Large groups of birds often attract attention. Readers often report flocks of hundreds or even thousands of robins, blackbirds and crows.

Those are not random gatherings of birds -- they flock for many reasons. There must be advantages, such as safety in numbers, but there are compromises, as well.

Birds forage more efficiently in large flocks than they do as individuals. Food sources often occur in large, unpredictable ephemeral patches. Many searchers working in unison find food patches faster than solitary feeders. Thus, members of a group typically have better access to food than individuals.

Furthermore, living in a large flock is safer than living alone. Though large groups are conspicuous and easy for predators to find, groups have many eyes and members are vigilant. They detect predators at greater distances and flee before a real threat materializes.

And if a hawk or falcon attacks, it's safer to be in a group. Flocks under attack gather into tight flight formations and make incredibly precise twists and turns. The result is that predators are confused -- it's difficult to focus on one individual.

Another reason evasive flocks form tight groups is that every individual tries to get to the center of the flock. The center of a group is the safest position because predators usually attack individuals on the edge of a group. A flock member on the edge is easier to separate from the group.

Large roosting (sleeping) flocks enjoy similar advantages. Owls might visit huge roosting flocks of crows, but they take only individuals. So over the course of a winter an owl might take 100 birds from a large roost, but if there are 50,000 birds in the roost, odds favor survival. And again the safest position in the roost is in the middle, so winter roosting flocks tend to be large. Typically older, more dominant birds take the safer interior spots, leaving younger, less dominant birds to the more vulnerable, peripheral positions. This results in constant bickering and competition for the best positions until all settle in for the night.

Another advantage to winter roosts is that flock members can huddle shoulder to shoulder on cold nights to reduce heat loss.

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