Have you noticed that songbirds are getting bigger? Good. Neither had I. But it’s happening, according to an article by PRBO Conservation Science biologist Rae Goodman and colleagues recently published online by the journal Global Change Biology. (Has enough time elapsed that we don’t have to say “formerly Point Reyes Bird Observatory” any more? These people may have a branding problem; maybe they should hire another consultant, or try a contest.) The differences are subtle; we’re not talking about chicken-sized song sparrows. They are, however, measurable and consistent—and may be related to global warming, if in an unexpected way.
The data Goodman et al analyzed came from long-term bird banding records at two locations: 40 years for the Palomarin Field Station down the road from Bolinas, 27 for the Coyote Creek Field Station near Alviso in the South Bay. Combining both data sets, wing length of banded birds has steadily increased at a rate of .024 to .084 percent per year. (I told you it was subtle.) Changes in body mass were not always significant, but when they were the trend was similar to wing length: .040 to .112 percent per year.
It would be reasonable to wonder if the wing-length trend, at least, had something to do with migratory behavior. Apparently not: there was no difference between the rates of change in long-distance and short-distance migrant species, or between local breeders and those that nested north of the banding sites.
Both trends came as a surprise. “It’s one of those moments where you ask, ‘What’s happening here?’’ said San Francisco State biologist Gretchen LeBuhn, a co-author. That’s mainly because they ran counter to other studies, most in Europe and the Middle East but one in Pennsylvania, that appeared to demonstrate size decreases in a similar range of bird species. And that result made sense in terms of Bergman’s Rule.
Bergman’s Rule (note that it isn’t quite a law), named for the 19th-century German biologist Karl Georg Lucas Christian Bergmann, holds that within a given species of warm-blooded vertebrate, subspecies from cold climates tend to be larger-bodied than subspecies from warm climates. Why? The ratio of surface area to body weight decreases as body weight increases, so a large body loses proportionately less heat than a small one. The song sparrow provides a good illustration of Bergman’s Rule: the subspecies in the Aleutians is a relative monster, although still not chicken-sized.
So the association between warmer climate and shrinking birds was intuitive. If natural selection had been favoring large-bodied individuals better able to survive the rigors of cold regions, warming would reduce the selection pressure. More runts would live to adulthood, breed, and pass along their small-body genes. They might even have an adaptive advantage over their large-bodied relatives, in which case the proportion of small-bodied individuals in the population would increase. It’s classic microevolution.
But why would birds in the Bay Area, which is warming along with the rest of the state, buck the trend? Remember that climate change isn’t just a matter of the mercury rising. It also involves an increase in the frequency of extreme weather events: winter storms, floods, droughts. Maybe larger-bodied birds are able to build up larger fat reserves to see them through extreme climatic events. Maybe change is increasing the nutritional value of the birds’ food, either plants or plant-eating insects.
That’s what the authors speculate, at least. Their article doesn’t address the wing-length increase at all. Maybe the trait is genetically linked with body mass. Maybe not.
It would be nice to know what species are represented in these data sets. That information, unfortunately, is in a spreadsheet that I couldn’t persuade UC’s computers to open. All I can tell you from the article itself is that the seasonal highs were 45 species in spring at Palomarin and 38 in spring at Coyote Creek. The extent of overlap is unknown.
I suspect the piece will fly below the radar of the climate change denialists. We shouldn’t be that surprised if not all the biological trends associated with global warming are in the same direction. Nobody ever said it would be simple.