Darwin's finches are different closely related species which Darwin discovered on the Galapagos Islands. Darwin's voyage on the "Beagle", and the finches in particular, are known to have influenced his thinking so that he would later produce a basic theory of evolution by natural selection. Darwin reasoned that there had to be a common ancestor. Later, extensive research was done by Peter and Rosemary Grant. The birds are all about the same size (10–20 cm). They mainly differ in the form of the beak. The beak is adapted to the food they eat. The birds are all brownish or black. They have short rounded wings and a rounded tail that often appears cocked to one side. Most male finch mature to a solid black color, while the females mature to a drab grayish color. Exceptions are made for the Vegetarian and Tree Finches the males never become completely black rather they have a black head, neck and upper breast. Warbler, Woodpecker and Mangrove Finches have more of an olive color.