Winter World: The Ingenuity of Animal Survival - Bernd Heinrich (2003)
By the end of July summer is drawing to a close. The autumnal symphony of cicadas and katydids has not yet started, but most birdsong has stopped. A hush comes upon the land. The insect songsters are still in their larval stages, and the young birds are out of their nests. Family ties are already broken and birds of many species then forge alliances with other juveniles, to form wandering bands along with their parents. Those vagabond bands include the huge populations of red-winged blackbirds and common grackles from the marshes. I meet them occasionally along the river-bottom cottonwoods and box elders. Flocks of tens of thousands of grackles, and sometimes of redwings, move through the late summer and fall woods like a giant steamroller, or perhaps functionally they are more like a giant vacuum cleaner.
A vanguard will land ahead of the flock turning the freshly fallen leaves, looking for food, while others hurry along behind. The rear birds keep flying to the front to avoid the freshly searched ground and so they move forward, in a rolling action. Noisy black swarms of them occasionally loiter in the trees, joining and leaving the fray. And so they stay as flocks for most of the fall and winter as they travel ever farther south. They presumably migrate only as far as they need to find food. In early spring the blackbird crowds reappear. These birds disband from their flocks only for the short period of about three months after they return in early spring, and then some of them become semicolonial to nest.
While these birds leave and spend most of their time in flocks in more southerly regions, other species come to New England from their breeding grounds on the tundra and taiga of the Canadian shield where they live in pairs. They become gregarious and wander in flocks only as winter approaches. Following their food supply of seeds and of winter berries that vary hugely in kind and amount from year to year, they make only unpredictable annual appearances.
Most of the winter visitors from the north are finches—redpolls, pine siskins, evening grosbeaks, red crossbills, white-winged crossbills, pine grosbeaks—that depend on tree seeds. But at least one fruit-eater, the Bohemian waxwing, and a grass-or weed-seed-eater, the snow bunting, also come in tight flocks from the north. Our resident goldfinches, which are solitary in the summer, also form their own winter flocks, but they stay. The purple finches stay only sometimes, and they form loose, small flocks. The cedar waxwings that nest here in the summer also form winter flocks, separate from their close cousins, the Bohemian waxwings. Flocking up for winter is a common phenomenon. What accounts for it?
There is not likely just one explanation. As with communal roosting at night, there are instead many interrelated ones, and their relative importance has been much debated in the scientific literature. The advantages of joining a flock probably include the “many eyes” effect of detecting danger, the previously mentioned “selfish herd” effect, to reduce predation risk as well as the learning effect of taking advantage of what others have experienced. Flocking specifically in the winter may reflect either dietary differences between summer and winter, or inability or reduced opportunity to wander in flocks while rearing young.
In the summer most northern birds must feed their young a high-protein diet so that they will grow quickly to adulthood. That means they must hunt for hidden and highly dispersed food, principally insects. In their hunting, individual initiative is at a premium. In the winter they can switch back to high-energy food, such as fruit or seeds, and many of these foods are in widely dispersed but large clumps that many pairs of eyes can locate more easily than one, where sharing costs little, and where there is risk to feeding alone when exposed in the open winter environment. But why do all the seed-and fruit-eaters segregate into their own species-specific flocks?
It probably relates to diet as well. Most of the seed-eaters are specialists for specific kinds of seeds, and by joining a flock they pool information of what is relevant to them specifically. For example, redpolls and goldfinches feed on birch seeds that are much too small for evening grosbeaks, so grosbeaks must forage separately. Snow buntings feed on the seeds of grasses, sedges, and other field plants. Evening grosbeaks have the strong thick bills with which to crack white ash seeds that the small finches lack. Crossbills have bills specifically adapted for prying apart the bracts of spruce and pine cones. Pine siskins have long thin bills suited for reaching the seeds under the bracts of hemlock cones. Given the different seed preferences and different tool kits needed to reach them, it pays for members of each species to join up and travel with its own kind. There are, though, glaring exceptions to the truism that “birds of a feather flock together.” They are the resident birds of the winter woods that feed on insects. All over the world, insect-eating birds form conspicuous multispecies flocks. For about the last ten years I have kept a tally and made observations of these flocks in the winter woods in Maine because it seemed odd that these birds, which are solitary in the summer, would so dramatically change their behavior in the winter. Why would very different kinds of birds that feed on very different insects, and almost never on clumped seed or berry bushes, follow each other around in winter only?
A flock of goldfinches eating birch seeds.
Multispecies bird flocks are not unique to the winter woods of Maine. In the hot lowland forests of Tanzania in East Africa, I used to search for the noisy groups of a certain forest weaverbird, and having found a flock I would invariably see several species of bulbuls, barbets, and flycatchers traveling along. One theory for these groupings of insect-eating birds is that some birds of the group act as beaters that provide prey to another. For example, woodpeckers on the trunks of trees may chase off insects that fly off and that are then available to be captured by the specialists on flying insects such as flycatchers. It’s the same idea as cattle egrets following buffalo to catch the insects they scare up, or some dragonflies following a large mammal through the grass, as happened to me in Botswana. (The dragonflies even chased me when I ran.) A second nonexclusive reason is safety in numbers. More eyes alert for predators such as snakes or hawks means less attention needs to be diverted to vigilance and more can be devoted to food-finding instead.
Like the bright-orange-and-black weaverbirds that I used to search for as markers of congregations of many interesting birds in Africa, so I look now for chickadees in the Maine winter woods. The other species commonly associated with a winter flock of chickadees near my cabin are two or three golden-crowned kinglets, a pair of red-breasted nuthatches, a pair of brown creepers, and sometimes also a pair of downy woodpeckers.
The chickadee flocks are probably the primary attractors for the other four bird species, because those species are almost never associated with each other in the absence of chickadees. I have never found creepers with woodpeckers, nuthatches with creepers, although all three are often alone. They either seek out and follow chickadees, or chickadees follow them, and the latter seems unlikely because a flock of chickadees cannot follow a half-dozen other species at the same time. Chickadees are also always by far the most numerous, the most noisy, and the most visually conspicuous members of any mixed-species flock. They make the most distinct “target.” It is hardly possible to miss a chickadee flock, but it is hard to find the often quiet, unobtrusive, and hidden brown creepers, nuthatches, kinglets, or the downy woodpecker or two that may be with them.
The insect-eating winter birds minimize competition among themselves because each species forages on different trees, different parts of the same tree, or different prey. However, I’m doubtful that the beater effect, which may appropriately apply in the summer or in tropical forest flocks of presumably nonbreeding birds, would apply for these winter groups. Frozen insects are immobile and will not be chased off to become a target to a flock member. That leaves the many-eyes hypothesis for predator protection as a reasonable alternative.
I suspect flocking is advantageous at any time of the year, but it is constrained in the summer when the birds are tied to a nest site. In winter, when a limited food supply becomes a factor for survival, flocking may also become evermore advantageous, provided there is no competition for food. That is because it permits the birds to pay more constant attention to searching for food, and less on vigilance for predators.
Kinglets in a mixed-species flock in winter (with downy woodpeckers, brown creepers, red-breasted nuthatches, and chickadees).
In the Maine winter woods I almost always find the golden-crowned kinglets in groups of two to five individuals. Despite their small flocks, the group cohesion in kinglets is remarkable. The Austrian ornithologist Ellen Thaler, studying captive kinglets near Innsbruck, found that the birds make special calls when approaching their sleeping place. These calls attract members of the troop of kinglets foraging together. A second assembly call draws the group into a cluster. Once together, the birds in the center of the cluster hunch their heads down into their shoulders with their bills pointing up. The birds at the edges tuck their heads back and to one side under their wing feathers. The group takes about twenty minutes to get into position in warm weather, but they bunch up in only five minutes when it is cold, although mated pairs and siblings always bunch up with each other in seconds. Apparently the kinglets recognize family members by voice and they are less inhibited to huddle with them than with strangers.
The average number of kinglets per winter troop near my cabin in Maine is probably too small for either the many-eyes or the selfish-herd hypothesis to apply. But if huddling is necessary for surviving cold nights, as can be deduced from physiological studies showing considerable energy savings of two birds huddling together, then a couple may be enough. However, body warmers are not likely to appear magically at dusk if each bird is moving at breakneck speed all day through the dense woods, looking for the nearly invisible caterpillars it likes to eat. Attracting and keeping vocal contact with others throughout the day may be a key component of their winter survival, especially in dense coniferous woods where kinglets are not only rare but almost invisible. The availability of body warmers at dusk cannot be left to chance; losing only one or more members per troop might doom the rest to freezing to death on some cold nights, especially after a day of poor foraging. If so, then it is no surprise that no kinglet winter flock, even a tiny one, is ever silent for more than several seconds at a time. The birds try to keep in contact. If they should get separated from a traveling companion, then it’s a safe bet that by finding a noisy chickadee flock, they would soon meet up with another of their kind. That, at least, is how I reliably find kinglets.