The ins and outs of acclimatisation: Imports versus translocations of skylarks and starlings in 19th century New Zealand

New Zealand is home to around 40 alien bird species, but about 80 more were introduced in the 19th century and failed to establish. As most of these introductions were deliberate and documented in detail by the acclimatisation societies responsible for them, New Zealand bird invasions are often used as a model system to unravel what determines the outcome of introduction events, especially the role of propagule pressure. However, the credibility of these data was challenged recently, as different authors have reported different numbers of liberated birds. This discrepancy has several causes. Using introductions of Eurasian skylark (Alauda arvensis) and Common starling (Sturnus vulgaris) as examples, we show that the most important issue is that not all liberated birds were imported from overseas, and so import records underestimate the total propagule pressure for particular regions. There is evidence for the import to New Zealand from overseas of 361 skylarks and 619 starlings, versus at least 1491 and 1678 individuals, respectively, being translocated to other regions within the country. The majority of liberated birds in some regions of New Zealand were translocations from other parts of the country where the species had already previously established. Nelson was the main source of translocated skylarks, while Otago was the main source of translocated starlings. Canterbury, Hawke’s Bay and Wellington were the main recipient regions for these translocations. Our findings may have implications for analyses of propagule pressure, as well for studies of population genetics and spread of alien birds in New Zealand