Evolution of Beaks Gives Rise to New Love Songs, Species Diversity in Darwin's Finches
Writer: Margrit McIntosh
UA dept. of ecology and evolutionary biology
When birds' beaks change to match new foods, their songs also change, potentially affecting their ability to recognize mates, according to new research.
In the 11 January issue of Nature, Jeffrey Podos describes how changes in the shape and size of the beaks of Darwin's finches, driven by fluctuations in which foods are available, also affect how well they sing.
Podos conducted the study while a post-doctoral researcher at University of Arizona 1998 - 2000. He just returned from field work in Ecuador to his faculty position at the University of Massachusetts.
Podos found that large, thick beaks designed for crushing hard seeds are not as "tuneful" as slender beaks adapted for picking up insects. Because many birds rely on each others - songs as a way of identifying suitable partners, changes in song may affect their ability to recognize potential mates, which in turn could lead to evolution of new species.
Biologists have long recognized the importance of courtship signals in determining who mates with whom, potentially affecting whether a species splits into two or remains as one. But until now it has not been clear how exactly a changing environment could affect these courtship signals. The key that Podos found to unlocking this mystery is the beak of the bird, which is used in two very different jobs: feeding and singing. Changes in the food environment do not directly cause new bird songs, but beaks that evolve in response to changing foods can't necessarily make the same sounds they used to.
In particular, a bird with a large, thick seed-crushing beak can't make the rapid and precise beak movements that a bird with a slender beak can. Building on previous research that showed how beak shape and size affects bird song, Podos devised an ingenious way to measure the "skill" of a song.
A song's trill rate (syllables per second) and frequency bandwidth (range of high and low tones) are inversely related -- as one goes up, the other goes down. But within this limitation, some birds are able to create songs that maximize both qualities. Podos measured vocal performance by how far a particular song deviates from this maximum. By measuring beaks and recording songs of different finch species, and different individuals within a species, Podos was able to show that as beaks grow larger, song performance worsens.
In keeping with the theme of the UA program under which Podos did his research, an NSF Research Training Grant in the Analysis of Biological Diversification, Podos was able to incorporate the degree of relatedness among the species he studied into his results. In the past, researchers comparing species treated each individual species as a separate entity. But because species are interrelated, their characteristics are not independent of each other. Using a method called independent contrasts analysis, Podos tracked the relationship between beak size and vocal skill along the branches of the evolutionary tree of the finches, strengthening the validity of his results.
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