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Northern Invaders

Lately I've been paying more attention to the birds visiting my backyard feeder, and I'm sure I'm not the only one doing so. As the end of fall approaches, lots of birdwatchers in the northeastern United States begin to wonder whether the "winter finches" will appear. They're thinking about several species of colorful finches that nest in the boreal forests of Canada and, every few years, head south in large numbers. Years can go by with scarcely more than a handful of evening grosbeaks, red crossbills, white-winged crossbills, or redpolls appearing in places like Massachusetts, New Jersey, or Pennsylvania, and then suddenly, flocks of them will show up at backyard feeders throughout the Northeast, gorging on sunflower seeds and other treats. It's enough to brighten even the bleakest winter day. Earlier this fall, observers in Canada were reporting a major southward movement of white-winged crossbills, raising the hopes of birdwatchers in the US, but thus far, very few crossbills have crossed the border. Ornithologists have correlated these southward "irruptions" with the abundance of the cone crops of various northern trees. White-winged crossbills, for example, feed extensively on the seeds inside the cones of spruces and hemlocks (indeed, their oddly shaped bills are designed to extract those seeds). In the autumn, if the cone crop is small, or if the cones have shed their seeds due to unusually warm, dry weather, the crossbills will abandon their northern haunts and head south. The related red crossbill specializes on the seeds of pines; it too will head south if the pine cone crop is inadequate. Redpolls are particularly fond of birch seeds and are thought to migrate in response to fluctuations of that food resource. My personal favorite is the evening grosbeak, a magnificent golden-yellow bird that reminds me of a goldfinch on steroids. It likes the seeds of maples, box alders, ashes, cherries, and pines. Once largely restricted to the western United States, it expanded its breeding range into eastern Canada and northern New England about a century ago. As recently as 30-40 years ago, large flocks visited the northeastern and central Atlantic states every winter. But something mysterious has happened to the grosbeak population, for the species has virtually disappeared as a winter visitor to my part of the world. When three individuals turned up at a wildlife sanctuary near Princeton last winter, birdwatchers from throughout New Jersey flocked to see them. Although none of the winter finches appears to be in danger of extinction, they do pose an interesting conservation dilemma: How do you protect species that have no regular migratory season or pathway, species that appear to wander for hundreds of miles? No one knows. In the meantime, we birdwatchers keep our fingers crossed that they will appear in our backyards this winter...and we will wish them a safe journey back to the boreal forest when spring arrives. What do you think? Leave us a comment. ———- David Wilcove is professor of ecology, evolutionary biology, and public affairs at Princeton University and one of the world’s leading experts on endangered species. He is the author of No Way Home: The Decline of the World’s Great Animal Migrations.