New Zealand's native falcon isn't one species but two sub-species that live separately on the North Island and South Island.
That's been confirmed by new research that examined the body sizes and genetic makeup of the karearea and found strong evidence for two distinct sizes within the species, with the North Island form being smaller than the South Island one.
The Massey University scientists behind the just-published study say that knowing this will help prevent conservationists from accidentally breeding the two together in an effort to boost the karearea's low numbers.
The karearea, found in most of New Zealand south of Waikato and on our $20 notes, is the only surviving endemic raptor species in the country.
The two subspecies have been named Falco novaeseelandiae novaeseelandiae, in the South Island, and Falco novaeseelandiae ferox, for the smaller North Island form.
Professor Steven Trewick, who co-authored the study with postgraduate student and falcon enthusiast Lena Olley, said karearea vary considerably in size and colouration, over and above the differences between the males and females that are typical of raptors, and this variability has caused confusion since its earliest observation in the 1870s.
"Differences in size and other attributes among spatially separated populations could represent adaptation to local conditions and by recognising two distinct subspecies in karearea, we will be able to identify the patterns of diversity within the species and understand the distinct evolutionary ecology of each."
Karearea use many habitats including bush, coastline and estuary, open tussock land, farmland and exotic pine plantations throughout New Zealand, but are absent from the far North.
The southern falcons are associated more with open habitats, while northern falcons are strongly associated with native forests.
"Remarkably the boundary between the size clusters coincides closely with the Cook Strait, which is a geologically young feature of the New Zealand environment," Trewick said.
"This finding supports an informal conservation management strategy to avoid translocation and crossbreeding in captivity of falcons from the two islands."
The Department of Conservation estimates there are between 5000 and 8000 karearea left, but this number is uncertain.
Major threats have come from habitat change associated with expansion of pastureland but falcons do survive in some rural environments and make use of introduced bird species as prey.
Predation by introduced mammal pests including cats remains a problem, as is competition for resources.
Falcons have however known to fare well in exotic pine plantations, something that Massey researchers are now investigating.
The new study, which appeared in the journal IBIS, the International Journal of Avian Science this week, was supported by the Department of Conservation through assistance with sampling and a grant from the Taxonomic Units Fund.
New Zealand's native falcon, karearea
• Capable of flying at speeds over 100 km/h and catching prey larger than itself
• Found in most of New Zealand south of Waikato, along with some offshore islands including Auckland Islands
• Threatened by predation, habitat loss, disturbance, development impacts, human persecution, electrocution
• Now understood to consist of two subspecies living separately in the north and south islands