Fifty birds flew home last month. Now, 50 may not seem like much and flight might not sound all that unusual for birds, but we're talking about the critically endangered rowi (Apteryx rowi), New Zealand's scarcest kiwi species and one of the world's rarest flightless birds.

Rowi have had a rough time in the wild. Invasive predators such as European stoats (Mustela erminea) and domesticated cats and dogs nearly wiped the species out in their sole habitat, the Ōkārito forest on New Zealand's South Island. The remaining birds lay about 80 eggs a year, only 40 of which hatch. Of those 40, four manage to survive predators long enough to reach six months of age. On average, just two wild rowi make it to adulthood every year. After that they are usually safe, but a 5 percent survival rate is not a good start.

That's where the New Zealand Department of Conservation (DOC) comes in. A few years ago the DOC started removing eggs from the wild, hatching them in a nearby nursery and then flying them hundreds of kilometers away to a secure facility where predators could not reach them. The point of the Operation Nest Egg program is less to hatch new birds than to let them grow up in safety until they are large enough to defend themselves. At about one kilogram, adult rowi are actually pretty tough and can easily counter predators with their sharp beaks and claws.

Operation Nest Egg has worked. In 1997 the rowi population was fewer than 200; by mid-2012 that number had grown to 375. The 50 one-year-old birds flown back to the Ōkārito sanctuary last month, plus three other birds released earlier this year, brings the population up to about 400 in their native forest, plus another few dozen that have been released on predator-free Mana Island. The DOC hopes to hit 600 birds by 2018.

The newly released rowi will not breed until they reach around four years of age. Until then many of them will be tracked via transmitters attached to their legs. The devices also help Operation Nest Egg locate the birds that do breed so that even more eggs can be taken in from the wild and saved, and this success story can continue to build.

Photo courtesy of the New Zealand Department of Conservation