Mixed-species Bird Flocks Optimize Benefits

flock of pelicans and seagulls on shore during daytime

Biological Strategy

Birds

Sam Stier

Image: Julia Volk / Pexels / Free non-commercial use

Coordinate by Self-Organization

To create and maintain a healthy community of individuals and ecosystems requires that living systems coordinate their activities. Coordination doesn’t necessarily mean that there’s a leader orchestrating what happens. In nature, coordination is usually achieved through self-organization. In a flock of geese flying in a V-formation, for example, there’s no lead goose controlling where all of the others fly. The flock uses this formation because each goose gains energy from air vortices created by the goose in front of it. The lead goose doesn’t gain that benefit, so when it tires, it moves back and another goose takes the front position.

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Birds

Class Aves (“bird”): Eagles, hawks, sparrows, parrots

Birds are evolutionary engineering marvels. They are descended from dinosaurs, but are far from our idea of heavy, scaly reptiles. Of the specific adaptions that set them apart, most notable is flight—although some mammals can fly, birds take the prize for abundance in the skies. Many birds have hollow, lightweight skeletons and specially-designed wings to help them stay aloft. They also have feathers made of keratin that help them stay warm, attract mates, and improve navigation and aerodynamics in flight. In contrast to their dinosaur ancestors, they lack true teeth and have replaced them with specialized beaks and bills.

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Simple rules allow mixed-species bird flocks to find food and minimize predation

Seeing bees, crows, or other animals of a single species living together is a common sight. Seeing flocks of birds made up of a mix of species is much rarer and so all the more intriguing. Researchers have discovered that flocks made up of a mix of species have many advantages. For example, they can locate food patches together but then eat different foods within the patches. This way they balance the need for finding food with the need to reduce competition. Many different species can also monitor risks from predators in different ways and at different times.

But how do the individuals in each flock of mixed species make decisions that help the flock work well together? Individual birds in a flock of mixed species show a few different tendencies: (1) One tendency is to move from areas with a low density of birds to areas with a higher density of birds. (2) Another tendency is to move from an area with a low density of birds to an area with a medium density. (3) A final tendency is to avoid sites that have less than one-third or more than half of the same species.

These behavioral rules balance competing interests among the flock. Overall, these rules: (1) Help keep the flock together. (However, if this was the only rule, flocks would all aggregate on one small site, creating too much competition.) (2) Keep the flock together, while reducing both the risk from predators and competition while foraging. (3) Help place birds in sites that are good for foraging and in sites with enough diversity to reduce competition and minimize the risk from predators. Because the third rule is based on flock proportions rather than absolute numbers, these decision making rules can scale with varying flock size.

Last Updated May 14, 2020

References

“However, where the group size varied, our rule seemed to generalize to ‘avoid sites below one-third’ for conspecifics and ‘avoid sites below one-half’ for heterospecifics…This suggests that birds may be adapting their decision-making rule to their local social environment in order to generate a similar response across species.” (Farine et al. 2014: 179).

“Thus, by exploiting social information in its broadest definition (basing decisions on the behaviour of others or being attracted to popular choices), animals could be using a general rule that satisfies combined needs to reduce risk, such as through dilution, and gain information about the environment, such as finding the best sites in which to forage.” (Farine et al. 2014: 179).

Journal article

Collective decision making and social interaction rules in mixed-species flocks of songbirds

Animal behaviour | Farine, D.R., Aplin, L.M., Garroway, C.J., Mann, R.P. and Sheldon, B.C.

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