By gathering together, crows find safety in numbers. Crows have a cooperative defense behavior known as mobbing. If one crow sees a predator, it will give an assembly call. Every crow within earshot will quickly fly to the source and attack the predator. If a crow has already been attacked, it will give a distress call, summoning other crows to its defense.
Crows are also highly social.
They have up to 25 different calls and seem to use them all. Crows may roost in cities as protection from the weather. During winter nights, urban areas tend to be five to ten degrees warmer than the surrounding countryside. Cities, with their smorgasbord of overflowing trash bins, are ready-made crow cafes, offering savory eating for birds that aren't particular about their diet. One of Missouri's biggest crow roosts was in the trees around city hall in St. Joseph.