Winter World_ The Ingenuity of Animal Survival - Bernd Heinrich [98]
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.
Redpoll.
Evening grosbeak.
Pine siskin.
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