Winter Roost

To me, winter hardship means getting up for work in the morning before the kitchen has hit 60 degrees. I suppose I could encourage the furnace to awaken earlier, but hot coffee usually provides heat enough. When I glance out the window to the first grey winter light of a Minneapolis morning, I inevitably see a few crows headed southwest, flying in dips and swoops as though it's windy, even when calm and 10 below. I would rather not be that cold, I think, clutching my hands around the warm mug.

But crows are winter roosters here in this part of the state. Ravens stay in the north woods year round, but their smaller cousins migrate slightly south, to dense aggregations where scavenging is good. I see them in the evenings too, a steady stream flying northeast on the last light of day, seemingly endless. But just where they go, I have never known. So I endeavored to find out.

Common sense would suggest that they head for the river, where plenty of trees line the banks and the closest thing to a natural order reigns. I planned to head down there some evening, tracing a line, quite literally as the crow flies, from our neighborhood to the Mississippi below St. Anthony Falls and the University. So imagine my surprise when driving home from work last night, just after the early dusk of a pre-solstice sunset, to notice a cloud of crows circling the I-94/35W commons, and alighting en masse in the trees of the neighborhoods that overlook this urban tangle.

The ecologist Bernd Heinrich has written a wonderful book about the habits of denizens of the snowy north, Winter World : The Ingenuity of Animal Survival, which will provide answers to anyone who has wondered how turtles get by for six months without taking a breath, or speculated about the marginal caloric economy of a tiny chickadee. I was pleased to find that he confirmed my experience--biologists have noticed over the past 50 years that crows have begun to prefer roosting in cities in the winter, and choose the brightest, noisiest places. Heinrich speculates that they may have learned to do so because such an environment discourages the crow's top predator, the great horned owl.

But why do they get together in the first place? Protection via group awareness is one likely reason. But Heinrich's theory, the result of detailed observations of ravens in the winter woods, is that corvids (crows, ravens, blackbirds, and grackles are good examples) benefit from communal information-sharing about the location of food sources in the lean months. Imagine the mother lode that might satisfy the largest conventions: Heinrich writes that some crow roosts out west contain several million individuals! But even our urban gathering could draw hundreds of thousands of birds.