Cooper's hawks — on the rise

From the September 27 Washington Post "Urban Jungle" feature:

During the summer, Cooper's hawks  spend much of their time in heavily wooded areas, waiting for a chance to swoop through the foliage and seize a dove, robin or chipmunk.

But now they have begun to migrate, and the solitary, crow-size hawks are easier to see, especially in the mornings, sailing south.

They may reach the Florida Keys, where they tend to congregate before perhaps flying on to Cuba or the Yucatan for the winter.

Cooper's hawks are in the Washington area year-round, but winter's birds come here from farther north.

A century ago, Cooper's hawks were condemned as chicken killers and shot on sight. But a combination of conservation efforts and a steady increase in older forest cover has boosted the population.

By taking feather samples from a migrating first-year Cooper's hawk, a scientist can determine the latitude where the bird was raised. The feathers contain small amounts of the hydrogen isotope deuterium, which occurs in concentrations distinctive for certain latitudes.

Feathers from older birds are a less-reliable gauge of latitude, perhaps because older birds often grow new feathers as they migrate, and the reserves they use to build new feathers may have come from prey caught at various latitudes.