Sunday, November 29, 2009

Robins and Global Warming

Earlier this year, the National Audubon Society (NAS) released a report entitled: Birds and Climate Change: Ecological Disruption in Motion.

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):

Steller's Jay
Fox Sparrow
House Finch
Purple Finch
Pine Siskin
Red-breasted Nuthatch
"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)
Steller's Jay
Purple Finch
Pine Siskin
Boreal Chickadee
Red-breasted Nuthatch
American Three-toed Woodpecker
Spruce Grouse

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)
Ring-billed Gull
American Robin

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)
Pygmy Nuthatch
Marbled Murrelet
Virginia Rail
Spruce Grouse

NOT SURE (I don't spend time in the winter range, and am not sure of what factors might be involved)
Varied Thrush

GOOD GLOBAL WARMING CANDIDATES? (These species require open water; obviously warming temperatures would allow them to remain further north)
Ring-necked Duck
Red-breasted Merganser

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.

Robin snags an earthworm. During warmer months, robins prey heavily on worms and other small animals; in colder months they switch to a diet heavily dominated by fruit.

Female American Robin surrounded by the fruit of Amur Honeysuckle, Lonicera maackii, upon which it and dozens of other robins were gorging. This photo was taken in Columbus, Ohio, on the Columbus Christmas Bird Count, December 2008. The massive amount of non-native honeysuckle fruit now available is enticing robins to winter further north in ever greater numbers.

Bush honeysuckles, native to Eurasia, were originally planted for their pleasing aesthetic qualities - showy, shiny fruit and pretty fragrant flowers - as well as wildlife food. These plants now serve as a prime example of the folly of intentionally introducing non-native species, as they have proliferated to the point where native plants have been adversely impacted, ecological webs fractured, and bird populations have been influenced to winter north of where they ought to be.

Perhaps the first academic botanical text to mention Ohio's worst invader honeysuckle, Amur Honeysuckle, was E.L. Braun's Woody Plants of Ohio, published in 1961. Here is what Ms. Braun said about the plant 48 years ago: "Reported only from Hamilton County, where it is becoming abundant in pastures and woodlands".

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.

This map of Amur Honeysuckle is hot off the presses, courtesy of Rick Gardner, botanist for the Ohio Department of Natural Resources. Like all good plant maps, it is based on specimens, and shows the continued invasion of Amur Honeysuckle. Based on my travels, I am confident in saying that the plant is present in all of Ohio's 88 counties, even if specimen records don't yet document that. Furthermore, it is absolutely abundant in many areas, producing robin food by the bushel. Not just Ohio, either - honeysuckles are on the rampage in many areas.

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.


Anonymous said...

Thanks for expressing your opinion. Perhaps others will look more closely at other global warming claims for logical explanations.


Jason Kessler said...

Thanks for the sane, considered analysis.

Wally said...

That makes a lot of sense. Amur honeysuckle and other invasive shrubs with winter berries are everywhere, and there are so many of them. It's no wonder that the birds would start tapping into this new food source. It's kind of surprising that there aren't even more of them doing it.

Scott said...

I really enjoyed this post. It is this kind of analysis that needs to happen in order to understand climate change. Too many of these studies start out with finding data to support a hypothesis. Instead of using the data to to formulate a hypothesis.

Buford Nature said...


plainbirder said...

Thanks for not jumping on the emotional bandwagon of global warming. I don't deny climate change, but a sane common sense approach is refreshing instead off shoveling everything in the the hopper of global warming.

nina at Nature Remains. said...

We have a ton of honeysuckle in our woods. Each year I make it my goal to tackle a few more of the very persistent bushes, but I know they have me beaten in the big picture.
This year seems to be an especially "good" year for berries, too--they're fatter, more numerous, and big bird magnets.
In the last couple of years, we've suddenly had the huge flocks of cedar waxwings--gorging on these berries.
But I love them for the birds.

Kirk Mantay said...

Very well reasoned. I thought the Audubon "study" was a good anecdotal look at these patterns.

"Anecdotal" often shows us the most likely path, given a need for adaptive management strategies and our lack of time to develop better data. "Anecdotal" is a good wildlife management tool for the short term.

However "anecdotal" is too often put forward as science, which it is not.

Kathleen said...

Great points. The explanatory power of these various hypotheses could be examined further, possibly by looking at correlations among the bird overwintering increases and these factors. For example, the honeysuckle hypothesis would be supported if areas where honeysuckle (or other invasives with similar bird-dispersed fruit) existed were correlated with an increase in robin bird counts and places where vegetation has remained relatively uninvaded by these shrubs did not have an increase in robins. Winter temperatures and human population density (and associated food provisioning) could be examined in the same way.

Jason Kessler said...

I have to say:

I thought your posting this was going to result in a real donnybrook. Raising questions about the real nature of the apparent current warming trend tends to arouse as much clamor as discussing abortion or smoking.

But look at all the supportive comments you've received!

I think it's a testament to your well thought-out and presented arguments, as well as the fact that nature people are more attuned to thinking about The Bigger Picture than the average guy in the street who, with the best of intentions, allows himself to be swayed by what I call "bad science.".


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