Kiwis, under normal conditions, do not fly. But this week 20 young members of the rarest kiwi species were special guests on board a military helicopter, flying across the Tasman Sea on their way to their new habitat off the coast of New Zealand.

Rowi (formerly known as Okarito brown kiwi, Apteryx rowi) are in the middle of a rare conservation success story. The flightless birds were only recognized as a separate species in 1994, raising the total number of kiwi species to five. But all kiwi species are endangered, their populations severely threatened by dogs, cats and invasive species such as European stoats (Mustela erminea). Fewer than 200 rowi existed in 2007, but intense conservation efforts in the past few years have nearly doubled that to 375. The New Zealand Department of Conservation (DOC) hopes that this week's move will soon result in even more growth for the population.

The 20 juvenile rowi will now make their home on predator-free Mana Island, formerly a sheep and cattle farm, which the DOC and other groups have been restoring for more than 20 years. The tiny island—just over two square kilometers—will provide what the DOC calls an ideal breeding location relatively free of human interference. "We expect that the absence of predator pressure, better breeding conditions and less competition for territories will ensure that the Mana Island rowi produce a high number of chicks that can eventually become part of the home population in Okarito," Iain Graham, a ranger with Operation Nest Egg, said in a prepared statement.

Operation Nest Egg has been responsible for rearing these 20 juveniles since spring 2010, when their eggs were removed from the Okarito Forest on New Zealand's South Island, where the species remains at risk from stoats and other predators. Full-size rowi can defend themselves against stoats, but young birds cannot. The chicks were raised on predator-free Motuara Island, where Operation Nest Egg routinely raises juvenile birds until they are big enough to defend themselves, at which point they are released back into the wild.

Breeding won't start immediately—the birds released Tuesday range from 12 to 18 months of age, and rowi don't usually breed until they reach age four—nor is it guaranteed. "A full third of adult rowi do not breed—and of those that do, half the eggs left in the wild do not hatch," Kristina Ramstad, a postdoctoral fellow at Victoria University of Wellington, said in a release last month. She is studying the genomics of rowi and little spotted kiwi (Apteryx owenii), the two rarest kiwi species, as part of an effort to improve their mating potential, and suggests that the birds could be suffering from an inbreeding depression due to their limited population size.

But for a species with a population this small, any breeding is good breeding. The BNZ Save the Kiwi Trust aims to increase the number of rowi to 600 in the next few years.

Previously in Extinction Countdown:

Photos courtesy of the New Zealand Department of Conservation