Butterbutts: Yellow-rumped Warblers at MJC and the Tuolumne River show Evolution in Action

In the days of my ignorance (actually not a thing of the past), all small birds were sparrows of some kind. Imagine my surprise when I started paying attention and discovered the dozens of colorful and interesting birds that weren't sparrows (most of them). Among the most beautiful are the little Yellow-rumped Warblers (Setophaga coronata), apparently also known as...butterbutts.

These are pretty little birds right now, but in a few weeks they will take on even brighter plumage as the breeding season commences. I may not see them as often as they tend to move up into the forests at that time. Right now they are flitting about in the trees in great numbers on my campus (all the following pictures), and along the Tuolumne River below where I live in Waterford (the two pictures above).

There is an interesting story concerning these little birds. Until 1973, they weren't "Yellow-rumped Warblers". They were an eastern species called a Myrtle Warbler, and a western species called the Audubon's Warbler. There are color differences, and differences in their calls. But...in 1973 they were combined into a single species because of a narrow region where the range of the two species overlapped. In that narrow region the two species were observed interbreeding and producing young, a long-term definition of a biological species. So now they are officially considered to be subspecies along with two others, the Black-fronted Warbler in Mexico, and the Goldman's Warbler in an isolated part of Guatemala. They're all Butterbutts, for the time being.

But change is afoot. Genetic research is far more advanced than it was in the 1970s, and DNA evidence indeed suggests that the four butterbutts should be considered species rather than subspecies. Part of the argument involves that fact that while they hybridize in that small zone of range overlap, the area of interbreeding has not expanded over time, as if the hybrids were not really viable.

These birds are a great example of how natural selection and evolution works to change and develop new species. As with the Scrub Jays that I discussed the other day, isolation is the driving force leading to changes. During the ice ages, the original populations of the warblers were separated by glaciers, and they remained isolated in the aftermath. The populations were subjected to different environmental conditions and different food sources, and over time little differences developed. Some of the variations allowed particular individuals to survive and pass on their genes to subsequent generations, and over time, the differences accumulated to the extent that they could no longer successfully interbreed. In other words, they were new species.

Because of the new research results, the North American Classification Committee of the American Ornithologists' Union may very well declare the four subspecies to be species once more. It's arbitrary in one sense, but the fact that there are blurry lines between possible species actually supports the idea of evolutionary change. There are simply not always sharply defined boundaries as divergence takes place.

In any case, these beautiful birds may be right in your own backyard. In ours they stayed away from the feeder and hung out in the Mulberry trees around the corner of the house, so I never thought to search for them. Nowadays, knowing where to look, I see them all the time. Have I mentioned that they aren't sparrows?