Beaks change songs in Darwin's finches.

The 14 species of Galapagos finches that have inspired evolutionists since the days of Charles Darwin may reveal yet more. The birds may have evolved different courtship songs as byproducts of beak changes, suggests Jeffrey Podos of the University of Arizona in Tucson.

Standard theories of how species arise suggest such a possibility: Mating signals diversify as aftereffects of animals' adapting to different environments. The idea sounds good, but evidence to support it has been "limited," as Podos puts it.

Well-known work over 3 decades by Peter Grant and Rosemary Grant of Princeton University linked beak size to shifting food supplies. Droughts favor birds with big beaks, which can crunch through even the tough-to-eat leftovers of skimpy food supplies. When rainfall returns to normal and small-seed plants abound, however, the big-beak birds, with their bigger bodies, face a disadvantage because of their high metabolic requirements.

The Grants found that, depending on the weather patterns, beak sizes can shift a few percent from one generation to the next.

Podos analyzed vocal-tract physiology as well as the courtship songs of nine finch species. The ones with chunkier beaks repeated syllables more slowly and in a narrower set of tones than did species with daintier beaks, he reports. Podos' work, therefore, shows that the weather helps write the finches' songs