When I was getting my biology degree, I was very much focused on ecology. Life on this planet is a complex, shimmering web of interaction and interdependency; pluck one strand, and the whole world vibrates. As organisms, we all affect both each other, and also the abiotic world around us. Some species, like humans, have bigger effects than others, but the reality is that you cannot study one organism without, at least in part, accounting for the others that live around it.

Later, when I was working as a curriculum developer, I wrote climate science lessons that viewed the issue through an ecological lens. See, while there was still mainstream “debate” over whether the planet was warming, wildlife around the globe was already actively responding to changes that a lot of people either didn’t notice, or were able to dismiss in their own minds. By focusing on ecosystems, we were able to show, over a decade ago, that the planet was warming, and that the effects of that warming were already measurable in the wildlife around us. I still think it was a good project, but the US public education system has little room to try new things, and is utterly clogged with testing. Add to that the difficulty in getting funds for this sort of work, and it’s hard not to feel like we were doomed from the start. That perspective on climate change, however, is still useful.

At the time, a lot of the research we were looking at related to changes in migration timing, ecological mismatch, and species range shifts. More recently, scientists have been tracking changes in body size and shape, driven by the warming of our planet. The latest example is a study showing that birds, at least in the Americas, are getting smaller, with longer wings:

The study combines data from two previously published papers that measured body-size and wing-length changes in a total of more than 86,000 bird specimens over four decades in North and South America. One study examined migrating birds killed after colliding with buildings in Chicago; the other looked at nonmigrating birds netted in the Amazon.

Though the two datasets are nonoverlapping in both species composition and geography, and the data were collected independently using different methods, the birds in both studies displayed similarly widespread declines in body size with concurrent increases in wing length.

Now, a new analysis of the combined data has revealed an even more striking pattern: In both studies, smaller bird species declined proportionately faster in body size and increased proportionately faster in wing length.

“The relationships between body size and rates of change are remarkably consistent across both datasets. However, the biological mechanism underlying the observed link between body size and rates of morphological change requires further investigation,” said U-M ornithologist Benjamin Winger, one of the study’s two senior authors, an assistant professor of ecology and evolutionary biology, and an assistant curator at the Museum of Zoology.

Both the Chicago and Amazonian studies attributed the reductions in species body size to increasing temperatures over the past 40 years, suggesting that body size may be an important determinant of species responses to climate change.

Birds hitting buildings is actually a serious problem, with around one billion killed every year in the US alone. It’s also helpful for science. I worked in a natural history museum in college, and part of my job was turning dead animals into study specimens. This was not taxidermy – that’s a whole art form in itself, and if I attempted it, my work would probably end up on Bad Taxidermy. No, what I did was skin them carefully (mostly birds), treat the skin, and stuff it with cotton wrapped around a wooden dowel, creating a sort of a preserved bird on a stick. These specimens are kept in drawers, so that they can be studied, and most of them came from people bringing in roadkill or window-killed birds. Natural history museums basically have libraries of dead plants and animals, along with data about them, that allow us to study the past, and compare it to what’s going on now. It’d be best if we could cut down on the death, but in the mean time, we might as well learn from it, right? I don’t miss the smells, though.

Getting back to the point of this post, the researchers also discussed the implications of the faster change in smaller birds:

It could be that smaller-bodied birds are adapting more quickly to evolutionary pressures. But the available data did not allow the U-M-led team to test whether the observed size shifts represent rapid evolutionary changes in response to natural selection.

“If natural selection plays a role in the patterns we observed, our results suggest that smaller bird species might be evolving faster because they experience stronger selection, are more responsive to selection, or both,” said co-senior author Brian Weeks, an evolutionary ecologist at the U-M School for Environment and Sustainability.

“Either way, body size appears to be a primary mediator of birds’ responses to contemporary climate change.”

So, if larger-bodied birds are responding more slowly to global change, what’s the prognosis for the coming decades, as temperatures continue to climb?

“Our results suggest that large body size could further exacerbate extinction risk by limiting the potential to adapt to rapid, ongoing anthropogenic change,” said study lead author Marketa Zimova, a former U-M Institute for Global Change Biology postdoctoral researcher now at Appalachian State University.

“In contrast, the body-size effect on evolutionary rates might increase persistence of small taxa if their rapidly changing morphology reflects a faster adaptive response to changing conditions.”

It’s important to remember the broader context in which this is happening. Specifically, the fact that bird populations are declining dramatically. It’s kind of neat to see natural selection in action like this, but it’s important to remember that this isn’t birds “changing their body sizes”, it’s the death or decreased reproductive success of birds that, in this case, are too big, or have wings that are too short. What I said about squid and lizards last October applies here as well:

When the average limb length of a Caribbean anole population changes, that doesn’t mean that we’ve got the same number of lizards and they all just have different legs. It means all of the ones with different proportions died. If you lay tens of thousands of eggs at a time, like the squid, then your population can probably bounce back pretty quickly if a few of you adapt to changing conditions. For those of us who reproduce more slowly, a drop in population like that means that it will take that much less to kill off everyone that remains.

If bird populations were stable as this change took place, then I don’t think there would be much cause for concern, but they’re not. They are adapting to climate change, but combined with habitat destruction, pollution, and pesticides, that may not be enough for many species. The world is changing around us, and every species on the planet is responding to it, ourselves included. Whether we are able to survive will depend on how quickly we adjust, and how much we do to slow down the warming. At the moment, it’s not looking good.

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