Flock (birds)

A flock is a collective gathering of individual birds. These birds come together to forage for food or to travel across distances. While many flocks consist of a single species, mixed flocks are also common. Mixed flocks contain two or more different species living and moving together. Often, the species in these groups share similar morphological characteristics. This means they have similar body shapes and sizes. Flocking is a vital behavior that helps birds manage the balance between finding resources and staying safe from predators.

In mixed flocks, birds often divide labor through different behaviors. One type of bird is known as a sally. Sallies act as guards for the group and catch prey while flying through the air. The other type is called a gleaner. Gleaners find their food by searching within dense vegetation. These two roles create a helpful cycle for the whole group. When gleaners move through plants to eat, they flush insects out into the open air. This action increases the foraging rate for the sallies. As more food becomes available in the air, the flock will often have more sallies than gleaners.

Safety from predators is one of the most important reasons to join a flock. Living in a group allows birds to spend less time and energy searching for danger. One method of protection is the use of alarm calls. When a bird recognizes a threat, it can use vocalizations to warn its conspecifics, or fellow members of the species. This provides an acoustical cue that helps the flock react quickly. Some birds even use a strategy called mobbing. For example, black-capped chickadees will surround a predator and attack it in a mob-like fashion. This behavior forces the predator to leave the area.

Young birds often learn these essential survival skills from older members. In the nonbreeding season in Chile, Peruvian thick-knees form flocks with an average of 22.5 birds. These groups include both adults and youngsters. Researchers have observed young thick-knees learning anti-predator strategies from the adults in the flock. This learning process ensures that juveniles are better equipped to handle threats as they grow. While some believe flocks help birds spot predators, others believe the sheer size of the flock simply decreases a predator's success rate.

Flocking also changes how birds find and share food. Some birds follow an information-sharing model. In this system, the first bird to find a reliable food source alerts the rest of the flock. This allows the entire group to benefit from the discovery. There is also a phenomenon called the beater effect. This occurs when birds in a flock capture prey that was made vulnerable by an unsuccessful bird. However, flocking has social costs. A social hierarchy can exist where dominant birds deny food to subordinate birds. Additionally, some individuals may act as scroungers. Scroungers wait for others to find food rather than acting as producers who actively search.

Even hunting can become a highly organized team activity. The Harris's hawk uses an intricate hunting system involving groups of 2 to 6 birds. The group splits into smaller units to enclose a prey animal, such as a rabbit. This coordinated attack allows the hawks to hunt much larger animals than they could alone. By working together, they also decrease the amount of energy spent on each hunt. This ensures that every hawk in the group can eat from the catch.

Large-scale flocking can create spectacular natural events. In Denmark, there is a biannual phenomenon called sort sol, which means "black sun." This occurs when massive flocks of European starlings gather during their migration. The birds create complex, moving shapes against the sky as they gather to rest and eat. These enormous numbers help the starlings decrease their risk of being attacked by hawks. While increasing flock size can lead to more aggression between individuals, the benefits of group living remain a central part of avian life.