- The northern spotted owl (Strix occidentalis caurina; spotted owl) is of conservation concern and endemic to mature forests of the Pacific Northwest. Adult survival has a strong effect on population growth rate, but juvenile survival and recruitment are also important components of population change. Despite the importance of this life stage, estimates of survival in the first year of life, factors related to survival, and the contribution of juveniles to later life-history stages and population stability are unknown. A better understanding of stressors during these missing life stages can provide insights on long-term population demographics. Environmental variation may affect survival of juvenile spotted owls, thus understanding how increasing temperatures and precipitation affect the less–studied life–history stages of this threatened species will give us insight and considerations for future management, particularly in areas predicted to experience increased temperatures and storm severity as climate patterns change.
The development of new tools and modeling techniques allows researchers to look for new ways to evaluate animal populations. Individual physiology is a common tool for ecologists interested in how an individual’s environment affects life history and demographics. Corticosterone is a steroid hormone integral to a variety of physiological pathways and resource allocation, such as metabolism, immunity, and cognition. Thus, it is associated with life history and environmental changes as well as fitness metrics in many taxa. In avian species, circulating corticosterone diffuses into developing feathers (FCORT) and can be extracted, measured, and used as an indicator of challenging conditions faced during feather growth that may have life-long ramifications. In addition, models that incorporate changes in ecological states or life history stages increase the accuracy of demographic models for spotted owls. Using multi-state mark recapture models, it is possible to estimate survival for non-territorial floaters, the probability of recruiting into the territorial breeding population, as well as evaluating individual and landscape characteristics that may affect these important demographics. Here I focused my study on juvenile spotted owls captured and banded after fledging but prior to natal dispersal on seven demography study areas in Washington and Oregon during 2001–2017. The goals of this study were to determine: 1) the environmental or individual variables associated with variation in juvenile feather corticosterone and 2) the relationship between corticosterone extracted from feathers and survival of non-territorial birds or recruitment into the territorial breeding population.
First, I quantified FCORT levels in 4,720 feathers from 1,056 juvenile spotted owls and used an information-theoretic approach to examine the environmental, temporal, and individual factors related to patterns of FCORT (Chapter 2). The final model set contained four competitive models with the negative relationship of juvenile mass at banding as the most important predictor of FCORT. Increasing temperatures and higher amounts of precipitation prior to and during fledging was related to increased FCORT, suggesting that this period may present an energetic challenge for juvenile spotted owls. The relationship between FCORT and precipitation was modified by juvenile mass, where increasing mass decreased the positive relationship between FCORT and precipitation, suggesting that some component of body mass, such as fat stores or surface to volume ratios were important for juveniles while fledgling.
Second, I used long-term, capture-mark-recapture data across 17 years and seven study areas in Oregon and Washington to estimate apparent survival and recruitment of 1,061 non–territorial and territorial spotted owls (Chapter 3). I also examined the relationship between demographics and juvenile feather corticosterone levels, juvenile mass at banding, day of year on which a juvenile was banded, and barred owl presence. Of the 1,061 owls included, 381 of those eventually recruited into the territorial breeding population. The mean age of recruitment was 2.2 years, but almost half (49%) recruited into the territorial population at age one. There was weak to moderate support for a difference in survival for non-territorial and territorial birds, and survival of both territorial states varied annually. There was a quadratic relationship between apparent survival of non-territorial floaters and feather corticosterone, with the highest survival observed for those birds with intermediate levels of corticosterone. Non-territorial survival also had a positive linear relationship with juvenile mass at banding and a negative linear relationship with banding date. These results also indicated that recruitment probability decreased over the course of the study and heavier juveniles at banding had a higher probability of recruiting into the territorial population.
This study established a link between developmental conditions, individual physiology and long-term demographics. My findings indicated that the fledging period is an important and sensitive period in spotted owl development, in which small climatic changes can be associated with changes in physiology and these physiological differences can be associated with survival probabilities. In addition, juvenile mass explained variation in physiological differences in juveniles as well as bolstering survival and recruitment. Understanding the long-term ramifications of developmental conditions is important to building accurate demographic models, predicting population stability in response to environmental changes and informing conservation strategies. The first step is identifying the landscape variables associated with individual variation in development and the connections to long-term demographics.