Barn Swallows and Natural Selection

Barn swallows are the fighter planes of the bird world. Sleek, compact, and highly maneuverable, they perform impressive aerial acrobatics—mostly while on patrol for flying food, but sometimes, it seems, just for the sheer exhilarating fun of it.

A migratory species, our swallows depart each year long before winter, and return the following spring around tax time. I marvel that they journey thousands of miles in each direction, yet always manage to find their way back to our barn. They arrive on the scene with an excited chatter that seems to say: "We're back! Did you miss us?"

The swallows know who their enemies are. When our cat ventures into the yard, they mount wave after wave of harassing sorties, swooping down in daring solo dives, and pulling up—afterburners ablaze—mere inches from the cat's head. The cat feigns aloof indifference. Silly birds.

Barn swallows are a decidedly mixed blessing. Their prodigious consumption of flying insects helps control the local mosquito population, and for that I am grateful. But they're also prodigious defecators inside the barn, where they make their nests among the joists and rafters. Huddled inside those overhead nests, the fledgling chicks point their hinder ends over the nest edge and let loose on the barnscape below. I keep a large sheet of cardboard over my riding mower lest it become encrusted in swallow guano, but not everything inside the barn is so well protected.

Today, while operating that very mower, I conducted a short meditation on swallow nests and natural selection.

One might be excused for suspecting that swallows, when choosing nest sites, are ever mindful of the cat and other climbing predators (such as snakes). Indeed, swallow nests appear purposefully situated out of reach of even a determined cat. Does the bird-brained swallow engage in careful deliberation when deciding where to build his nest? Probably not.

Nor was the barn swallow endowed by some hypothetical creator (presumably after he created barns) with an instinctual ability to choose good nest sites.

I'd argue that the "instinct," if you wish to call it that, is the product of the same process of natural selection that has determined all of the swallow's morphological and behavioral characteristics. A simple example will illustrate.

Most swallow nests in our barn are, as I said, high up against the loft joists. They tend to be over a clear span, away from anything that a predator could climb to obtain access to the nest. Some protrusion from the joist, such as an electrical junction box or even just a nail, supports the nest, which is constructed of mud and grass and "glued" around the supporting protrusion.

This year, however, one hapless swallow built his nest on the tip of a stick that was leaning against the inside corner of two walls. The nest was sturdily constructed, but was only four feet off the ground. It was strange indeed to see a swallow nest that was actually below chest height. While there was still no way to climb to the nest, the cat could have jumped and pulled it down. Perhaps the cat didn't notice or didn't care.

Fortunately for the greater race of swallows, the dogs did notice and did care. It turns out that the dogs are every bit as interested in swallow chicks as you'd expect the cat to be. It was a simple matter for a golden retriever on hind legs to help herself to the nest and the chicks.

Now, suppose there is some configuration of genes that normally directs swallows to build their nests high and out of reach. That configuration came about by chance mutation, but since swallow nests so-built increase the probability that chicks will survive and reproduce, those genes are readily propagated to later generations.

Maybe those genes were not properly expressed in our unfortunate swallow, perhaps, again, due to chance mutation (genetic mutation is an exceedingly common occurrence).

We can easily see how the "normal" genetic expression we've come to expect in swallows tends to protect their chicks. And it's equally easy to see how a genetic misfire that leads to a poor nest site places the chicks at risk. In fact, genetic mutations that result in decreased survival tend to be rapidly extinguished from the gene pool.

In the parlance of evolutionary biology, the swallow's environment "selects for" nest sites that are high and out of reach by favoring genes that result in such sites. Since such sites are the ones that most reliably allow swallow chicks to survive and reproduce, genes which promote good nest sites will be more readily propagated to future generations than those that don't.

At least that's how it looks from my mower.

Copyright (C) 2007 James Michael Brennan, All Rights Reserved