crow rookery

We live near a crow rookery. When the sky darkens and the wind roars down from the north, the sky is filled with crows heading for a safe roost. Flock after flock flash overhead flapping and soaring, their wings a dark pattern against the white sky.
I once read that when flocks of  Passenger Pigeon flew overhead, the sky was darkened for minutes at a time, and when they descended from the sky to perch, great branches broke from the trees under their weight. That isn't so hard to imagine after seeing hundreds of crows fluttering on swaying branches as far as the eye can see.