Why Do Birds Flock Together?

First posted on 10-06-2008

Kimberly Smith, professor of biological sciences in the J. William Fulbright College of Arts and Sciences, replies:

Vertebrates commonly form aggregations: fish form schools, mammals form herds, and birds form flocks. In birds, flock formation is generally associated with cooperative food hunting, information exchange and protection from predators. While there are disadvantages to being in close association with lots of other animals, the fact the flocking is so common in birds means the advantages outweigh the disadvantages.

An example of cooperative hunting would be flocks of White Pelicans that swim single file, body to body, driving schools of fish from deeper water into shallower water, where the fish can be scooped up in the pelicans’ pouches. Another example is associated with the Northern Mockingbird, the state bird of Arkansas, which defends fruit trees during winter as a food source. These very aggressive birds are able to deter individual birds, but flocks of American Robins and Cedar Waxwings are able to overwhelm the mockingbird, allowing them to steal the defended fruits.

Information exchange has to do with knowledgeable birds and naïve birds. Knowledgeable birds have some information about the environment that naïve birds lack. For instance, possibly the first blackbirds leaving a roost at dawn are the ones that discovered good feeding sites the previous day. By forming flocks leaving the roost, naïve birds are led to good feeding sites by knowledgeable birds. While information exchange was been clearly demonstrated in some animals, such as honeybees, studies done on flocking birds is equivocal.

The third reason for flocking was proposed by William Hamilton in “Geometry of the Selfish Herd” in the early 1970s. He suggested that the best strategy against a predator is to put another individual between you and the predator. If everyone tries to do this simultaneously, flocks, herds and schools should form in the presence of predators. Evolutionarily, it is safer to be in a group than to be alone. Traveling in a group may make it difficult for a predator to pick out a single prey item and flushing as a flock, as in ducks, might startle the predator, allowing most individuals to escape. Thirdly, it might be impossible for a predator to attack a flock without doing physical harm to itself.

The selfish aspect of his model is that it should be safer to be in the middle of the flock and riskier to be on the periphery, such that each individual should be trying to be in the middle. Therefore, individuals on the outside should be constantly trying to move into the flock and individuals in the middle should be trying to maintain their position. This causes animals in herds, schools and flocks to constantly change positions, leading to fast changes in direction. In bird flocks being actively pursued by another avian predator, such a falcon, this can lead to spectacular gyrations in the sky, as the flock moves up and down, side to side, and even flips backwards over itself as everyone tries to get in the middle.