The chaffinch, or, Fringilla coelebs, is one of the most common birds in Europe, but it is not a homely one. Blue-capped, red-breasted, and black-white feathered, the petite male bird looks as if it was painted using a paint-by-number set – except for the Hamburgler-like dark band across its eyes. The female bird is duller in color than her male counterpart, as is common in most species in which the female is burdened with chick rearing and so must be choosy about which male is good enough, or colorful enough, to father her young.
The chaffinch’s song, often transliterated as “susk-WEET,” is a species-unique tune. Just male chaffinches sing it, calling to their Juliets to come to the balcony, or the bush, or the park bench, or wherever. The bantam Juliets then recognize the singing as from one of their own species’ Romeos, instead of one of the dozens of other species tweeting garbled prose.
“It’s a long-distance mating signal,” says Dr. Lachlan. “The song is often the first point of contact for a female looking for her mate and her mate.”
For years, sensitive listeners had suspected that chaffinches on the European mainland and chaffinches out on the islands did not sing their melodies quite alike. But just what that difference in song was had been unknown, says Lachlan. Unknown, too, was if this seemed bird accent was happenstance, a chance aberration in the bird’s song each time they arrived at a new island. Or, was there a pattern to how the birds appended their song each time they relocated?
What if something more than chance was driving the changes in the birds’ song?
Beginning what was to be 15 years of research, Lachlan recorded the songs of 723 male birds in 12 different populations from the European mainland, the Azores, and Canary Islands. He then used an algorithm to divide the bird’s song into syllables. Next, he used another algorithm to assess the randomness of each bird’s song: After the bird sang syllable “A,” for example, how predictable was the next syllable?