When I was a child, the indisputable sign of spring was my father triumphantly announcing, “I saw a robin today!” We lived in the Great Lakes area, west of Toledo, Ohio, where the damp cold of winter seeped into your bones and clouds hung low and gray for months on end. Dad hated winter–today we would call it seasonal affective disorder–and he kept close tabs on the robins. In the early spring nothing made him more jubilant than spying his first robin–and sooner than anyone else. Every year he had counted at least fifteen by the time I glimpsed my first one.

I moved away from winter just after college and spent twenty years in the Bay Area, where I missed the robins of my childhood. Yes, there are robins in coastal California, but they wear a faded shade of orange on their breasts instead of the proud rusty red of the robins of the Midwest. So I was delighted when I moved to Boulder a couple of years ago to find the robins in the Rocky Mountain region wear the same deep-bright costume as the ones I grew up with.

One thing has puzzled me, though. Why am I seeing robins all winter? Just last week, walking my dog along our neighborhood street, I heard a clatter of robins and looked up to find one small tree covered in robins. I counted at least eight before they flitted out of sight. Why are they spending the whole winter here? Are the Colorado winters so much milder than Midwestern ones?

My questions were answered yesterday when I tuned in to a webcast from the Audubon Society covering a new report, “Birds and Climate Change” (updated in 2014 here). The webcast was superb—an interactive slide show and live presentation plus questions from a few of the eight hundred people registered for it. You can watch the archive of the presentation here (no longer available).

Dr. Greg Butcher, Audubon’s Director of Bird Conservation and a coauthor of the report, showed that from analyzing the past forty years of data from the Christmas Bird Count, Audubon scientists have found that many birds are moving northward as a result of warmer winters. We’re not talking about the extremes of the birds’ territories, we’re talking about the “center of abundance” for each species, the point at which half of the birds are wintering north and half are wintering south of it. Of the 305 species studied, 177 (58%) moved their center of abundance north during that time.

The average distance birds moved north was 35 miles. Robins, though, have moved way north–206 miles farther north than they wintered forty years ago. Dr. Butcher said that this year Audubon is getting calls from people in Minnesota and Bozeman, Montana, asking why they are seeing robins in the middle of winter. I haven’t checked with my relatives in Ohio, but no doubt they too are enjoying the sight of robins.

My father would be thrilled to see robins all winter, and I have to confess I like it too. But the reason is alarming: global warming. Over the same forty years of the Christmas Bird Count, the average winter temperature in North America has increased steadily. The warming temperatures correlate with the northward movement of birds, making it clear that climate change is the culprit.

One class of birds has not moved north in the past forty years: the grassland birds. Birds that nest and forage in grasslands, such as burrowing owls or meadowlarks–birds we enjoy here at the edges of the prairie along the Front Range–are remaining in place, but this is not a good sign because it indicates there is nowhere northward for them to move. Here in Boulder if you hike in the open space you will hear western meadowlarks trilling their liquid song from the tops of low bushes or fence posts. But their numbers are dwindling because their northern habitat keeps disappearing due to suburban sprawl and global warming. Their cousin, the eastern meadowlark, is in serious decline, its numbers decreasing by 72% over the past forty years.

On their website Audubon includes a petition that you can sign (no longer available) to call on lawmakers to support legislation that reduces greenhouse gases and our dependence on oil. Petitions don’t have “teeth”; they don’t ensure that policies will change. We’re the only ones who can guarantee that–by making changes in our lives to use less oil-based energy.