How do small-tree finches illustrate adaptive radiation?

Frank W is correct. Using the Galapagos finches as an example, there are now at least 13 species on the islands, each filling a different niche on different islands. All of them evolved from one ancestral species - a ground-dwelling, seed-eating finch from the mainland of Central or South America, which colonized the islands only a few million years ago. From this one migrant species, finch species have become adapted for different diets: seeds, insects, flowers, the blood of seabirds, and leaves. This process, whereby species evolve rapidly to exploit empty ecospace, is known as adaptive radiation.