'a flock of crows' is a correct and usable phrase in written English.
You can use it to refer to a group of crows, for example: "We heard a loud cawing overhead and when we looked up, we saw a flock of crows flying away."
We were joined by a flock of crows.
And once a flock of crows just flew in out of nowhere.
We parked along a country road, lit cigarettes, cracked the window open, and watched the November night rush in like a flock of crows.
They include a flock of crows flying into the camera, the devil suddenly lunging forward from his throne and a host of disembodied spirits flitting around.
But the Japanese designer did not just put women in black like a flock of crows flying across the brash 1980s.
One September afternoon in Portland, a flock of crows snatches Prue McKeel's baby brother and drags him into the Impassable Wilderness, a forbidden and supposedly uninhabitable woodland just across the Willamette River.
The script, written by Vitti, involved, among other things, a flock of pesky crows and a shareholders' meeting at the Springfield power plant.
It looked as if a flock of quarrelsome crows had flown in through an open window, gorged themselves, and flown away again.
A flock of hungry crows in May could easily peck through our family's vision of feasting on those delectable little ears of Golden Bantam come August.
Martin Amis once ruled this particular roost, but now it's tempting to imagine a flock of upstart crows squawking as they carve up the territory.
Claims that Rufus the Hawk, Wimbledon's headline-hogging bird of prey, has been bullied this week by a flock of SW19 crows have been refuted by his handler Imogen Davies.
