flock

成群的鸟儿在空中飞舞。

A ___flock of birds is dancing in the air.

每到春天，鸟儿们成群结队地飞回北方。

Birds ___flock back to the north every spring.

山坡上有一大群羊在吃草。

There is a large ___flock of sheep grazing on the hillside.