Home & Garden Columns

The Bluebird of Hostility: Getting an Evolutionary Edge

By Joe Eaton, Special to the Planet
Tuesday June 06, 2006

Unless you’ve been living in a cave since 1979, you have undoubtedly seen the Mad Bluebird. It was captured by aspiring wildlife photographer Michael L. Smith on a cold February day in Maryland. The subject, a male eastern bluebird, feathers fluffed out, sits on a fence post glowering at the camera. The Mad Bluebird has been very good to Smith, enabling him to quit his day job as an electrician. Over 100,000 signed prints have been sold, and the image appears on calendars, coffee mugs, and all kinds of tchatchkes. The royalties by now must be considerable.  

That bluebird’s actual emotional state at the time is, of course, open to conjecture. But the image came to mind recently when I read about a really ingenious study of our own local species, the western bluebird, that appears to demonstrate a connection between the evolution of behavioral traits—in this case, aggressiveness—and physical characteristics.  

What scientists mean when they talk about evolution depends on the scale of the process. Macroevolution is what drives the dramatic changes that cross major taxonomic boundaries: fish into four-limbed amphibian, feathered dinosaur into bird, hoofed land mammal into whale, ape into hominid. Microevolution is more subtle. It’s what Peter and Rosemary Grant spent years studying in the Darwin’s finches of the Galapagos, as chronicled in Jonathan Weiner’s book The Beak of the Finch: incremental changes in the size and strength of the bird’s beaks, tracking the vagaries of climate—El Niños and La Niñas—that determined the kinds of seeds that were available for food.  

Give it long enough, and microevolution can produce a new species. You can imagine a scenario in which a population’s lifestyle becomes so specialized that it no longer interacts with its parent stock and becomes reproductively isolated. But it’s just as likely to act as a stabilizing force, with small changes varying around a long-term norm. As the Grants found, incipient species can begin to diverge, then merge back if the environmental forcing conditions reverse themselves. 

How does all this apply to western bluebirds? Renee Duckworth, an evolutionary ecologist at Duke University—and, as a loyal North Carolina alumnus, it pains me to admit that anything good can come out of Duke—did her field work in Montana. She found that bluebirds varied in aggressiveness, although I’m not sure how that was scored. (And yes, I’ve seen bluebirds being aggressive; not long ago I watched one chasing an interloping house wren away from its nest tree). The more aggressive birds seemed to get the choicest territories, in open meadows. Those lower in aggressiveness made do with closed forest areas. 

Those two environments make different physical demands on a foraging bluebird. In meadows, bluebirds hover above the grass to snag insects; in forests, they glean bugs and berries among the branches of trees. Duckworth measured the two populations and discovered that the among the aggressive meadow-nesting birds, individuals with longer wings and tails—better suited for hover-foraging—succeeded in raising more offspring than their shorter-winged-and-tailed neighbors. 

In Darwinian terms, the longer-winged birds were more fit than the others. Evolutionary fitness isn’t just about personal survival—that would make it the tautology that creationists claim it is. It’s about how many copies of your own genes you leave in the world. If more aggressive, longer-winged bluebirds have more offspring, those traits will increase in frequency within the meadow-nesting population. (Wing and tail proportions seemed to make no difference for the nestling-survival rates of the forest-nesters). 

So, according to Duckworth, aggressiveness drives habitat choice, which affects physical proportions. Could this process ultimately turn meadow-nesting and forest-nesting bluebirds into different species? Not likely, because neither habitat is stable over the long term: forest fires keep shaking up the mix. The isolation that is a key part of the speciation process is only temporary. 

It’s an intriguing set of findings: a salutary reminder that behavior evolves too, and that differences in behavior can translate into physical differences. You have to wonder how much our own evolution owes to some remote ancestor having been bolder, or more curious, or more socially-skilled, or just plain meaner than the competition.