As any serious birdwatcher can tell you, the saying about birds of a feather flocking together oversimplifies birding behavior. Some bird species have a strong proclivity for flocking while others tend toward living solo or in couples. Even among birds with a tendency toward group living, flock size can vary greatly. Depending on the type of bird, a flock may consist of three to five birds, while others gather together in tens, hundreds or even thousands.
We usually think of robins hopping across the grass in spring and summer, either solo or in pairs, on the hunt for worms and insects to eat. However, fall and winter tend to bring on flocking behavior for these birds. They flock either to migrate, or if they overwinter, to roost and forage for berries together. A flock of robins tends to contain about 20 to 30 birds.
Crows tend toward very social behavior, roosting together in large groups. Flocking together enables crows to protect themselves from predators. A group of crows will often aggressively chase off a hawk or owl from their territory. Since crows not only consume carrion but have been observed ganging up on a sick animal to kill it, the term for a group of these members of the corvidae family is a murder of crows. Crows congregate in large families during the warmer months, and in colder weather hundreds of crows will flock together.
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One annual congregation of birds is so large as to draw flocks of human tourists. Tens of thousands of migrating sandhill cranes annually gather along a 75-mile stretch of the Platte River in central Nebraska. The early spring spectacle attracts birdlovers who come to see the huge gathering of these large birds who stop en route to their northern spring and summer homes. Cranes, like many other bird species, migrate in large groups for safety from predators. However, the degradation of wetlands means fewer places for migrating flocks of cranes to find sustenance on their journeys.