The native falcon karearea appears to be separated into two subspecies - one in the North Island and the other in the South, new research suggests.

"We found that North Island karearea were significantly smaller than South Island karearea, and despite some variance that is to be expected from natural variation and measurement error, the abrupt difference in size was striking," the researchers said.

There was an association between low producing grassland and larger karearea, and indigenous forest and smaller karearea.

Barry Harcourt A swamp harrier, also known as the harrier hawk.

A larger proportion of the South Island was above the tree line, with the dry eastern area mostly lacking tall forest today, the study, published in IBIS, the International Journal of Avian Science, said.

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The paper was written by Massey University Professor Steven Trewick and postgraduate student and falcon enthusiast Lena Olley.

Robert Charles/Fairfax NZ A morepork, or ruru.

For at least the past 5 million years the Southern Alps had generated a rain-shadow to the east of the South Island, supporting dry forest and high country grasslands, the paper said.

Natural fires or those set by early human settlers resulted in the loss of forest generally in the east and regeneration was slower than in the wetter west.

In the North Island the rain-shadow was less pronounced and a smaller proportion of the landscape was above the tree line. Smaller falcons may be at an advantage in forested habitat.

Breeding success in falcons was strongly influenced by the quality of the birds, so evolutionary shifts were likely to be rapid.

Karearea use many habitats including bush, coastline and estuary, open tussock land, farmland and exotic pine plantations, but are absent from the Far North.

The Department of Conservation estimates there are between 5000 and 8000 karearea left, but that number is uncertain.

Trewick, from Massey's Institute of Agriculture and Environment, said differences in size and in other attributes between populations living in different areas could represent adaptation to local conditions.

"Remarkably the boundary between the size clusters coincides closely with the Cook Strait, which is a geologically young feature of the New Zealand environment," he said.

"This finding supports an informal conservation management strategy to avoid translocation and crossbreeding in captivity of falcons from the two islands."

The karearea is the only endemic New Zealand bird of prey still surviving. Three other endemic species, Eyles' harrier, Haast's eagle and the laughing owl became extinct following human colonisation, the research paper said.

Two native raptors shared with Australia, the swamp harrier or kahu and the ruru had fared better.

The swamp harrier, which appeared to have become established in New Zealand after human settlement and after the extinction of Eyles' harrier and Haast's eagle, was one of the few native New Zealand birds to have increased in abundance since human colonisation.

The closest living relative of karearea is the South American aplomado falcon.