Eighteen years ago, the puaiohi was a diminishing breed in the Alaka'i Wilderness Preserve, the 10,800-acre rain forest on Kaua'i.

But the endangered, brown birds that can easily fit in the palm of an adult's hand are making a comeback with the assistance of scientists who are repopulating the preserve with a once-plentiful species.

Yesterday marked the 11th time in the last decade that juvenile puaiohi have been introduced into the preserve, an effort that those guiding the project say is showing success. A total of 23 youngsters who were born and raised at the San Diego Zoo are to be let go in two groups over the next two weeks.

"The superstars in this conservation effort have been the puaiohi themselves," said Alan Lieberman, the zoo's conservation program manager.

The puaiohi, which live on the floor and in the low bushes of the wilderness preserve and sport a white ring around their eyes, are considered endangered, as are 26 other endemic forest birds in the state. Another 21 such species are already extinct.

For the last decade, scientists from the zoo, the state Division of Forestry and Wildlife and the U.S. Geological Survey have been working to introduce more puaiohi to the preserve. Only 200 were estimated to live there in 1994.

Under the program, puaiohi eggs are periodically removed from the forest and sent to the zoo's Keauhou Bird Conservation Center, where they hatch and the birds are cared for. They are then repatriated on Kaua'i — almost 180 puaiohi as of now — where they begin the process anew.

"This beautiful forest bird has been our most reliable breeder, producing nearly 300 chicks since the beginning of the program," Lieberman said.

There are currently an estimated 500 puaiohi in the preserve, said David Leonard, a wildlife biologist with the state forestry division. The first group of new puaiohi was placed in the preserve a week ago, he added.

The preserve, however, is not all paradise. The 21/2-ounce birds must contend with non-native predators, such as cats, rats and possibly mongooses. Other nonnative animals — pigs, deer and goats — either trample or eat the underbrush where the puaiohi live. Mosquitoes carrying avian malaria and pox also are a threat.

Still, Lieberman said that the puaiohi population is "generally going up." He stressed, though, that scientists will pay attention not only to the numbers of puaiohi in the preserve but the quality of their habitat as well.

He noted that non-native plants, such as ginger, are pushing out native species the puaiohi thrive in.

To that end, the puaiohi and many other plants and animals should benefit from a new, ecosystem-based approach the federal government is taking toward the preserve.

Late last month, officials proposed adding 48 species that live only on Kaua'i to the endangered list at one time.