Conspecific and congeneric interactions shape increasing rates of breeding dispersal of northern spotted owls

Abstract

Breeding dispersal, the movement from one breeding territory to another, is rare for philopatric species that evolved within relatively stable environments, such as the old-growth coniferous forests of the Pacific Northwest. While dispersal is not inherently maladaptive, the consequences of increased dispersal on population dynamics in populations whose historical dispersal rates are low could be significant, particularly for a declining species. We examined rates and possible causes of breeding dispersal based on a sample of 4,118 northern spotted owls (Strix occidentalis caurina) monitored in seven study areas over 28 years, 1990 to 2017, in Oregon and Washington, USA. Using a multistate mark-resight analysis, we investigated the potential impacts of an emergent congeneric competitor (barred owl Strix varia) and forest alteration (extrinsic factors), and social and individual conditions (intrinsic factors) on 408 successive and 1372 non-successive dispersal events between years. The annual probability of breeding dispersal increased for individual owls that had also dispersed in the previous year and decreased for owls on territories with historically high levels of reproduction.Intrinsic factors including pair status, prior reproductive success, and experience at a site, were also associated with breeding dispersal movements. The percent of monitored owls dispersing each year increased from ~7% early in the study to ~25% at the end of the study, which coincided with a rapid increase in numbers of invasive and competitively dominant barred owls. We suggest that the results presented here can inform spotted owl conservation efforts as we identify factors contributing to changing rates of demographic parameters including site fidelity and breeding dispersal. Our study further shows that increasing rates of breeding dispersal associated with population declines contribute to population instability and vulnerability of northern spotted owls to extinction, and the prognosis is unlikely to change unless active management interventions are undertaken.

Range‐wide sources of variation in reproductive rates of northern spotted owls

“…We found strong evidence that northern spotted owl site occupancy is declining range-wide and that the species is at immediate risk of extirpation from large portions of its geographic range. This finding is not unique to this study (Dugger et al, 2016;Franklin et al, 2021;Yackulic et al, 2019), and other research has pointed to evidence of an extinction vortex (Gilpin & Soulé, 1986) for northern spotted owls (Franklin et al, 2021;Jenkins et al, 2021;Miller et al, 2018).…”

“…$$ There is evidence that the previous year's occupancy state can strongly influence current year occupancy in northern spotted owls (Dugger et al, 2016; Yackulic et al, 2019), providing a motivation for distinguishing among $$$ {\uppsi}_{i,t+1}^{\left[0\right]} $$$, $$$ {\uppsi}_{i,t+1}^{\left[1\right]} $$$, and $$$ {\uppsi}_{i,t+1}^{\left[2\right]} $$$ in our model structure. In addition, many bird species (e.g., Greenwood & Harvey, 1982; Johnson et al, 1992), including spotted owls (Jenkins et al, 2021), show greater fidelity to breeding sites where they successfully reproduced the previous year. These considerations led to the prediction that $$$ {\uppsi}_{i,t+1}^{\left[0\right]}\ll {\uppsi}_{i,t+1}^{\left[1\right]}<{\uppsi}_{i,t+1}^{\left[2\right]} $$$.…”

Integrating new technologies to broaden the scope of northern spotted owl monitoring and linkage with USDA forest inventory data

“…GNN models are mapped predictions of forest structure and composition across broad landscapes based on FIA plot data and Landsat imagery (Bell et al, 2021). The spotted owl monitoring program has a long history of using GNN data as territory-scale habitat covariates for modeling the effects of forest disturbance, structure, and tree species composition on spotted owl dispersal, distribution, and population trends within the eight historical study areas (Jenkins et al, 2019a(Jenkins et al, , 2021Franklin et al, 2021;Davis et al, 2022;Rockweit et al, 2022). By integrating next generation survey methods throughout the entire spotted owl range, the monitoring program could further leverage the potential of range-wide FIA-derived models as spatially explicit predictors of spotted owl distribution and population change.…”

“…This was a reasonable possibility at the time of monitoring program development because northern spotted owls have consistently been found to be an obligate of older forests with large trees (Forsman et al, 1984;Wilk et al, 2018;Jenkins et al, 2019b;Sovern et al, 2019). However, competition with, and displacement by, invasive barred owls have disrupted spotted owl population and territorial dynamics including the strength of the relationship between old forest and demographic performance (Dugger et al, 2005;Jenkins et al, 2019aJenkins et al, , 2021Yackulic et al, 2019). The habitat quality of old forest for northern spotted owls is degraded if barred owls are present (Lesmeister et al, 2018).…”

Using passive acoustic monitoring to estimate northern spotted owl landscape use and pair occupancy

“…These persistent population declines are driven primarily by continued loss of old forest due to large wildfires and timber harvest on non‐federal lands as well as by competition and displacement by congeneric barred owls ( Strix varia ), a recent newcomer to the Pacific Northwest (Davis et al, 2022; Franklin et al, 2021; Lesmeister et al, 2018). Barred owls have been linked to changes in spotted owl behavior and ecology such as reduced niche space, increased adult dispersal, and decreased survival and reproduction (Jenkins et al, 2021; Jenkins, Lesmeister, Forsman, et al, 2019; Jenkins, Lesmeister, Wiens, et al, 2019; Lesmeister et al, 2018; Rockweit et al, 2022; Wiens et al, 2021). Without management intervention, competition by barred owls in combination with declining habitat conditions is expected to cause extirpation of northern spotted owls from parts of their range in the coming decades (Franklin et al, 2021; Yackulic et al, 2019).…”