Crows are very social and can gather in roosts of over 500,000 birds. They are beneficial to humans as they eat many insects that are harmful to agricultural crops. There are a couple of great books about these extraordinary birds: The Gifts of the Crow and In the Company of Crows and Ravens . Learning more about them will provide a perspective that is much different from the “pest” or “nuisance” that some call them.
Aside from humans, the greatest enemy of crows are Great horned Owls. During the day, the crows can outmaneuver the owls but this is just the opposite at night.