables - in their legs. They have a particularly long tendon that stretches all the way from their thighs, over their knees, down over their legs, round their ankles and under their toes.

This nifty setup means that when they're snoozing their bodyweight causes them to keep their knees bent. This pulls the long tendon tight and subsequently their claws close tightly on whatever perch or branch they're resting on. - largestfastestsmartest.com