Given their large population and the seasonality of most of their range, American Crows vary widely in their social organization.  In general, when not breeding, crows may form flocks that number in the hundreds or thousands.  At one time, the flocks of crows on the Great Plains had millions of individuals.  Many species, including herons, starlings, and swallows, form such large roosts; several potential hypotheses may explain this behavior, though ultimately the actual reasons are unclear (McGowan).  The first hypothesis suggests some kind of order: crows roost together to share information about food resources and follow others to food-rich areas (the "information center," according to McGowan).  Another hypothesis states that these large aggregations are a byproduct of the crows gathering where the resources are.  McGowan also speculates that roosts allow first- or second-year crows to find a mate for the breeding season, joking that the groups let the individuals "cruise the singles flocks" (Boyce 2002, McGowan).

The American Crow's flocking behavior may confer other advantages as well.  Langley (2001) has found that crow flocking behavior allows crows to overpower red-tailed hawks (Buteo jamaicensis) at roadkill sites.  This can be an important source of protein for many populations of crows, and their ability to dominate others species for such resources could be highly adaptive.

Photo Courtesy of Marcus G. Martin

Photo Courtesy of Marcus G. Martin

The groups may also provide predator protection; my father has observed crows in the pasture of our farm “setting sentinels” on fence posts while the other foraged, giving the advantage of advanced predator detection.  Crows are well-known for their mobbing behavior.  McGowan describes them as a “woodland alarm system” (2001).  Raptors are the usual victims of the crows’ racket; if a crow hears a conspecific mobbing a Great Horned Owl or Red-tailed Hawk, it will join the fight (McGowan 2001).  Crows even have specific calls—the “harsh caw,” the “assembly call,” the “dispersal call,” the “modified scolding call,” the “inflected alarm call,” among others—that refer to different predator situations (Verbeeken and Caffrey 2002), though McGowan writes crows make "lot ["of different calls"], but most of them sound like 'caw'".  Mobbing behavior, both to prevent predation and to overpower predators at a kill may very well  be a “selfish herd” arrangement.