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By Root 1336 0

a dead virus-infected crow is found, “all" the crows may disappear in that area. But there is no reason to conclude that “the first thing that happens when the West Nile virus invades an area is that all the crows die—" (“The Silence of the Crows,” in the Washington Post, 30 August, 2002), much less that we are “witnessing the disappearance of the crow from the American landscape." I see no evidence that these birds are disappearing from “the American landscape"; there is no evidence that they are being depleted either by mutual warring or by the virus. If anyone finds a virus-killed crow, it is most likely to be simply where there are a lot of crows—a communal roost, and since roosts are strictly temporary, lasting only through the winter, it is therefore a given that the crows disappear after some die. We don't need another round of government bombing to kill 200, 000 crows in a roost to get rid of a few sick ones.

19

WINTER FLOCKS

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