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Birds of a Feather

October 16, 2009

Every year at this time, one cannot help but notice that most landbird species spend the day foraging in groups. For example, tree-dwellers such as chickadees, nuthatches, kinglets, and woodpeckers arrive at our feeders in a group. So do ground-dwellers such as juncos, sparrows, and finches. Indeed, birds not of a feather forage together at this time of year. Why don’t they forage alone? In what way would individuals do better (in the eyes of natural selection) if they had a genetic makeup that predisposed them to forage near others, rather than lead a more solitary existence?

One hypothesized advantage is that flocking behavior may promote an increase in feeding efficiency. An individual that is programmed to forage near other individuals may realize a greater rate of food intake than a solitary forager because the many eyes associated with a group are more likely to spot a food “hot spot” than is a single pair of eyes. Also, individuals who forage near one another may copy the foraging style or location of the more successful foragers. Interestingly, there is very little evidence in support of this idea.

Another possible advantage of foraging near others is that an individual stands a better chance of avoiding being eaten by a bird predator, such as a Northern Pygmy-Owl, Merlin, or Sharp-shinned Hawk. The success of these bird predators depends entirely on the element of surprise. By flocking, the chances of at least one bird spotting an approaching predator are pretty good. When that individual takes off, all the others nearby are automatically warned, and they escape too! Numerous experiments with trained predatory birds have demonstrated conclusively that small birds in large flocks flush sooner (and are less likely to be caught) than the same kinds of birds who forage alone or in smaller flocks.

So, the next time a tightly grouped bunch of chickadees or finches descends onto your feeder to clean you out of bird seed, think about how flocking behavior may have helped most of them find the feeder to begin with, and how such social behavior may also help them survive the constant threat of predation by other birds.

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