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. Although 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 yr, 1990-2017, 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 1,372 nonsuccessive 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.

Keywords: Strix occidentalis occidentalis; Strix varia; barred owl; competition; dispersal probability; philopatric species; population stressors; spotted owl.

Fig. 1

Locations of the seven study areas within five ecophysiographic provinces within the northern spotted owl range in Oregon and Washington used to examine breeding dispersal trends, 1990–2017. Nesting and roosting cover map generated from Glenn et al. (2017).

Fig. 2

Cumulative percentage of historical northern spotted owl territories with barred owl (barred owl index; BO‐A) from 1990–2017 in seven study areas. Trend lines for ecophysiographic provinces shown.

Fig. 3

Conceptual framework for our multistate mark–resight model of northern spotted owl apparent survival ($\widehat{\mathrm{S}}$), resighting probability (p), and transition probabilities (ψ), between two potential states: a site faithful state (F) where owls remained on their previous territory and a breeding dispersal state (D), where owls moved from their most recent territory to another between observations.

Fig. 4

The naive annual estimates of northern spotted owl breeding dispersal during 1990–2017 increased across our monitored population (a), within Washington ecophysiographic provinces (b), and within each Oregon ecophysiographic province (c)–(e). Point size increases with annual sample sizes across all sites in (a) (n = 216–857 owls) and by study area in (b)–(e) (n = 7–210 owls observed annually).

Fig. 5

Estimated (a) successive (ψ_DD) and (b) nonsuccessive (ψ_FD) annual breeding dispersal transition probabilities with 95% confidence bands for northern spotted owls observed within 7 study areas from 1991 to 2016. Estimates were generated using the top model in Table 4 holding other variables at their study area annual mean values. Study area abbreviations: CAS, South Cascades; CLE, Cle Elum; HJA, H. J. Andrews; KLA, Klamath; OCR, Oregon Coast Range; OLY, Olympic Peninsula; TYE, Tyee.

Fig. 6

The probability of successive (ψ_DD) and nonsuccessive (ψ_FD) breeding dispersal transitions for northern spotted owls (during 1990–2017) varied with (a) study area, (b) amount of territory experience, (c) pair and productivity of individuals, (d) the proportion of territories with barred owl detections, and (e) the territory quality index. Estimates were generated using the top ranked model in Table 4 while holding all other variables at study area means; (b)–(e) were generated using study area means for H. J. Andrews. The open and solid points in (d) represent the value of BO‐A at H. J. Andrews in 1990 and 2017, respectively. Study area abbreviations: CAS, South Cascades; CLE, Cle Elum; HJA, H. J. Andrews; KLA, Klamath; OCR, Oregon Coast Range; OLY, Olympic Peninsula; TYE, Tyee.