Beagle Investigations Return with Darwinian Data

John recommended looking into the work of Robert Bowman and this is his section from the larger work about galapagos finches Notícias de Galápagos 1 (which would probably be helpful but most of the rest of the work is in Spanish) much of it is comparing the giospiza to other species of song birds, and goes into why the galapagos finch is the perfect subject for studying bird song in an evolutionary focus. 

This article is really nice because it connects song and beak morphology all in the realm of evolution. It's a quick read and good for beginners but also pretty interesting for anyone

Ecology and Evolution of Darwin's Finches by Peter R. Grant

Published by Princeton University Press 1999 

Overview of useful sections:

Chapter 9

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.

Chapter 14

Other Features

"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. 

The first article is a transition into Grants work concerning differentiation in bird songs. The article briefly discusses the allopatric divergence, and the interesting concept of song being a culturally inherited trait. Paragraph 22-4 delves into the song in relation to the hybridization of finch species due to song and how this revolves around the males. The end of paragraph 24 states "The barrier to gene exchange erected by song differences has been breached, and environmental change appears to have been the most important factor" It ends by explaining that the variety of finch songs likely has to do with both sexual selection along with a random shift in learned behavior. 

This article cited the works of Grant which are extremely relevant to the study of song and evolution however the publication is a full book and I could not track it down anywhere free so I will delve into the library when I get the chance.