Birds of a feather

the benefits of flocking

It is said that “birds of a feather flock together”, and indeed they do. We see gangs of crows—sometimes a hundred or more—foraging together on the wetlands or at the tideline. A skein of Canada geese flies overhead, talking all the time. Better yet, a little troop of trumpeter swans flies up the channel, their whiteness contrasting well against the dark mountainsides.

Why do such birds do things in groups? There are (so far) three main kinds of reasons.

When geese or swans fly together on extended flights, they usually do it ‘in formation’—about a meter apart in a line or a V, they gain aerodynamic advantages. A vortex forms at each wing tip, creating down-drafts and up-drafts. By flying at the appropriate distance from another goose and timing its wing-flaps to catch the up-drafts, a goose that follows another one gains lift and saves energy.

When crows or buntings or shorebirds forage in bunches, one possible benefit lies in protection from both aerial predators, such as eagles, hawks, or owls and ground-based predators, such as coyotes or dogs. A bird foraging alone must keep a sharp lookout for possible predators, and the time spent being vigilant decreases the time available for feeding. In a group, however, there can be many eyes watching and everyone can spend more time eating. In a flock of geese, grazing with their heads down, there is almost always at least one with its head up, being vigilant; a few minutes later, a different bird will be the sentinel. Furthermore, if a group of birds is startled into flight by an approaching predator, the whirl of many frightened birds might distract the predator, making it harder for the attacker to focus on a particular bird.

There may also be advantages in locating and exploiting a food resource. In some cases, particularly in species that are quite sociable year-round, if one bird finds a really nice patch of food, that bird might signal to its friends and relations to come join the feast, and the rest of the gang just follows. I suspect that crows might do this.

Sometimes, the foraging activities of one bird might make food more available to its opportunistic companions. A bird fossicking about in the wrack line, for example, might stir up any number of invertebrates, making them vulnerable to the next comer. Then every bird in a foraging group may take advantage of that and, in so doing, stir up more prey. Another example: dense squadrons of scoters cluster on the surface of a bay or channel often make what I call ‘chain dives’—one bird at the edge of the flock dives deep and all the others, in a line, follow the lead, moving up to dive in the same spot as the first diver. I have to wonder if the foraging activity of the first birds might loosen up a congregation of mussels, for instance, and make it easier for the later divers in the chain to pluck them up.

There are also times (and places) when birds that are NOT of the same feather flock together. These mixed-species flocks occur most commonly in forests around the world. In North America, mixed-species flocks form in winter, and commonly include chickadees, which are often the focal or ‘nuclear’ species, accompanied by golden-crowned kinglets, and maybe nuthatches or brown creepers or even a downy woodpecker. The various species in such a flock are not related to each other nor do they forage in the same ways, although they typically all eat insects and spiders found in the forest canopy. In our local forests, I have found mixed flocks of chickadees and golden-crowned kinglets in winter, but I seldom detect the other likely follower-species (of course, I simply might miss them!).

Obviously, the ‘many eyes’ idea applies to multi-species flocks. In many cases the birds in the flock are foraging at some distance from each other, maybe even out of visual contact, but the cohesion of the flock is maintained by vocalizations. Each bird may be foraging independently, but if one bird spots a predator, the whole flock may be notified to be on watch. By joining such a flock, and depending on the watchfulness of everyone, each bird might gain more time for feeding; this had been shown to occur in some situations. In a few cases, the foraging of one species might create feeding opportunities for others: for example, the tapping and bark-scaling of a downy woodpecker could make hidden bugs accessible to birds that glean surfaces.

As usual, many questions remain. Why do chickadees usually serve as nuclear species, drawing other birds together in a foraging flock? Do nuclear species benefit as much as their followers do? What triggers the formation of a mixed flock at certain times or places? How long do these flocks stay together, and what leads to the eventual breakup of the flocks? Under what circumstances might the foraging activity of one species benefit another? A curious naturalist might not have a ready answer to such questions, but formulating questions is usually the first step in finding answers. Sometimes finding an answer is not even the most useful outcome of questioning; the very act of questioning can stimulate new insights and perspectives.


Watching young animals

a bear family, ducks at war, a porcupine child, and flocks of young birds

The first part of July gave me some very nice opportunities to observe young critters just learning to make their way in the world.

Above the tram in Bear Valley, we watched a bear family foraging on green herbage. Mama was all business, but the cubs were more interested in playing. The two larger ones wrestled and rolled, boxed and bumped, flattening the vegetation after mama had forged ahead. Cub number three was a little smaller than its siblings but eventually trotted out of the thickets to follow the rest.

The forest at the top of the tram gave us a close-up look at a mixed-species flock of birds. First we saw a nuthatch, then a chickadee and a couple of siskins, and then the trees were full of a family of golden-crowned kinglets, including numerous fledglings. Accompanying this gang were two brown creepers, hitching up the tree trunks and acrobatically working along on the underside of branches. The entire flock foraged in plain sight for at least five minutes. Mixed-species flocks are thought to be advantageous to the participants, especially in keeping multiple eyes on the lookout for predators, but all the fluttering activity may also stir up insect prey.

On my home pond, we’ve had the duck wars. Three female mallards have brought broods of ducklings to forage along the pond margins and rest in the weeds on shore. Three broods, all of different ages, and the female with the oldest ducklings tended to rule the waters. She often chased all the others, sometimes very aggressively. Her ducklings were quite well feathered in mid July, and those of the next oldest brood were just beginning to show real feathers amidst the down. Then calamity struck—I heard a female quacking loudly and persistently, and I looked out to see that her brood had just been reduced from four to three. With the remaining ducklings closely huddled around her, she fussed continually for at least two hours, and I finally went out—to discover a sorry little pile of down under the trees on the far bank. I suspect a goshawk had found its lunch. The female went on fussing for another hour or more before accepting the new reality.

A friend and I were scrambling along the bank of one of the tributaries of Fish Creek one day. As the valley narrowed down to a canyon, we practically stumbled over a female porcupine with her offspring. Mama quickly hid her head under a log, leaving her spiny back bristling in our direction. Baby, on the other hand, fiddled around a few minutes, then clambered over a stick and slowly made its way between two tree roots. There if finally did the right thing (for a porcupine) and wedged its head into the fork between the roots and erected its defensive spines over the only exposed part of its little body. This little guy was a tad slow off the mark: baby porcupines can execute the typical defensive maneuvers almost immediately after birth. And this one had had weeks to practice. We left them in peace, of course.

Another friend and I led a guided hike up Gold Ridge on a nice but rather foggy day. There were some spectacular flower shows, and an assortment of marmots, including a young one just poking its head above the flowers. We also had a treat, in the form of a juvenile golden-crowned sparrow. The juvenile looked nothing like its nearby parent except in general shape; it had no strong black and gold crown stripes, but it did have conspicuous almost-golden spangles all over its back. This observation was special, because we’d never before seen a juvenile so close-up, even though this bird nests up there regularly.

We saw one female sooty grouse, with some wee chicks hidden in the low vegetation, and one rock ptarmigan, which might have been guarding an invisible brood. But both grouse and ptarmigan seem to be much less common on the ridge than they were just a few years ago.

Gray-crowned rosy finch adult. Photo by Bob Armstrong

On top of Gold Ridge, we hoped to find gray-crowned rosy finches that nest in the cliffs up there. And, in between swirls of fog, there they were. Adults were feeding fledglings at the edge of a remnant snow bank. The fog made it difficult to see plumage colors, and all the birds just looked black, but eventually we could discern some pattern and distinguish parent from chick. The juveniles were plump, active, and fully capable of doing their own foraging, but –in the way of all young songbirds—they wanted their parents to deliver.


a common bird with an uncommon memory

A friend and I stood in a small muskeg on Douglas, checking out some deer tracks, when little twitterings announced the arrival of chickadees. Two of them scoured the twisted trunk of a dead pine right next to us, but we soon saw that there were two more—no, four more—no, maybe six or eight more—thronging the foliage of nearby hemlocks. They might be finding overwintering spiders or insect pupae and even adults.

I enjoy watching the flock that hang out around my house, too, as they diligently pick out little black sunflowers seeds from the feeders and flit into the adjacent spruces. They may eat some of those seeds immediately but sometimes the return trip to the feeder happens so quickly that there was only time to stash the seed in a hand crevice for a later snack.

Watching these chickadees stirred me to dig for more information. The species that lives in our area is the chestnut-backed chickadee, which lives principally in the Pacific coastal rainforest. Of the seven species of chickadee in North American, four (including ours) breed regularly in Alaska.

Chestnut-backed chickadee. Photo by Bob Armstrong

All the chickadees share the habit of nesting in tree cavities. The cavities may be natural ones left by a broken branch or a woodpecker, or they may be excavated in soft, decaying wood by the chickadees themselves. Clutch sizes tend to be large, averaging seven or eight eggs per nest. Both parents care for the chicks, but in some cases, not all the chicks in a nest have the same father because, as in many other birds, some hanky-panky goes on! Predation on nest contents of cavity-nesting birds is commonly lower than for open-cup nesters, but nest predation on chestnut-backed chickadee nests can be as high as fifty or sixty percent of nests; red squirrels are a principal predator on eggs and chicks.

When chickadees forage through the canopy, they often hang upside down to glean from the undersides of twigs and leaves. They can do this very agilely, reportedly thanks to well-developed special leg muscles. Chestnut-backs often forage in flocks in winter, sometimes joined by other species such as nuthatches and kinglets. In the southern part of their geographic range, as many as fifteen other species have been recorded in mixed-species flocks with chestnut-backs.

Where chestnut-backs overlap with black-capped chickadees in Washington, their foraging patterns differ slightly. Chestnut-backs have slightly smaller bills and are more closely associated with conifers. They forage especially on foliage and twigs, while the blackcap forages more often on the bark of tree trunks and branches. Although both of them hang upside down to reach the undersides of leaves, the blackcap reportedly does so more often.

Chestnut-backs apparently have been much less intensively studied than blackcaps, which is arguably the best-studied songbird in North America. Because detailed information about chestnut-backs is hard to find, I thought I’d summarize some of the details about blackcaps, which range all across North America and nest in the Interior of Alaska. It seems likely that much of what is known about blackcaps also applies to chestnut-backs, but that remains to be ascertained. In the meantime, here is some cool stuff on blackcaps, particularly their winter flocking and its consequences, food storage behavior, how they get through the cold season, and their calls.

The winter flocks of black-capped chickadees are strongly hierarchical, with males generally dominant to females and older birds dominant to younger ones. Winter ranks have carry-over effects to the subsequent breeding season. Dominant male breeders are in better body condition than subordinates and tend to have better nesting success, at least in some habitats. They also participate more than subordinates in hanky-panky outside of the socially monogamous pair. Higher winter ranks of females also lead to their better survival in the breeding season.

Black-capped chickadees store food in fall and winter, each one stashing hundreds and sometimes thousands of seeds and insects in bark crevices or among conifer needles or in cracks in trunks and branches. Unlike red squirrels, which create piles of cones, the chickadees generally store items singly. They are very good at remembering where these items are stashed, being able to retrieve them after several days or even weeks, if one of the many potential thieves (squirrels, mice, nuthatches…) has not stolen them.

The part of the brain associated with memory is called the hippocampus, which increases in size in the fall, when food storage is a common activity. Blackcaps in the north store more food and have larger hippocampi than those in the southern part of their range. Blackcaps also have larger hippocampi than chickadee species that do less food storage.

Dealing with cold winter temperatures requires metabolic energy, and the colder the weather, the higher the metabolic costs. Blackcaps in Alaska are reported to have a metabolic rate about fifteen percent higher than those farther south. Heat generated by the muscular activity of foraging, during the day, also contributes to staying warm on winter days. At night, however, blackcaps allow their body temperatures to drop ten or twelve degrees centigrade, saving substantial energy; however, the blackcaps in Alaska are a bit different (of course?), and only let their temperature drop about three degrees. At night, they roost in cavities or in thick vegetation, usually singly.

The calls of black-capped chickadees may sound to us like indistinguishable twitterings. But not so, among themselves. They can distinguish the subtle variations in each other’s calls and identify specific individuals (as can many other songbirds). Furthermore, the “chick-a-dee” alarm call varies according to the relative risk from a potential predator that is visible; a different call indicates a predator that has been heard. Other species, such as nuthatches, eavesdrop on blackcap alarm calls and respond to the signal. Recent research has shown the male black-capped chickadees sing at higher pitches when the level of anthropogenic noise is high (for example, near heavy traffic, construction, logging activity, and so on), which may have consequences for breeding (as it does in some other songbird species too).