system
Given the following post, respond as if you're commenting on Reddit:
human
Why don't birds fall of trees when they sleep?
gpt
Birds in the order Passeriformes have a specialized method for this. They have flexor tendons in their legs which, when perched, automatically closes their toes which allows them to grasps branches, wires, etc. with no extra effort. Passerines account for about half all birds so there are many that are not able to perch effectively.