Demographic characteristics and trends of Northern Spotted Owl populations in Northwestern California

A contentious point in the controversy surrounding Northern Spotted Owls is whether populations are declining or stationary. We estimated age- and sex-specific survival probabilities and fecundity rates from 1985 through 1993 in a population of Northern Spotted Owls on public lands in northwestern California. We used mark-recapture models to estimate survival probabilities and ANOVA models to estimate fecundity rates. We estimated annual rates of population change using average estimates of demographic parameters in a 3- stage Leslie matrix and by estimating counts of owls over time. We found a significant decline in survival for owls ≤3-years old in mark-recapture models which only included this age-class. However, survival of owls ≤1 - year old was variable over time when all age-classes were included in the mark-recapture models. Estimates of juvenile survival and fecundity (with the exception of one year) were constant during the study period. Using our estimates of demographic parameters in a Leslie matrix, we estimated an annual rate of population change (λ) of 0.9656, which was significantly different (P = 0.019) from a stationary population. In contrast, trends in numbers of owls (λ = 1.000-1.009) were not different (P = 0.15-49) from a stationary population. A number of biases made estimation of population rates of change problematic and were possible explanations for the discrepancy between our estimates of λ. During the period of study, we concluded that the population of Spotted Owls in our study were not in dramatic decline, but may have been in either slight decline or stationary.