Why might different species of finch exist on different islands?

This question refers to the different species of finch found on the Galapagos Islands. The reason for the existence of different species of finch on different islands can be thought of as an example of adaptive radiation.
Adaptive radiation is a process in which one species evolves into multiple species in order to fill the niche in a given geographical region. It has been hypothesized that the first finch arrived on the Galapagos Islands around two million years ago. Currently, there are fifteen species of finches (all evolved from one original single species of finch) differing from each other in terms of body size, beak shape, song, and feeding behavior. These variations can be thought of as specialized species best suitable to the conditions prevailing on different islands. In other words, each species of finch fills a specific niche.
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