Haast's eagle, or Harpagornis moorei is known to us only from unearthed remains and markings left on the bones of prey, but the picture that emerges is quite impressive. Before seafaring Polynesians discovered a wayward fragment of Gondwanaland—now called New Zealand—there was an isolated ecology of birds, where most niches we associate with mammals had an avian counterpart.
The top predator was big enough to take down a 3 meter moa- the tallest of all known birds. Raptors normally hunt prey that they can carry, to avoid competition with other predators. But Harpagornis's only pre-Maori rival, pre-1250CE was a reptile, the tuatara, a lightweight at only a twentieth the eagle's estimated 10-13 kg. Too heavy to soar, with a measly 3 meter wingspan, the Harpagonis, the largest eagle known to exist, is thought to have navigated within the precolonial forest by flapping, and swooping on prey from high branches at up to 80 km/h.
Talons, a fearsome 6 cm long, are comparable to those of a tiger or a leopard, left extensive damage on moa bones found in swamps. Patterns on the bone suggest that that the eagles would land on the backs of the moa and then puncture internal organs, the beak shape suggesting deep evisceration, similar to a vulture's style.
Evidence for the eagle disappears around the 1500's where soil studies show the Maori had modified the landscape with fire and the youngest bones have not been found in more recent soil horizons, though there are always unconfirmed sightings, much as there are for the now extinct Thylacine. Another theory for its extinction relates to its taste for bipeds. It is conceivable that the early Polynesians were preyed upon by the eagle, though there is as yet, no supporting signature on human remains from the period. It can be assumed that the eagles were actively hunted by the Maori to reduce competition for the moa. Artifacts made from eagle bone indicate at least some reciprocal predation.
Extinction was inevitable once the primary lowland forest habitat of the South Island's eastern/Canterbury plains was cleared. It was the destruction of a contracting range since the last glacial retreat and the major prey—the extensive moa fauna were themselves hunted to extinction. Exact dates for extinction are uncertain but by the time European discovers arrived in New Zealand in the late 1700's the giant eagle was, at best, a thing of legend. Rock paintings have also been found that show a eagle-like bird and birdman.
The first evidence for the bird was found in 1871 in a swamp in Canterbury and described in 1872 by Dr Julius von Haast, who gave its Latin name and as an early New Zealand explorer and naturalist is commemorated in having his name connected with many things of geographical and biological significance.
Gill, Brian. New Zealand's extinct birds, Auckland, 1991
Hutching, Gerard. The natural world of New Zealand: an illustrated encyclopaedia of New Zealand's natural heritage, Auckland, 1998
Worthy, Trevor H. The lost world of the moa: prehistoric life of New Zealand, Christchurch, 2002
Moas - Lost Giants of New Zealand, Harper/Collins, New Zealand
http://www.vuw.ac.nz/lals/nzdc/NZWords%20no4.pdf.