Our winter woods are usually as still as death. The trees are bare; the ground is blanketed with dead leaves or snow; the dominant colors are gray, black, and white. The silence is broken only by the moaning of the wind through naked branches.

And then you hear the chickadees. Their buzzing chicka-dee-dee-dee call announces their arrival long before you see them flitting through the lower branches of the taller trees or in the crowns of the tall shrubs. Behind them, most of the time, trails a mixed group of nuthatches–especially white-breasted nuthatches– brown creepers, golden-crowned kinglets, and downy woodpeckers, all the birds of the winter woods gathered together in a single flock, noisily foraging on the bare limbs of dormant trees.

Mixed flocks like these are a common feature of bird life. In the tropics, you can find them year-round. Here in the misnamed temperate zone, they are a phenomenon of winter, forming each autumn and breaking up in spring.

The birds in these flocks are predominantly insect eaters, although they also eat seeds and fruits. Chickadees, in fact, are among the most common visitors to backyard bird feeders. Feeding on insects in the winter means searching for those in dormant stages–eggs, pupae, or hibernating adults–and that is principally what they are doing as they bounce from branch to branch through the forest.

Ornithologists use the word “aggregation” to describe a group of birds that have simply gathered in one place. Gulls at garbage dumps are aggregations. They are there because they can find food there, but the individual birds have very little to do with each other.

Flocks are social organizations with an internal structure, a set of relations that defines the positions of individuals in the group. At the center of each of our winter flocks are members of the genus Parus, the group of birds that includes chickadees and titmice. In the Chicago area, the tufted titmouse is usually confined to southern and eastern sectors. You find them commonly at the Indiana Dunes and in extreme southeastern Cook County, but not north or west of these areas. So the black-capped chickadee is the usual leader in these parts. From southern Illinois to the Gulf of Mexico, the Carolina chickadee replaces the black-capped.

Black-capped chickadees are sedentary birds that typically spend their whole adult life in one small area. An ornithologist named Susan Smith studied the lives of black-capped chickadees around Mount Holyoke College in South Hadley, Massachusetts. Using colored leg bands that enabled her to follow the movements of individual birds, she was able to construct a complete picture of the lives of her study population.

Winter flocks actually form at the end of summer. The birds have spent the breeding season living in pairs, each pair on its own breeding territory. At summer’s end, the young of the year begin to wander. Each year, she found that these young birds completely abandoned her study area. They were replaced each year by young wanderers from elsewhere. She had no way of knowing how far these wanderers had come.

Smith’s study area held about a dozen breeding territories. In fall, these territories would be consolidated into four winter flock territories, each sustaining three breeding pairs–which remained together all year–and varying numbers of young-of-the-year that had wandered in from elsewhere.

The birds create a dominance hierarchy with each rung in the status ladder occupied by a pair of birds. The bottom rungs of the ladder belong to the newly arrived young birds. Some young birds– Smith called them floaters–do not pair up. Instead they drift from flock to flock through the winter. Smith discovered that these floaters were essentially opportunists. Instead of pairing up as low birds on the totem pole, they would wait for mortality to eliminate a higher-status bird and then they would move in on the widow or widower and take over the position in the flock vacated by the deceased.

Parus birds are the essential nucleus of every flock. The nuthatches, creepers, kinglets, and downy woodpeckers cluster around them. If there are no Parus birds present, there are no flocks. The Parus birds lead the flock through the woods. The other birds watch them and follow after.

If you are approached by a moving flock, you will see chickadees in the lead and at the center and the other birds ranged in a sort of horseshoe shape around them. The birds are all searching for the same sort of foods, but they tend to concentrate on different parts of trees and to use different hunting methods.

Chickadees usually glean their foods from the smaller twigs out at the tips of branches. They often hang upside down to search the undersides of the twigs. Kinglets often hover near the tips of twigs. Nuthatches and creepers work the trunks and larger limbs. Creepers start near the ground and hitch themselves up the trunks. Nuthatches move down the trunks, their heads pointed toward the ground, their tails in the air. Woodpeckers use their beaks as chisels, digging food out of cracks the other birds cannot penetrate.

There is substantial overlapping in this search for food, but there is also change in response to the presence of other species. Chickadees tend to forage higher when titmice are present, and kinglets do more hovering when chickadees are present.

Douglas Morse studied mixed flocks in woodlands in Maine, Maryland, and Louisiana. He found that the dominance hierarchy was mainly within each species. Attacks, chases, and actual fights flared up between two chickadees or two kinglets far more often than they did between members of different species.

The speed of a flock’s movement through the woods was directly related to the size of the flock. Flocks with fewer than 10 birds moved at an average rate of about 100 meters an hour; flocks with 30 or more birds moved about four times that speed.

The birds are feeding as they move. The kind of quick once-over they give the trees as they pass is apparently a sound feeding strategy. We tend to think of winter woods as totally dormant and unchanging, but there is actually a lot going on. Even in January, unusually warm, sunny days can stimulate insects to get out and move around. Storms blow down trees and strip dead bark from standing snags, exposing eggs and pupae that had been hidden. The birds in these flocks don’t often feed on the ground, but they will search through pieces of bark blown from trees. Their winter strategy is to make repeated visits to all corners of their territory–which in a deciduous woods might cover about 25 acres–searching for newly exposed food.

But why make these visits in flocks? These birds are not cooperative hunters like wolves. They each hunt separately, and their chances of finding a cluster of insect eggs on a dormant bud would be just as good if they were alone. Indeed they might lose meals to other members of the flock.

The answer seems to be that traveling in groups provides some protection from predators, especially airborne predators: hawks, owls, and shrikes. The chickadees take a leading role in the defense of the flock. They are very alert to any threat, and experiments have proved that the other species in the flocks respond to chickadee alarm calls. These calls–and many other species make similar noises–are very high-pitched. They are concentrated on a narrow frequency with few overtones, and they have no sharp onset or ending. They just fade in and fade out, making it very difficult to locate the birds that are making the noise.

The birds respond to these calls by freezing where they are, or by diving into cover and then freezing. They remain still until they hear an all-clear signal that signifies that the hawk has flown. The dominant male chickadee is most likely to be the bird that sounds the all clear.

More eyes provide greater safety, and greater safety means more time to feed. A Rutgers graduate student named Kim Sullivan conducted a very elegant experiment on downy woodpeckers at the Great Swamp National Wildlife Refuge in New Jersey that nicely proved the latter point.

Downies feeding on a tree trunk stop from time to time to scan the skies in a characteristic head-cocking movement, alternately looking left and right, searching for approaching hawks. Sullivan counted and timed these movements and compared the numbers for lone birds with those for birds in flocks. She found that lone birds averaged 20 head cocks a minute. Birds with one or two companions averaged 13 head cocks a minute, and birds in flocks of three or more averaged only 6 head cocks a minute. In other words, lone birds spent more than three times as much time looking for predators as birds in the larger flocks.

The time spent on defense had a direct effect on feeding. Lone birds took an average of more than two minutes to find a food item. Birds in large flocks averaged about one and a quarter food items a minute.

So the birds that follow the chickadees through the winter woods benefit very directly from being part of a flock. They can spend more time eating and less time scanning the sky. They can rely on the vigilance of many eyes, and especially on the extreme vigilance of the chickadees.

But what do the chickadees get out of this? The likely answer is cannon fodder. They move through the woods protected on at least three sides by skirmishers from other species whose position at the edge of the flock makes them far more vulnerable to the attacking sharp-shinned hawk or northern shrike. Like human generals, they observe the wars from a safe position behind the lines.