I don't have specific details of the finches with me right now, so you might have to use your book for the exact details, but the specializations mostly have to do with their beaks.
Some of the finches developed long and narrow beaks to reach into tree trunks and eat bugs while other finches developed hard robust heads and beaks strong enough to crack nuts
The different adaptations of the finches allows for each one to fill its own niche.