Juvenile Northern Spotted Owls with higher mass and intermediate levels of corticosterone have greater long-term survival

Abstract

Abstract Early life experiences have carry-over effects that manifest in later life stages. Challenging rearing environments result in more energy invested in immediate survival and less energy on growth and maturation, which can decrease survival in both the short- and long-term. One way to measure differences in energy allocation to growth between individuals is through physiological indices, such as concentrations of the metabolic hormone corticosterone, and body condition. Corticosterone increases in response to challenges to homeostasis and mobilizes stored fat and muscle to meet energetic demands. Maintaining elevated corticosterone can result in poor body condition and decreased survival. Juvenile or pre-breeding age classes are typically a substantial portion of the total population and serve key functions in population dynamics in many long-lived species. Thus, understanding how the rearing environment may influence demographics across life-history stages is crucial to understanding larger population dynamics. Yet, demographic models and conservation planning often lack vital rate estimates for early life stages because, prior to breeding, animals cannot be effectively sampled (i.e., they are unobservable). We used corticosterone concentrations in feathers and body mass of fledged juvenile Northern Spotted Owls (Strix occidentalis caurina) as indicators of potential energetic limitations early in life and used a multi-state modeling framework to quantify the effect of these indicators on apparent annual survival prior to claiming a territory (pre-territorial birds) and the probability of recruiting into the territorial population. Apparent annual survival for pre-territorial birds was higher for birds with greater mass, earlier banding dates, and intermediate levels of corticosterone. Birds with greater mass at banding were also more likely to recruit into the territorial population. Our results demonstrate the importance of early development and traditionally excluded life-history stages on long-term demographics. Although early life stages are difficult to observe, monitoring and conservation efforts may be improved by increasing studies on pre-territorial animals, control of Barred Owls, and conservation of forest structures important for Spotted Owls. This may contribute to increased juvenile survival and recruitment.