Twenty California condors in northern Arizona and southern Utah have died since March and half of the endangered birds tested positive for a strain of avian flu, according to authorities.

Four condors are still receiving supportive care and have shown improvement, but wildlife officials told The Arizona Republic that they’re worried the recent outbreak could potentially spread to other condor populations.

So far, authorities say, the virus hasn’t been detected in the other condor populations in California or Baja California, Mexico.

The Peregrine Fund, a group that manages wild populations of the bird, estimates there are 116 wild condors flying over Arizona and Utah and occupying the landscape within Grand Canyon National Park, Zion National Park, Vermillion Cliffs National Monument, on the Kaibab Plateau and in surrounding areas.

"The condor is slow to mature, taking up to eight years before they can produce young, and with an average of one young every other year, the rate of replacement for a loss in the wild is a big impact," Chris Parrish, president CEO of the Peregrine Fund, told the Republic. "Given the total number of birds we eventually lose and the age structure of those lost will have a tremendous impact on the recovery effort, likely to take decades."

The California condor has been protected as an endangered species since 1967.

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Fewer than 25 condors remained in the wild by 1982 until an effort was launched to capture the remaining birds and start a captive breeding program.

The first condor was released into the wild in 1995 and the first wild-born condor arrived in 2003.

Before the recent string of deaths, the National Park Service said only 334 condors remained in the wild.

According to avian disease experts, it’s likely that migration patterns are aiding the spread of avian flu during this current outbreak.

They said avian flu historically dies out after a season, but this strain has been spreading for a longer period and has affected birds since last spring.

Virologists aren’t sure why the strain is persisting longer than normal, and studies are still underway to understand the cause.