- Northern Spotted Owls (Strix occidentalis caurina) are a long-lived forest owl and range-wide declines in their numbers have resulted in the species being listed as threatened under the endangered species act. While many studies have been focused on population trends and reproductive performance of Spotted Owls from different age-classes, none have examined age related performance or lifetime reproductive success of individual owls. Using data from known age Spotted Owls on four long-term demography studies in Oregon and Washington, I conducted separate analyses to examine the functional relationship of age and reproductive success, measured as the number of young fledged (NYF), and to examine lifetime reproductive success.
In my age-specific analysis, I used a mixed models approach to account for repeated measures on individual owls. I found that the standard 3-level age-class approach (1-year-old, 2-year-old, adult) often used in Spotted Owl research was a poor fit relative to curvilinear and threshold models that allowed for age-dependent variation beyond age 3. A quadratic age effect was more often supported for males,
whereas a threshold effect indicating a linear increase in NYF from ages 1 to 4 was most supported in the analyses of female data. Females tended to achieve a maximum in reproductive performance at earlier ages than males, and there appeared to be a negative relationship between the age when a maximum in mean NYF was reached and overall fecundity, as reported in earlier studies. Temporal variability in numbers of young fledged at each age was best modeled with a categorical year variable as opposed to a cyclic biennial ("even-odd") year effect.
Lifetime reproductive success of Spotted Owls, measured as the total numbers of fledglings and recruits produced by individuals, varied widely. For owls with relatively complete data, the number of lifetime fledglings ranged from 0 to 20 and the number of lifetime offspring that were observed as recruits within study areas ranged from 0 to 7. There was a significant positive relationship between the number of lifetime young fledged and the number that later recruited locally. Seventy five percent of females and 67% of males bred at least once. Whereas 17% of females and 16% of males produced 50% of the offspring fledged by each sex, only 9% of females and 7% of males produced 50% of the banded young that were later observed as recruits. Thirty nine percent of females and 30% of males produced no fledglings and 64% of females and 69% of males produced no local recruits. Thus while most owls fledged at least 1 offspring, most did not produce any fledglings that recruited locally during the study. Cumulative proportions of individual owls that first bred at different ages indicated that females tended to initiate their breeding at earlier ages than males. Whereas 36% of females bred first at ages 1 or 2, only 19% of males bred first before
age 3. Of the owls that bred, 98% of females and 91% of males bred at least once by age 6. Compared to owls on the three Oregon study areas, owls on the Cle Elum Study Area in the eastern Cascades of Washington bred early (>50% by age 2), had higher mean numbers of fledglings (>1) at most ages, and had short mean lifespans (6 years). On the Oregon study areas, owls first bred at later ages (>50% at age ≥3), had lower mean numbers of fledglings (0.4–0.7) at most ages, and had longer mean lifespans (7–9 years). These patterns appear consistent with a compensatory relationship between reproduction and survival that was suggested in at least one previous study. Life history theory is also consistent with the idea that where lower and more variable non-juvenile survival occurs (as has been documented on Cle Elum), selection pressure for earlier breeding and greater offspring production at each attempt are to be expected. Nevertheless, it is unclear if local conditions such as prey abundance, harsh winter conditions, or predation pressure act proximately to influence reproduction and survival of Spotted Owls in these studies, or if the variability in patterns of age-specific reproductive success and components of lifetime reproduction on these study areas reflect adaptive life history responses among populations of Spotted Owls. It is likely that both plasticity and life history adaptations underlie the differences and patterns that were revealed, but tests of these hypotheses were beyond the scope of my study.