Helicopter capture-and-releases in October and December of one of the most threatened species of California's bighorn sheep in the deserts of Southern California marked the beginning of a more intensive effort to help save the precarious population. Already included on the state's threatened species list and under consideration for federal endangered species status, Peninsular bighorn sheep numbered only 380 individuals in a 1992 count, down from an estimated 1,170 sheep in 1979. The steady decline in their numbers has been attributed in part to a century of diminishing habitat and water diversions, as well as to the prolonged drought and a respiratory disease that affects young lambs. By monitoring movements of the sheep with the help of radio collars and periodic medical examinations, researchers hope to discover information that will lead to better protection. As part of a three-year collaborative study headed by UC Davis wildlife veterinarian Dr. Walter Boyce, the research team recently placed 43 radio collars on selected bighorn sheep from a herd in Anza Borrego Desert State Park. About 60 more collars will be placed on the bighorn in the summer and fall of 1993.