Found the book!!
Ecology and Evolution of Darwin's Finches by Peter R. Grant
Published by Princeton University Press 1999
Overview of useful sections:
Song and Bill Morphology as Species Cues
Song is the way of expressing ones species from over a large distance, which beak shape is how it would be expressed in the short distance
Beyond Species-Recognition: Mate Choice
In this section, Grant reports on 14 females he watched, after observing the songs of their fathers. Though statistically speaking 2-3 of the females should have mated with males that shared their fathers song if it were random, not a single female did. This implies that song could act as a prevention method against inbreeding to keep the gene pool healthy.
"The distribution of sound energy in songs varies in such a way that it might be transmitted best in the particular configuration of vegetation in which the song is sung." In other words, the plants in an area have a large effect on the songs of the finches that live there.