A flock is a gathering of individual birds to forage or travel collectively. Avian flocks are typically associated with migration. Flocking also offers foraging benefits and protection from predators, although flocking can have costs for individual members. Bird species living in a flock are able to capture prey, likely injured, from an unsuccessful bird within its flock. This behavior is known as the beater effect and is one of the benefits of birds foraging in a flock with other birds. By far most common collective noun used to describe a group of pigeons is a flock of pigeons.

Source: Wikipedia