Biological invasions are not only events with substantial environmental and socioeconomic impacts but are also interesting natural experiments, allowing the study of phenomena such as the cultural evolution of bird song following introduction. We took an excellent opportunity to compare the distribution of dialects of the yellowhammer Emberiza citrinella, a small Eurasian passerine, in its native source region (Great Britain) and invaded range (New Zealand) more than hundred years after relocation. Recent field recordings (including those provided by volunteers within a citizen science project) were complemented by those from archives, each assigned to appropriate dialect by visual inspection of a sonogram, and the resulting spatial patterns of dialect distribution were interpreted using historical data on the yellowhammer invasion. The two countries differ markedly in the composition and distribution of dialects. New Zealand populations sing a greater number of different dialects, seven in total, five of which were not detected in the current British population, but have been reported by previous studies from the continental Europe. Two identified localities of capture (Brighton, Sussex, UK) and release (Dunedin, Otago, NZ) differ even more strikingly, having no dialects in common. The largely sedentary nature of yellowhammers allows for two mutually exclusive explanations for European dialects being detected in New Zealand but not in Great Britain: 1) the corresponding song types have emerged de novo in New Zealand, through convergent cultural evolution; 2) the dialects have disappeared from Great Britain, while being preserved in New Zealand. Indirect evidence from the widespread occurrence of these dialects in continental Europe and the reported stability of yellowhammer song, supports the latter explanation. We suggest that the yellowhammer dialect system is an avian equivalent of a phenomenon already noted in human languages, in which ancient words or structures are retained in expatriate communities.