A lynx born in Colorado has given birth to two kittens, a major milestone in the state's ambitious attempt to reintroduce the elusive cats, researchers said Tuesday.

It was the first documented case of a Colorado-born lynx giving birth since the reintroduction program began in 1999.

The cat, born in 2004, gave birth to two males in mid-June, buoying hopes that lynx will develop into a self-sustaining population in the state.

"From here on out, we're just waiting to see if we can maintain a good survival rate," said Tanya Shenk, the Colorado Division of Wildlife's lead researcher on the program.

The news was tempered, however, by a dramatic reduction in the number of births this year. Colorado Division of Wildlife researchers found four dens with a total of 11 kittens, down from 18 dens with 50 kittens last year.

Biologists are puzzled by the decline and studying possible reasons. Shenk said one possibility is that recent releases of adult cats could have disrupted the cats' social structure. But that is only speculation, she added.

"We're not seeing any indication of the animals being in poor body condition. There seems to be sufficient prey," she said.

The four dens where kittens were found were normal, Shenk said.

Researchers believe other kittens were born but not counted this year because their mothers do not have radio collars and cannot be tracked.

"I don't think we've found all the kittens any of the years," Shenk said.

The last two winters, biologists and trackers found additional kittens missed during the summer count. They typically start searching for newborn lynx in mid-May after flight crews monitor the cats' movements from the air.

Biologists take genetic and blood samples from the kittens and put microchips under their skin so they can identify them if they catch them again later.

The animals' death rate hasn't increased, and most of the cats are staying in established territories, said Rick Kahn, the Division of Wildlife biologist who heads the lynx program.

Lynx, designated an endangered species by Colorado, were wiped out in the state by 1973, victims of trapping, poisoning and development.

A total of 218 lynx from Canada and Alaska have been released in southwestern Colorado since 1999, and at least 78 are confirmed dead. More than 80 births have been documented since 2003, and researchers believe the total number of lynx in the state is holding steady at about 200.

Four of the first five transplanted lynx starved to death, prompting a torrent of criticism from both advocates and opponents of reintroduction.

The division quickly changed strategies, keeping the animals caged a few weeks to fatten them up instead of releasing them immediately. The delay also meant the animals would hit the high-country slopes later in winter, when more prey is available.

The state has spent $3.5 million since 1997 on the restoration program. The funds have covered the costs of transporting the cats from Canada, the researchers who study the animals and ensure their health, radio collars and monitoring, trapping, and pilots.

Kahn said no additional lynx will be released in Colorado next spring because of the decline in births.

"Given the high number of lynx currently out there we want to give them an opportunity to settle down and establish a stable social structure," he said.

The agency will assess the need for further releases and make recommendations to the Colorado Wildlife Commission, which oversees the division, next year.