Social scenes in the sky.

Some people hang out together for fun. Birds do it to survive.

Compared with humans, you might not think of birds as social animals. (I mean, you hardly ever see geese hanging out at the mall.) But look skyward when great flocks of birds migrate toward their feeding grounds. Many of these birds not only fly and feast together, but also breed in huge colonies and join forces to fend off predators.

All these bird-to-bird interactions are examples of social behavior, says orinthologist Joanna Burger. While some birds successfully "wing it" on their own, others depend on the multiple benefits of a group to survive.

POWER OF THE PACK

Take mealtime. If a solitary foraging bird is not wary, it can become lunch for a hungry predator. But when birds forage together, more eyes are on the lookout, offering each member more security--and more time to chow down.

Techniques to ward off predators can vary species by species. Squawking Scarlet macaws, foR instance, fall silent when they sense a hawk lurking in the treetops. Seconds later, the group explodes into a screeching, wing-flapping riot. The sudden uproar--much louder than any individual bird's shrieks--often confuses the predator and chases it away.

Other flocks get physical. Ravens, for instance, have been known to gang up and mob predators such as vultures.

Starlings use another defensive tactic. One scientist has observed flocks of starlings bunch together in the air when attacked by a falcon. This strategy appears to deter the falcon by increasing its chances of colliding with the crowd and suffering a disabling injury (a snapped wing feather, say).

Starlings are usually highly social, sticking together day and night. But flocks of most species aren't as cohesive, convening only at strategic times during the year. For instance, about one-tenth of the world's bird species form dense colonies during their breeding seasons. This strategy helps individuals protect their young, ornithologist Burger says.

For example, if a preying hawk spots a solitary nesting bird, it's almost certain that the hawk will overpower the individual and snatch its young. But if that bird is nesting with 500 others, the odds are only 1 in 500 that its offspring will be snatched.

ENERGY SAVERS

Another benefit of nesting in huge colonies is energy conservation. Penguins, for example, conserve heat energy by huddling together. Their feathers collectively trap heat, raising the temperature in the center of the flock as much as 10 [degrees] C (18 [degrees] F). In addition, with less of each penguin's body surface exposed to the cold air, even individual penguins on the fringe can reduce their heat loss by 50 percent.

Flocking may also help birds conserve the energy they need for long-distance migrations. Some scientists theorize that certain flight formations, such as a single-file line or a V, give birds in a flock an energy-saving edge. Like a lead race car or bicyclist in a race, a bird at the head of a single-file flock cuts through the air and creates a partial vaccum in its wake. Birds following the leader in this vacuum face less air resistance, the head-on force of air pushing them back. Result: The birds fly more efficiently. They also take turns leading the flock. In this way that energy-guzzling task doesn't fall on one individual, Burger says.

Birds cling together for a strategic cause: their own survival. The pictures on these pages will help you recognize the strategies when you see them. Why not head outdoors and check out the many survival strategies of flocks in your piece of the sky?