Peter R. Grant quotes:

Thus mating of females was strictly along the lines of paternal song.

Males transmit signals in courtship through behavioral displays.

Almost nothing is known from hybridization studies about the inheritance of courtship behavior of females, or of their responsiveness to particular male signals.

Plumage features constitute a major component of courtship signals.

To summarize, the particular song a male sings, and the behavioral responses of females to song and morphological signals, are not genetically inherited in a fixed manner but are determined by learning early in life.

Islands are known to differ in the food supply available to ground finches, mainly seeds.

Genes that underlie the capacity to receive, use and transmit information are the evolving properties.

Exchange of breeding individuals between two populations tends to homogenize their gene pools.

The independent role of morphology in mate choice is revealed by the rare instances where the usual association between song and morphology is disrupted.

Species can be recognized by their morphological characteristics and songs.

Closely related species of birds are also chromosomally similar.

We observe closely related species in sympatry and infer how they evolved from a common ancestor.

The divergence of songs in the new population away from those in the progenitor population would only be prevented if these processes were balanced by repeated immigration and subsequent breeding: song flow.

Thus the genetic basis to the origin of bird species is to be sought in the inheritance of adult traits that are subject to natural and sexual selection.

The theory of founder effects does not explain how novel features like plumage traits arise.

The process of speciation is completed with the cessation of genetic exchange.