One species was probably hunted to extinction by the first settlers.
Worldwide there are six species of swan (in the genus Cygnus). In Europe, you mostly encounter white swans, such as the Mute Swan (C. olor). But occasionally, a black swan may swim of fly by. This species – conveniently called the Black Swan (C. atratus) – naturally occurs in Australia and New-Zealand and has been introduced in other parts of the world. In fact, it is the only native swan species Down Under. But that used to be different. Scientists studied fossils from the period 1280-1880 and concluded that there used to be two species of Black Swan in Australia and New-Zealand.
Nicolas Rawlence (Otago University) and his colleages succeeded in extraction DNA from these fossils. The genetic material they obtained was vastly different from the current Black Swan population. It was even so different that the researchers concluded that they were dealing with another species. This new – but extinct – species was named Poūwa after Maori legend (the Maori are the indigenous population on New-Zealand).
The extinct species is also morphologically different. On average, these birds were bigger and heavier compared to their extant relatives. If you manage to put a Black Swan on a scale (something I wouldn’t try in the breeding season), the balance will read four to nine kilograms. The Poūwa is a bit heavier, between six and ten kilograms. Moreover, it also had shorter wings and longer legs. This suggests they it might have been on its way to flightlessness. Losing the ability to fly is a common phenomenon on islands. Think of the Flightless Cormorant (Phalacrocorax harrisi) on the Galapagos Islands, with its tiny wings it cannot take to the skies.
How did the Poūwa go extinct? When Europeans arrived on New-Zealand in the eighteenth century, there were no swans. Probably, the Poūwa was hunted by early settlers that reached New-Zealand from Polynesia around 1280. Remains of Black Swans in archaeological sites suggests they were on the menu. So, in the eighteenth century there were no swans on the islands, but now there are. What happened in the meanwhile? In 1860, Europeans introduced Black Swans on New-Zealand although some birds might have flown over from Australia at their own pace.
Another recent study on these Black Swans suggested that some Poūwa’s survived and interbred with the newly introduced Black Swans from Australia. However, this study – by Valeria Montano and colleagues – is based on microsatellites and might lack the statistical power to provide strong evidence for this scenario. For more information on this debate check out the correspondence following the publication of the ancient DNA study (see here and here).
Rawlence, N.J. et al. (2017) Ancient DNA and morphometric analysis reveal extinction and replacement of New Zealands’ unique black swans. Proceedings of the Royal Society B. 284: 20170876.
Montano, V. et al. (2018) A genetic assessment of the human‐facilitated colonization history of black swans in Australia and New Zealand. Evolutionary Applications. 11(3), 364-375.