The Beak of the Finch: Chapter 3 Summary & Analysis

Peter and Rosemary Grant were both interested in the variation of animals and plants from a very young age. Darwin, too, studied variation from a young age, focusing on a study involving barnacles in 1846. While a colony of barnacles might appear homogenous (or the same), Darwin found remarkable variations between his specimens. He became frustrated, wondering where he should draw the lines between different species. The study confirmed for Darwin that “there is no species barrier”—in other words, that one species can shade into another. Darwin assumed that this shading (or splitting) of species was a process too slow for him to observe. But in reality, it’s a process that can indeed be seen happening each day in places like the Galápagos.

As ornithologists studied Darwin’s finches, some split them into species—others lumped them together. Today, most taxonomists consider the 13 species of Galápagos finches to be a single family or subfamily, divided into four genera. In the first genus, the birds live in trees and eat fruit and bugs; in the second, they live in trees and are herbivores; in the third, the birds live in trees but look and act like warblers (another species of bird entirely); in the fourth, the birds live on the ground. Within the ground finches, there are three distinct species (G. magnirostris, the largest; G. fortis, the medium; and G. fuliginosa, the smallest). Within each of those species, there are variations in the finches’ beaks, but it is said at the Galápagos research center that only God and Peter Grant can tell the finches apart on sight. 

In the 1970s, Peter Grant had begun to wonder why some species of animals and plants were hypervariable (prone to lots of variation), while others weren’t. In order to study this phenomenon, he needed to observe a hypervariable species over a long period of time—and Darwin’s finches were ideal for his purposes. Peter and Rosemary first traveled to the islands in 1973, bringing along their young daughters. There, they found the birds to be extremely tame—the birds did not recognize humans as predators due to their isolation, and they would perch on the Grants’ shoulders or eat out of their hands. The finches, it seemed, were studying the Grants, too. 

That first year, the Grants hopped between seven different islands, meticulously measuring the finches. They found that while most species of birds elsewhere in the world have beaks that are so identical in shape and size that only four in 10,000 might have beaks that deviate from the mean, on the Galápagos, about four in 100 finches had beaks of different sizes. Even Darwin, the Grants quickly learned, had not realized how extremely variable his finches were.