Authorities say they have confirmed that lead poisoning killed half of the eight California condors that died last winter in Arizona and Utah.
The Idaho-based Peregrine Fund says two other condors suffered fatal trauma possibly caused by a predator between last December and February and two carcasses were unrecoverable.
Condors encounter lead when eating animals that have been shot with lead bullets.
Officials with the Peregrine Fund's condor reintroduction project say 72 condors currently fly in a range that stretches from Arizona's Grand Canyon to southern Utah's Zion National Park.
There were just 22 condors when a program was started in 1996 to save the endangered species from extinction.
Arizona is trying to reduce the toll on condors by providing vouchers for lead-free bullets and other rewards for hunters who dispose of carcasses properly.