A collective noun is a word that describes a group of people, things, animals, etc. Below are some collective nouns (shown in boldface):
a herd of sheep
a choir of children singing
a family of doctors
a bouquet of flowers
a flock of birds
In American English collective nouns are more often singular, and so a singular verb is used with them. (In British English they are more often plural, and so a plural verb is used with them.) Below are some sentences showing collective nouns with singular verbs (shown in italics):
A herd of sheep is grazing happily in the field.
A choir of children sings beautifully on TV.
A family of doctors was gathered at a reunion.
A bouquet of flowers sits on the table.
A flock of birds flies over our heads.
Sometimes when the members of the group are doing different things, or are not acting together as a unit or with a shared purpose, a plural verb is used. A singular verb is still correct in these examples. Below are some examples of this:
A herd of sheep are scattering in all directions across the field.
A choir of children are all from different schools.
A bouquet of flowers were strewn around the room.
If you’re unsure whether to use a singular or plural verb with a collective noun, it's safe to use a singular verb.
To read about plural collective nouns (more than one group of things) click here.