I wrote about the nene a few months ago, and the following is an abridged version of that post.
Perhaps 500,000 years ago, a pregnant Canada Goose, or a small flock of geese took a wrong turn. A really wrong turn. Somehow, flying along the coast of North America, the leader maybe decided to take a short cut, and it went poorly. Soon the shoreline was lost to view, and for days, not a single bit of land could be seen. I can imagine the flock becoming smaller and smaller as the weaker members fell behind and were lost. After days of nothing but ocean, an island came into view. The tired geese landed and began looking for food. None of the plants were particularly familiar, but some proved edible and a few of the geese survived. For all we know, some of them rested up and left for further journeys, but a few stayed around. They took up residence on the volcanic island in the tropics of the Pacific Ocean, a vastly different environment than their summer home in the Canadian Arctic. The isolated volcanic edifices are today called the Hawaiian Islands.
During the millennia that followed, different flocks of geese took up
residence in different ecological niches, such as lava flows, grassy
slopes, and woodlands. Isolated groups began to diverge, and before long
at least three species developed on the islands. One of them, the Giant
Hawaiian Goose (species name not yet established), was big, almost four feet long. A lack of predators on the island made flight an
energy-intensive but unneeded luxury, so the wings became smaller as the
bird evolved to larger size. The goose was flightless. This worked fine
for a long time, but humans ultimately arrived on the islands, and
probably exterminated them, either by hunting, or by introduction of
egg-eating rats, or by habitat destruction.
The second species was the nene-nui (Branta hylobadistes).
It was also a big bird, but had larger wings so that it was still
capable of weak flight. Like the Giant Hawaiian Goose, it became extinct
soon after the arrival of humans on the islands. It has been
characterized as a bird in evolutionary transition, in the midst of
losing the ability to fly, but it went extinct first. Possibly because it couldn't fly well.
The flying goose, the one that survived was the nene (Branta sandvicensis). The nene's wings were smaller than its Canadian ancestors, because it didn't need to migrate thousands of miles during the change of seasons. But it retained enough flying ability to escape from the island's human invaders. It differed from the Canada Goose in several other respects, including longer legs, reduced webbing between its toes, and a more erect posture. It is adapted to living on lava flows and grasslands, and was less dependent on wetlands than other geese.
It survived, but no longer thrived. Rats ate their eggs and chicks, and later on cats and mongooses were brought to the islands. The species was devastated, dropping from an estimated 25,000 when first European contact was made in 1778 to thirty individuals in 1952. A captive breeding program began, and populations were established on their ancient habitats on Maui and Kauai (where mongooses were never introduced). The wild population is now around 1,950 with another thousand or so in captivity. They are still highly endangered, but their prospects are slowly improving. They have been named the state bird of Hawaii.