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.