'flocks of birds' is correct and usable in written English.
You could use it in a sentence like this: The sky was filled with flocks of birds taking flight as the sun began to set.
Flocks of birds materialized out of the mist.
Flocks of birds move, acrobatic, across the sky.
It's September, which means 'tis the season for giant, mesmerising flocks of birds.
In Little Neck Bay, he saw flocks of birds hovering over disturbed water and drew nearer.
Dried-out salt lakes, flocks of birds, sheep same colour as the land.
The flocks of birds and clouds that mix together in the background.
The exterior is of cast aluminum, with interlocking gray and black pieces evoking flocks of birds.
"We were kayaking through flocks of birds and the water was like glass," she said.
Another example appears in flocks of birds that post sentinels to warn of danger.
The Wildflower Meadow, Central Park: milkweed; daisy; columbine; flocks of birds; toasted oats; wood; honey; my childhood; bark.
His numerous grandchildren, like hungry flocks of birds, attacked the mangoes while they were still green and sour.
