The plan to re-establish the Hawaiian crow in the wild suffered a blow as three of the five birds released into the East Hawaii wilderness were found dead last week of unknown causes.

Conservation officials said Tuesday that they recaptured the remaining two birds and moved them back into an aviary until they can figure out what happened through medical examinations of the carcasses.

The five captive-raised Hawaiian crow, or alala, were released into the Puu Makaala Natural Area Reserve near Hilo on Dec. 14 after an extended transition period in which they lived in an outdoor aviary set up in the wilderness.

After the release, officials reported that the birds quickly adjusted to their new surroundings, eating from feeders placed in the area.

John Vetter, a wildlife biologist with the state Department of Land and Natural Resources Division, said some level of mortality is expected when reintroducing a species back into the wild.

“The initial days of release are always the most difficult stage of any release program, and the level of uncertainty is also highest with the first release cohort,” Vetter said in a news release.

Puu Makaala Natural Area Reserve was said to be ideal for the alala. Conservationists worked to preserve and protect native plants and species within the 18,730-acre protected area for two decades, leaving a habitat similar to the one where alala lived before their numbers began to decline.

The Hawaiian crow has been extinct in the wild since 2002, preserved only at the Keauhou and Maui bird conservation centers managed by San Diego Zoo Global’s Hawai‘i Endangered Bird Conservation Program. At last count 130 birds were living in captivity.

Bryce Masuda, the program’s conservation manager, said condolences for the loss have come from around the world.

“The loss of these three birds is difficult for the entire community, including the many people who have cared for these birds since their hatch and have worked steadfastly to prepare for their release,” he said in a statement.

The released birds, all males, were examined for suitability and subjected to a battery of health tests in preparation for the long-awaited release, officials said.

They also underwent predator-avoidance training to help to teach the danger represented by the Hawaiian hawk and to allow them to respond with evasive action.

Scientists said anti-predator behaviors in wild animals can be lost after only a few generations in captivity, so it’s likely the remaining alala do not know they need to avoid the hawk.

The birds were moved to a flight aviary in mid-October, allowing them to acclimate to the forest before they were transferred to a smaller aviary in the forest one week prior to the release.

Each bird was outfitted with tiny radio and GPS transmitters and was being monitored daily.

A second release of seven mostly young females, also hatched this year, was planned for January. Officials were planning to release 12 birds a year over the next five years.

Scientists say alala are an important part of the life of the Hawaiian forest because they help disperse native plant seeds. They also hold significant value in Hawaiian culture.

Recently shown to be adept users of tools, the clever alala use a vast array of calls and vocalizations, more than any other raven or crow.