Why don’t birds fall off their perches when they are sleeping?
Believe it or not, it takes less effort for a bird to stay on its perch than to let go of it. When a bird lands on a perch or a tree branch, tendons in its legs automatically tighten and its toes grip tight. This “locking mechanism” keeps the bird from falling off its perch when it falls asleep.

http://www.yesmag.bc.ca/Questions/BirdPerch.html

This tendon locking mechanism (TLM) exists opposite the proximal phalanges of each toe and pollex of many bats.

Since many birds have a TLM similar to that of bats, it is an excellent example of the convergent evolution of a feature brought about by similar functional pressures on birds and bats.

http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&d...