Darwin's finches are a group of 14 related finch species (subfamily Geospizinae, family Fringillidae) discovered by Charles Darwin on the Galápagos Islands, each slightly different from the others and none similar to the finches of North America. Seeing this diversity in one small, related group of birds, Darwin wondered whether they were descended from a single early species. These observations helped him formulate the theory of evolution. Darwin's finches differ from one another primarily in shape and size of the bill. Males of most species are at least partly black. The grayish brown females usually lay four eggs in domed nests built in cacti and shrubs. The woodpecker finch, Camarhynchus pallidus, uses cactus spines to extract larvae from cacti and trees.