Founder effect and cultural evolution of songs in an isolated population of chaffinches, Fringilla coelebs, in the Chatham Islands

Abstract

English chaffinches introduced by humans to New Zealand last century have, subsequently colonized the Chatham Islands about 800 km to the east. Cultural evolution in songs of the Chatham Island population was investigated by comparing a sample of 30 song, types with six equivalent samples from sites in New Zealand. Statistical analysis of 13 variables recorded on each song type showed that the basic structure of song has not changed significantly since chaffinches colonized the Chathams around 1900. Unlike the New Zealand birds, the Chatham birds have not developed, elaborate end phrases to their songs. Comparisons within and among syllable pools at each site suggest that the processes of cultural transmission and cultural mutation have been unaffected by colonization history. The major finding of the study is that the syllable pool in the isolated Chatham population is depauperate relative to those in the New Zealand population. The Chatham island songs, however, have not diverged greatly because 18 of the 20 Chatham syllable types are also present in New Zealand. It is concluded that the Chatham Islands were colonized by a small number of founders who had already learnt their syllables and songs from adult conspecifics in New Zealand. This appears to be the first evidence of the effect of a bottleneck of small population sizes in reducing the diversity of the syllable pool in an isolated population.