The climate of North America has changed dramatically over the last 40 years, with the greatest effects occurring during winter. There’s been a shift to milder temperatures, but climate models predict more variable and intense precipitation, not unlike the snowy February we just experienced. These models predict that temperatures will continue to rise over this century and that the most profound climate changes will occur in more northerly latitudes.

We expect that some species will expand their ranges northward as the climate warms. Such expansions are nicely documented by Christmas Bird Count data and Breeding Bird Survey data. Expanding species include turkey vultures, tufted titmice, Carolina wrens, northern mockingbirds, blue-winged warblers and northern cardinals.

In a recent article in Global Change Biology, Karine Prince and Benjamin Zuckerberg have taken a broader approach to examine changing bird populations in the face of climate change. Rather than concentrating on single species, the authors take a community approach to examine the impact of winter climate change. A community is simply all the species found in a given area.

Prince and Zuckerberg used Project Feeder Watch data for their analysis. PFW entails regular censuses of feeders from early November until late April. Observers report the birds visiting their feeders regularly over two-day periods. Over 10,000 PFW sites are sampled each year in the U.S. and Canada, with each generating many checklists each year.

For their study, the authors confined their analysis to eastern North America south of the 50th parallel, which passes through Newfoundland. They restricted their analysis to counts between Dec. 1 and Feb. 8. Their work was based on data from over 30,000 PFW sites from the winter of 1989-90 to the winter of 2010-11.

The authors first used the Christmas Bird Count to calculate the average minimum winter temperature of each species (Species Temperature Index). This value was obtained by taking the average of the average minimum winter temperature within each Christmas Count circle where a species occurs. The result is a number that represents the minimum average temperature near the middle of the range of a species.

The authors then calculated a Community Temperature Index (CTI) by averaging the STIs for all species found in a particular PFW community. CTIs were calculated two ways: by averaging the STIs of all species in the community, and by weighting the CTI calculation by abundance of the species in the community.

The authors expected that CTI values would increase over the time period, reflecting the northern incursion of more southerly species with high STI values. In fact, that’s the pattern that was found using both types of CTI measurements. The analysis shows winter bird communities in eastern North America are becoming dominated by species adapted to warmer climates. Many species are contributing to the changes in the CTIs, with chipping sparrows and yellow-rumped warblers most important in the Southeast, and eastern bluebirds and Carolina wrens in our region.

FLYING WEASELS

I expect you have seen the online picture of a weasel riding a woodpecker in flight. If not, here’s an article with photos: http://slate.me/1FRvZFH

These photos are not the first documentation of a flying weasel. Author and wildlife artist Ernest Thompson Seton reported that someone shot an eagle and found the skull of a weasel attached to its throat. The best guess is that the eagle swooped down on the weasel, intending to make it a meal. The would-be-prey must have latched onto the throat of the eagle and never let go. The eagle carried the reminder of that ill-considered attack for the rest of its life.

This report is recounted in Annie Dillard’s lovely essay, “Living Like Weasels” at: http://bit.ly/1gV7OIm.

Herb Wilson teaches ornithology and other biology courses at Colby College. He welcomes reader comments and questions at

whwilson@colby.edu