It didn't take much of a perusal to cause red flags to fly up. Basically, NAS studied 305 species of North American birds using the past 40 years of Christmas Bird Count data as their primary stock, and claim that the majority of the study subjects are expanding their winter ranges northward. The report garnered good press, which I suspect was the primary intent. However, the report has also been criticized as overly broad-reaching in its conclusion that global warming has caused widespread northward range expansions of birds.
I rarely use this blog to criticize, but the NAS has put this report forth under the mantle of good science; therefore it is only fair to dissect it. And dissect the report in detail I will not, but the claims of tying in some of these alleged range expansions with global warming is worth a look. Mind you, buried in their report is a brief disclaimer stating that other factors may also be responsible for range expansions, but the overwhelming message is that climate change is to blame.
For the record, I do want to say that I certainly believe man-induced weather changes are occurring - I don't believe it possible to wreak the ecological havoc that our species is and not have such consequences. But jumping on sensationalist climate change bandwagons and using shoddy claims to back a PR campaign for purposes of a fundraising/membership drive is not too cool, either.
Gorgeous male American Robin. According to NAS, robins have expanded their wintering range northward by 206 miles in the last 40 years. I wouldn't dispute that. The real questions is... why? I'll offer what may be a more justifiable and documentable theory than global warming in a minute.
Of the 305 study species in the NAS report, 64% have advanced their winter ranges northward, according to the report authors' analysis. The other 36% went southward, apparently. But of the northbound birds, most species' expansion is seemingly inconsequential; possibly statistically insignificant. We're talking one, two, maybe ten or twenty miles in some cases.
So, I culled the heavy-hitters - species that have moved over 200 miles northward. There are nineteen of them and it's an interesting list. Global warming is the NAS position for these expansions; I will offer a few other possible explanations. I'm saving the robin for last, but here are the other 18 species grouped under alternative hypotheses that might allow species to expand northward in winter or be more likely to be detected (some species are included in more than one category as multiple factors may be involved):
"Rufous-sided" Towhee (Eastern and Spotted are lumped)
INCREASED OBSERVERS AND REPORTING ON CBC'S (An example: for the 1st 10 years of the 40 year study period, the Canadian province of Ontario averaged 27 counts and 725 0bservers annually. For the second quarter, this ballooned to 38 counts and 1,192 observers. In the third 10 year period there were 50 counts and 1,283 observers. In the final quarter there were 63 counts and 1,754 observers annually. These factors, along with increased bird feeding, would certainly seem to play a role in keeping birds north, and increasing the detection and reporting of them)
American Three-toed Woodpecker
INTRODUCTIONS (Species either accidentally or intentionally liberated and still actively expanding their ranges)
House Finch (eastern populations result of 1940 release in New York; still expanding in areas)
Wild Turkey (Fish and Game agencies are actively reintroducing turkey in many areas)
INCREASED FOOD SOURCES (either waste associated with expanding human population, or non-native plants)
INCREASE IN BIRDER SOPHISTICATION (better knowledge of winter distribution and where to find birds; increased use of boats for pelagic species; I-pod and audio technology to locate secretive species)
NOT SURE (I don't spend time in the winter range, and am not sure of what factors might be involved)
GOOD GLOBAL WARMING CANDIDATES? (These species require open water; obviously warming temperatures would allow them to remain further north)
Now, let's get to the case of the American Robin. The photo above shows the "new" woodland understory of Ohio and an ever increasing area of the Upper Midwest. Any of several species of bush honeysuckles (Lonicera spp.) are overrunning the landscape, providing millions of juicy berries that not long ago were unavailable. If you are a frugivorous (fruit-eating) bird, this means new feeding opportunities.
The map above is from a 1995 publication, The Dicotyledoneae of Ohio: Part 2, by botanist Tom Cooperrider. We can see that a tremendous expansion of Amur Honeysuckle has occurred in the 34 years since Braun's note.
Global warming undoubtedly is causing impacts to some birds, other animals, and plants. But tarring a whole suite of bird species with the global warming brush, as NAS has done in their report, is misleading. Many other factors come into play, and it is only fair to analyze those as well.