There is an interesting and potentially tragic purity in the way other life forms respond to our warming climate. They do not debate the matter. They simply shift as the contours of their habitat shift. Even the distribution of slow-moving species - like trees - is changing. And more mobile species? They offer vivid testimony about the alterations in our world.

According to the Audubon Society, data from the past 40 years of the annual Christmas Bird Count - a three-week census of American bird populations - shows a striking northward movement among a majority of species.

The boreal chickadee has moved 450 kilometers north, almost out of the range of the lower 48 states. The marbled murrelet, a seabird that breeds inland, has moved 580 kilometers north. The wild turkey has gone about 640.

These population shifts make it look as though many bird species can easily adjust to a warming world. And some of these shifts may be opportunistic in nature - taking advantage of open ground and new food sources in regions that used to be snowed over in winter. But ultimately birds cannot migrate out of their habitats.

A boreal species pushed farther and farther north comes eventually to the end of the plants (which move far more slowly) that it depends upon for food. A grassland species cannot simply decide to become a woodland species. And the entire species mix that defines suitable habitat is changing as well, bringing with it the risk of extinction.

The important thing to remember - as we notice an absence of purple finches at our feeders - is that we are not merely witnesses of these striking shifts. We are the cause of them, and it is our responsibility to do all we can to mitigate them.