Owls on the move: Changing dynamics of breeding dispersal for northern spotted owls

Julianna M. Jenkins, Pacific Northwest Research Station, US Forest Service, Corvallis, OR, Damon B. Lesmeister, Pacific Northwest Research Station, USDA Forest Service, Corvallis, OR, Katie M. Dugger, Oregon Cooperative Wildlife Research Unit, U.S. Geological Survey, Corvallis, OR and J. David Wiens, Forest and Rangeland Ecosystem Science Center, US Geological Survey, Corvallis, OR

Background/Question/Methods
Breeding dispersal, the movement from one breeding location to another, is generally uncommon for philopatric species that are long-lived and evolved within stable environments. Individual decisions regarding breeding dispersal are highly dynamic and regulated by a series of trade-offs between possible costs and benefits of dispersal. Trends in breeding dispersal movements over time may signal changes to population or environmental conditions. We examined annual detections of 4118 marked adult northern spotted owls (Strix occidentalis caurina), a philopatric species in old-growth forests of the Pacific Northwest, USA, during 27 breeding seasons (1990–2017) to assess adult dispersal patterns. During the study spotted owl populations were in decline and faced increasing competition from invasive barred owls (S. varia). Our specific objectives were to evaluate: (1) rates of breeding dispersal in northern spotted owls over 28 years (1990–2017) on seven demographic study areas spanning five ecophysiographic provinces in Oregon and Washington; (2) whether rates and possible factors associated with the rate of breeding dispersal have changed over time; (3) if temporal changes could best be correlated with extrinsic (e.g., forest cover, forest disturbance, barred owl presence) versus intrinsic factors (e.g., years of experience on a territory, prior breeding success); and (4) test whether there were differences in survival for dispersing owls. We used multistate mark-resight models to estimate apparent survival, resighting rates, and transition probabilities between two states: a site faithful state and a dispersal state.
Results/Conclusions
Our sample included 408 successive and 1372 non-successive dispersal events between years. Adult dispersal rates, rates of repeated dispersal, and dispersal distances increased dramatically over the study period. We did not detect differences in survival between our dispersal states. Early in the study, intrinsic factors (e.g., pair status, reproductive success) were primary drivers in spotted owl dispersal, but in later years barred owl occurrence was an increasingly strong effect. Our results illustrate the profound impacts barred owls have on spotted owls, which evolved breeding philopatry under environmental stability and low levels of interspecific competition. While dispersal is not inherently maladaptive, increasing rates of breeding dispersal associated with population declines contribute to population instability and vulnerability of northern spotted owls to extinction, and this forecast is unlikely to change unless active management interventions are undertaken.