In comparison with the nearest land mass, Australia, New Zealand has relatively few species of land birds. For instance, morepork and laughing owl are our only owls (eight species of owl in Australia); we have only two hawks (24 hawks in Australia); one kingfisher (10 in Australia); and so on. The explanation of a fauna so limited – a characteristic of nearly all the forms of terrestrial life found in New Zealand besides the land birds – must lie in the long isolation of New Zealand from other lands. According to geological evidence New Zealand could not possibly have been reached by land since the Cretaceous Period approximately 70 million years ago. Amongst the most remarkable of New Zealand land birds are the moas and kiwis, and these must be of extremely ancient lineage, for when their ancestors arrived in New Zealand they were wingless. It is also probable that certain of the unique forest birds, like the moas and kiwis, were very early arrivals, but they came by flight and not along a land route. These are the families comprising the rifleman and other New Zealand wrens, the New Zealand “wattle birds” (huia, saddleback, and kokako), and the native thrush. These three families apparently have no close relatives in the world today.

It seems probable that the remaining members of the bird fauna arrived at intervals across the dividing seas by the process of aerial colonisation; bats, too, colonised after the country was isolated, although the last land link was severed too early to enable New Zealand to receive an invasion of non-flying mammals. Many New Zealand birds are only slightly different from members of the Australian bird fauna (e.g., fantail) and must therefore have arrived from Australia in comparatively recent geological times. The considerable number of Australian species recorded as stragglers in New Zealand during the past hundred years gives evidence in support of this hypothesis; it can easily be imagined that over long periods of time numbers of stragglers arrived, and some of these remained to become permanent members of the fauna. Within historic times there are records of seven such stragglers that have begun to breed in New Zealand – white-faced heron, royal spoonbill, grey teal, Australian coot, spur-winged plover, welcome swallow, and silvereye.

The bird fauna includes a number of remarkable species that have evidently been much longer in New Zealand and have only remote relatives in Australia or to the north (eastern Asia-Melanesia), e.g., wrybill, weka, and takahe. This group of older colonists includes flightless species, and these, unlike the moas and kiwis, must have developed this feature after their arrival.

The sea birds include some – mainly the essentially coastal gulls, terns, gannets, and certain shags – which, like the land birds, provide a comparatively recent link with Australia. But New Zealand's rich sea-bird fauna includes two groups, the penguins and petrels, which apparently originated in the south.