Invasion history preserved in a bird song: Is Yellowhammer memory more reliable than ours?
Yellowhammer (Emberiza citrinella) is a small passerine, widely distributed across Eurasia. In the 1870s it was introduced from Britain to New Zealand to sort out the problem with overpopulated insect pests. However, as this bird is rather granivorous, it became a nuisance itself. From the same period we have the first notion about the geographical variation of yellowhammer song. Since then, substantial research has been done on Yellowhammer dialects. Surprisingly, only 11 dialect types were described from the whole native range. The dialects can persist on a site for decades and thus potentially store some information about the past. New Zealand Yellowhammers have been isolated from their home country for 140 years, whilst acclimatisation societies kept records about shipments and liberations of these birds. Thus, it is possible to compare the “written” and “oral” history and see to what extent they match.
The presented project consisted of three steps. First, using documents of acclimatisation societies and newspapers we reconstructed the history of yellowhammer colonization of New Zealand. Second, with the help of volunteers we mapped the distribution of yellowhammer dialects in Britain and New Zealand. Finally, these two approaches were combined to get insights into the process of Yellowhammer invasion.
Using old documents of acclimatisation societies, such as annual reports, letters, cashbooks and minutes, and old newspapers we succeeded in reconstructing the history with surprising precision. It was possible to track down 25 ships with Yellowhammers on board, which departed from London towards 6 different ports in New Zealand. The localities of release are also either known, or can be limited to several candidates. We also identified one particular region in Britain that was the main source of Yellowhammers for South Island.
Analyses of recordings obtained by various means - citizen-science project (Yellowhammer Dialects, http://yellowhammers.net), sound archives and online storages, and our own field work – revealed surprising results. Not only seems New Zealand richer than Britain in terms of the number of dialects detected (7 versus 5), but four of those found in the invaded range are nowadays present in continental Europe but were not found in Britain.
The written and “oral” histories do not have to be in conflict, though. Several phenomena will be discussed that could explain the observed patterns, including population decline, independent cultural evolution, and sampling bias.