CHADRON, Neb. — Officials say all 21 lambs equipped with tracking collars for a study of bighorn sheep in northwest Nebraska have died.

Todd Nordeen of the Nebraska Game and Parks Commission says almost all died of pneumonia. Commission biologists have gathered from the carcasses tissue samples that will be tested to determine what is triggering the disease.

Nordeen knows of only one noncollared lamb in the Pine Ridge area, where birthing rates have been low for the past three years. No lambs born last year survived.

Officials think Nebraska has a population of about 300 bighorns, 115 of them in the Pine Ridge. The Wildcat Hills herds have been more successful at warding off diseases, growing to about 185 sheep after releases of bighorns from elsewhere in 2001 and 2007.

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