system
Given the following post, respond as if you're commenting on Reddit:
human
Eli5: How do bird legs work?

They look so thin and basic. How do they function?
gpt
Like human fingers, there are no muscle groups in them, just tendons. By relaxing or constricting the tendons, they move their joints, just like we move our fingers and toes.

Interestingly, some birds can wrap their claws around a branch and then relax. When they rest, their body weight can push down on the legs, and exert force on the tendons, which causes the tendons to reach their maximum length of stretch, and force the claws to squeeze around the branch.