When we talk about a group of people, animals or things we often use a special name for the group. We call these special names collective nouns . We usually use a singular verb after a collective noun, because we think of all the members of the group as one group, not as separate parts of the group.

The flock of birds is high up in the top of the trees.
(a flock of birds = many birds together in one group)

The school of fish swims in the river.
(a school of fish = many fish together in one group)