Tree finches, as their name implies, are largely arboreal and feed primarily on insects. They are paler than ground or cactus finches, with gray heads and wings and white or streaked breasts. Their bill shape is sharper than ground finches, and more useful for grasping -- better suited to their insect diet. 

Small tree finch

(Geospiza parvula) lives on all the major islands except Española, Genovesa, Marchena and Darwin

Large tree finch

(R. psittacula) is also rare or unknown on Wolf, Santa Fé, Baltra, and Pinzón and Rábida. Of note is that it is rarely found on the large, hospitable island of Floreana.

Medium tree finch

(G. pauper), which is as you might suspect mid-way in size between the other tree finches. However, its range is restricted only to Floreana Island, and some believe it may be a hybrid of the Large and Small tree finches found here as well. As noted, the Large tree finch is not a frequent resident of Floreana, and if the Medium species is a hybrid, it may have replaced the Large in habitat.