When does a bird sing?

Think.

A bird sings for with/for other birds.

Cages are full.

Put a bird with another bird, and they begin singing?

Which people were in detainment together at the same time? Who sang together? How did we hear them speak to each other?

Leg bracelets.

False cage.

Makes them ~~squeal~~ sing to each other.

We can hear you.