Because New Zealand is devoid of any native land mammals (except for the bat), the bird population has taken over many ecological niches that we are used to seeing filled by mammals. To fill these niches, many birds had to adapt by developing unusual characteristics such as "flightlessness, gigantism, low clutch sizes, criptique plumage patterns, and nocturnal behavior."
Most birds that show gigantism are herbivors. This is thought to be attributed to the low food value of plants. It is an advantage to be able to intake and process large amounts of food at once. A bigger size, therefore, means greater efficiency. Another advantage of being large is that "large animals use less energy per unit weight than small animals". Many large birds, such as the now extinct moa, the goose, the takahe, and the kakapo became flightless due to their increased size. This was only possible because New Zealand did not have any ground predators.The moa was a giant bird (up to 3m in height). It was part of the ratite family which also includes the kiwi, the ostrich, and the emu. This large bird resembled an ostrich, but had no wings. It was a herbivor and ate woody twigs, leaves, seeds of forest trees, and vines. It is thought to have become extinct about 400-500 years ago due to hunting (mostly by man) and destruction of their natural habitat.
There are also a number of smaller sized flightless birds such as the snipe rail, Finsch's duck. These birds were tempted into becoming flightless by the lack of ground predators.
Although native ground predators are lacking, many proficient avian predators can be found. These birds feasted on other birds, as well as insects, bats, amphibians, and reptiles. The presence of these avain predators caused the birds of prey to develope intricate camouflage patterns as well as to become nocturnal. Almost all of these predatory birds hunt during the day. The only exeption are the now extinct laughing owl and the owlet-nightjar.
One of the most impressive of the predatory birds was Haast's Eagle. This eagle, a forest dweller, was the largest and most powerful eagle in the world. The typical weight for a male eagle would have been 9-10 kilograms and 10-12 kilograms for females. The largest female weighed 9-10 kilograms and had a wing span of 2.8 meters. These eagles ate large birds, such as pigeons, geese, and even adult moas. In order to compture a moa, the eagle would perch itseld high on a branch and wait for it's prey to come within range. When this happened, it would dive at high speed and ram into the moa. The impact would knock the moa to the ground. The eagle would then use its 75mm talons to "crush and pierce the neck and skull of the immobilised prey". Because the eagle was the largest predator around, it could stay near it's catch for several days. Haast's Eagle became extinct several hundred years ago, probably due to the many enviromental changes due to man's presence.
The forest's of today are "quite, eerily so, and empty of animals compared to pre-human times". Many of the unique specialized specied were extremely vulnerable to the changes in it's ecology, and were all too easy prey to man, dogs, and rats.