These roosts sometimes hold several hundred up to a million birds. Big, noisy birds. Harrisburg, PA, had a crow roost of between 10,000 and 20,000 birds right in the capital complex. Acting like a swat team, the dispersal team shot pyrotechnics, cracker shells, and bird bombs every night while a truck with a sound system blared a tape of the distress calls of crows being attacked by Great Horned Owls. Well, we don't really know, but for one thing some of the biggest trees around are in city parks or cemeteries, and crows like big trees for their roosts.
Crows also like to congregate near lights because their night vision is poor.
The bright city lights give them a better chance of escaping from or driving away predators like Great Horned Owls. McGowan thinks that roosts are a place where crows exchange information. If a crow hasn't found much food that day, it might watch for the guys that look fat and happy and well-fed and then follow them to their feeding area in the morning. It is also the possible that social interactions go on in these roosts that help the birds figure out their place in the pecking order of crow society. McGowan explains that crows didn't move into towns and cities until the late 1950s and although they're opportunistic, he still wondered how they would adapt to city life.