Why do birds not fall out of trees when they sleep?

When a bird lands on a perch, these tendons tighten and so the toes lock around the perch. This involuntary reflex keeps a sleeping bird from falling off a perch. The tendons stay tight until the legs straighten. ... Falling asleep doesn't change the grip, as the weight of the bird keeps the leg in the locked position.

Explanation:

4 copper and gold, the california gold rush is why im pretty sure.

offline recommendation systems option b

because recommendation systems produce a list of two ways

1. through collaborative filtering

or

2. through content-based filtering