Valley Girls Know What They Want: Sexual Selection and Elevation in Finches

Each year as spring comes, opening your window will invite in a cacophony of ebullience from the male songbirds in your neighborhood. They find a good spot, puff out their chests, and sing their hearts out to any ladies of their species who might be listening. The females then choose the male with the best song, because he is more likely to be healthy and able to pass on good genes to his offspring. But odds are you live in an area that’s relatively low in elevation, unless somehow this blog’s gotten picked up by an Alpine sheepherder with a satellite internet connection. If you were, however, to travel up to very high elevations one spring to hike in alpine meadows or pine forests, you may notice that you’re not getting nearly as much of a show from the local bird populations. They don’t sing with as much gusto or showiness, even though they may look very much like the birds from back home. So what gives?

Emilie Snell-Rood, from the University of Arizona, and Alexander Badyaev, from Indiana University, did notice this. In a new paper in Oecologia [157(3):545-551] they are the first to prove this variation is a function of elevation. As their study group they chose a large group of finches, called the Cardueline finches, which contains 126 species with the widest elevational range of any living subfamily of birds in the world. Many of the finches that live at lower elevations have very close relatives at higher elevations, which sets up a perfect study system to test for elevation-based differences between species.

In their study Snell-Rood and Badyaev used sonographs to get a “picture” of each species’ song. With this they were then able to quantify different aspects of the songs’ showiness, for example number of notes, length, pitch range, etc. They then analyzed the relationship between these variables and the maximum elevation of the birds’ range. They were able to control for other factors that are known to vary with song complexity and might have otherwise confounded their results, such as body size, bill size, and habitat type. Once these confounding factors were controlled for, any existing differences between species’ songs would be more likely to be due only to elevational distances. They did find, in fact, that between “sister species,” the species with the lower breeding territory was more likely to sing elaborate, loud courtship songs than their counterparts higher up the mountain. Lower-elevation species also had longer songs with more notes. For an example of the kind of showiness in these carduelines, check out the song of the pine grosbeak, one of their lower-elevation species. Although I can’t find a file online of the song of its sister species—the crimson-browed finch—to use as a comparison, you can clearly hear the complexity and length of the pine grosbeak’s elaborate song.

The interesting thing about this variation in song complexity with elevation is that it correlates to differences in the breeding behavior and family lifestyles of these birds. The cardueline finches at lower elevation need more elaborate songs because there is more sexual selection in these species—the females spend more time evaluating and choosing mates, who are more showy in song and in plumage than the females. This is because, for these lower-elevation birds, the males don’t always really do much to help raise the offspring. Any contribution they will make to the next generation will be through good genes, which must be evaluated at the offset by the female through some proxy like song or plumage brightness. At higher elevations, a far more important predictor of offspring success is the amount of help it receives from its parents. Since food is much more scarce farther up in the mountains, both parents are required to forage for food to feed the baby birds. The male’s success lies in his ability to care for his young, not in convincing a female that he’s worthy of a copulation with her. He then does not need to invest as much in elaborate, difficult songs as his polygamous counterparts down in the valleys.

Next time you hear an effluence of birdsong through your window, or up on a hike in the mountains, try to pick all the songs apart as you listen and analyze them. As in most things in nature, there is more going on than it seems.