Social status, forest disturbance, and Barred Owls shape long-term trends in breeding dispersal distance of Northern Spotted Owls

Abstract

Dispersal among breeding sites in territorial animals (i.e. breeding dispersal) is driven by numerous selection pressures, including competition and spatiotemporal variation in habitat quality. The scale and trend of dispersal movements over time may signal changing conditions within the population or on the landscape. We examined 2,158 breeding dispersal events from 694 male and 608 female individually marked Northern Spotted Owls (Strix occidentalis caurina) monitored over 28 yr on 7 study areas to assess the relative importance of individual (sex, experience), reproductive (annual productivity, mate availability), and environmental (forest alteration, presence of competitor) sources of variation in breeding dispersal distance. Median breeding dispersal distance was 3.17 km, with 99% of all breeding dispersal events <37 km. Mean annual dispersal distances increased by 2.43 km in Oregon and 9.40 km in Washington between 1990 and 2017, which coincided with increases in annual detections of nonnative Barred Owl (S. varia). Frequency of breeding dispersal events, both among and within individuals, also increased over time. Female owls moved farther than males (median of 3.26 and 3.10 km, respectively), and birds with less experience (territory tenure) moved farther than those with more experience. Owls that were single in the year prior to dispersal moved 13–31% farther than those paired prior to dispersal. The greatest environmental change occurring over the course of our study was the expansion of Barred Owl populations. Breeding dispersal distance was positively related to Barred Owls in the study area and disturbance within the originating territory. While it appears that social factors continue to be important drivers of breeding dispersal distance in Spotted Owls, increased competition from Barred Owls and habitat alteration have a contributing effect. Increased breeding dispersal distances should be of concern for conservation efforts and considered in population monitoring because changing dispersal behavior may lead to higher rates of mortality and/or emigration from historical study areas.

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

“…The intrinsic factors of reproductive success, site and mate experience, and sex best explained variation in breeding dispersal behavior (Forsman et al 2002). More recently, breeding dispersal distances of northern spotted owls were found to be associated with forest disturbance and social status (Jenkins et al 2019a). Territory quality, as defined by site-level occupancy and reproductive history, has also been found to play an important role in dispersal behavior of the California spotted owl (Strix occidentalis occidentalis [Seamans andGutiérrez 2007, Gutiérrez et al 2011]).…”

“…Changes in forest stand characteristics through disturbance events (e.g., wildfire, timber harvest) may also influence breeding dispersal rates in northern spotted owls. Barred owls can displace spotted owls from historical territories (Kelly et al 2003, Pearson andLivezey 2007), which has contributed to an increase in breeding dispersal distances over time (Jenkins et al 2019a). In addition to displacement, interspecific territoriality and competition for limited resources with barred owls at high-quality sites may decrease the realized quality of territories (Wiens et al 2014) and lead to decreases in site occupancy by spotted owls (Yackulic et al 2019).…”

Older forests used by northern spotted owls functioned as fire refugia during large wildfires, 1987–2017

“…Large wild res are a severe threat to northern spotted owl habitat and populations (Clark et al 2011, Davis et al 2011, Rockweit et al 2017), yet the issue has been debated in the scienti c literature, especially when also considering other spotted owl subspecies (Hanson et al 2009, Spies et al 2010, Ganey et al 2017, Jones et al 2020, Lee 2020). In addition to wild re, multiple other stressors play a role in degrading the prognosis for persistence of northern spotted owl populations (Dugger et al 2016, Miller et al 2018, Jenkins et al 2019a, Wiens et al 2019. We approached this study to better understand the long-term and broadscale patterns of risk that high-severity re pose to spotted owls and their habitat because the extent and frequency of wild res that is expected to increase with climate change (Davis et al 2017, Halofsky et al 2020.…”

“…Here we refer to this forest type as nesting forest. We used maps of nesting forests that were generated by the NWFP monitoring program and have been used in many publications on spotted owl population dynamics and resource selection (e.g., Wiens et al 2014, Dugger et al 2016, Jenkins et al 2019a. Within the study there were large areas not capable of developing into nesting forest, mainly due to soil type, plant association, or elevation (Davis and Lint 2005).…”

Estimating nest‐switching in free‐ranging wild birds: an assessment of the most common methodologies, illustrated in the White Stork (Ciconia ciconia)

“…2015, Jenkins et al . 2019) or as non‐breeders (Williams & Rodwell 1992, Spendelow & Eichenwald 2018), or have been excluded explicitly (Blums et al . 2002, Ayers et al .…”