Demography of two Mexican spotted owl populations

The Mexican Spotted Owl (Strix occidentalis lucida) is a threatened subspec ies of the southwestern United States and northern Mexico. Although threate ned, little is known about its population status. We studied the demography of the Mexican Spotted Owl in one study area in Arizona and one in New Mex ico from 1991 to 1997. We estimated annual survival rates (phi), fecundity rates (b), and abundance (N) to test the hypothesis that population trends were stationary Although annual fecundity rates ((b) over bar = 0.494 for A rizona and 0.380 for New Mexico) and annual juvenile survival rates (<(phi) over bar> = 0.179 for Arizona and 0.109 for New Mexico) differed in magnitu de between the study areas, they exhibited similar temporal patterns. Annua l survival for territorial owls varied randomly in Arizona but declined lin early in New Mexico. Mean annual survival for territorial owls was 0.814 in Arizona and 0.832 for owls greater than or equal to 3 years old and 0. 664 for owls 1-2 years old in New Mexico. Based on survival and fecundity esti mates, the annual rates of change (<(lambda)over cap>) indicated that both populations were declining at greater than or equal to 10% a year. These es timates were corroborated by observed declines in abundance. Some regional factor may have been affecting fecundity whereas a combination of factors m ay have been affecting survival. Two possible reasons for the population de clines are declines in habitat quality and regional trends in climate.