What Is Nesting?

It means: The tendency to arrange one's immediate surroundings, such as a work
station, to create a place where one feels secure, comfortable, or in
control

When a group of birds are building their nests.

Also could mean when a dog or cat is pregnant and about to go in to labor. They will go through a nesting stage start scratching at the carpet or bedding to get comfy for labor and the babies.

Nesting is for birds to build their nest.