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Volume 120, 2018, pp. 530–542 DOI: 10.1650/CONDOR-17-164.1

RESEARCH ARTICLE

Geographic variation in natal dispersal of Northern Spotted Owls over 28 years Jeff P. Hollenbeck,1* Susan M. Haig,1 Eric D. Forsman,2 and J. David Wiens1 1

U.S. Geological Survey, Forest and Rangeland Ecosystem Science Center, Corvallis, Oregon, USA U.S. Forest Service, Pacific Northwest Research Station, Corvallis, Oregon, USA * Corresponding author: [email protected] 2

Submitted August 28, 2017; Accepted March 28, 2018; Published June 13, 2018

ABSTRACT The most recent comprehensive estimates of Northern Spotted Owl (Strix occidentalis caurina) natal dispersal distances were reported in 2002. Since then, Northern Spotted Owl populations have experienced substantial demographic changes, with potential attendant changes in natal dispersal distances, including temporal or geographic trends. We analyzed the natal dispersal of Northern Spotted Owls during 1985–2012 in Oregon and Washington, USA (n ¼ 1,534 dispersal events), to determine current natal dispersal distances and to evaluate potential trends that may inform management actions. Mean net dispersal distance (natal site to site of first attempted breeding) was 23.8 km 6 19.2 km SD, with females dispersing ~50% farther than males. Net dispersal distance varied by ecoregion (Washington Coast and Cascades, Washington Eastern Cascades, Oregon Coast Range, Oregon and California Cascades, and Oregon and California Klamath) but declined similarly in all ecoregions over time (~1 km yr1). Dispersal direction also varied by ecoregion, following coarse-scale forest habitat configuration, and was bimodal (north–south) in the Oregon Coast Range, south–southwest in the Oregon and California Cascades, and showed little directionality in the Washington Eastern Cascades, Washington Coast and Cascades, and Oregon and California Klamath. Long-distance dispersal events (.50 km) also varied by ecoregion (mean: 62.3–99.5 km), with most long-distance dispersal (8% of dispersers; distances up to 177 km) originating in southern ecoregions. We found no direct relationship between Barred Owl (Strix varia) detections near natal or settling locations and dispersal distance. These findings, particularly the declining trend of dispersal distances, may inform management actions aimed toward conservation of the Northern Spotted Owl. Keywords: Barred Owl, natal dispersal, natal philopatry, Northern Spotted Owl, Strix occidentalis, Strix varia ´ geogra´fica en la dispersion ´ natal de Strix occidentalis caurina a lo largo de 28 anos ˜ Variacion RESUMEN ´ natal de Strix occidentalis caurina fueron Las estimaciones globales ma´s recientes de las distancias de dispersion reportadas en 2002. Desde entonces, las poblaciones de S. o. caurina han experimentado cambios demogra´ficos ´ natal, incluyendo tendencias sustanciales con potenciales cambios concomitantes en la distancia de dispersion ´ natal de S. o. caurina desde 1985 hasta 2012 en Oregon ´ y temporales o geogra´ficas. Analizamos la dispersion ´ para determinar las distancias actuales de dispersion ´ natal y evaluar las Washington (n ¼ 1,534 eventos de dispersion) ´ neta (desde el tendencias potenciales que pueden apoyar las decisiones de manejo. La distancia media de dispersion sitio de nacimiento hasta el sitio del primer intento reproductivo) fue de 23.8 km 6 19.2, con las hembras ´ neta vario´ por ecorregion ´ (Costa y dispersańdose un 50% ma´s lejos que los machos. La distancia de dispersion ´ Cascadas de Oregon ´ y Cascadas Oeste de Washington, Cascadas Este de Washington, Rango Costero de Oregon, ´ y Montanas ˜ Klamath) pero disminuyo´ de modo similar en todas las ecorregiones a lo California, y Costa de Oregon ´ de dispersion ´ también vario´ por ecorregion, ´ siguiendo la configuracion ´ del ˜ La direccion largo del tiempo (~1 km/ano). ´ sud-sudoeste en las ha´bitat del bosque a escala gruesa – bi-modalmente (N-S) en el Rango Costero de Oregon, ´ y California, y con poca direccionalidad en las Cascadas Este de Washington y la Costa y las Cascadas de Oregon ´ de larga distancia (.50 km) también variaron por Cascadas Oeste de Washington. Los eventos de dispersion ´ (media: 62.3 a 99.5 km), con la mayor´ıa de las dispersiones de larga distancia (8% de las dispersiones) ecorregion ´ directa entre las detecciones de originańdose en las ecorregiones del sur (hasta 177 km). No encontramos una relacion ´ Estos hallazgos, Strix varia cerca de las ubicaciones de nacimiento y de establecimiento y la distancia de dispersion. ´ pueden apoyar las acciones de manejo particularmente la tendencia decreciente de las distancias de dispersion, ´ de S. o. caurina. destinadas a la conservacion ´ natal, filopatr´ıa natal, Strix occidentalis, Strix varia Palabras clave: dispersion

Q 2018 American Ornithological Society. ISSN 0010-5422, electronic ISSN 1938-5129 Direct all requests to reproduce journal content to the AOS Publications Office at [email protected]

J. P. Hollenbeck, S. M. Haig, E. D. Forsman, and J. D. Wiens

INTRODUCTION Natal dispersal (the distance moved by an individual from its natal site to its site of first breeding; Greenwood 1980) is a key demographic process for all organisms (Dobson and Jones 1985). Natal dispersal is a primary mechanism for maintenance of populations across species’ ranges (Paradis et al. 1998, Pulliam 2000), genetic population structure (e.g., gene flow and inbreeding; Tonkyn and Plissner 1991, McRae and Beier 2007), source–sink population dynamics, and colonization of new or unoccupied habitats (Amarasekare 2004, Pfenning et al. 2004). Juvenile dispersal strategies in birds have evolved in response to competition for resources, competition for mates, and inbreeding avoidance (Greenwood and Harvey 1982). Most bird species exhibit some degree of natal philopatry, settling in or near their natal territory as a strategy to increase individual fitness in patchy or dynamic landscapes (Russell and Rowley 1993, Daniels and Walters 2000). This could increase potential mate encounters while decreasing potential intraspecific competition that would result from remaining too close to the natal territory (McPeek and Holt 1992, Holt and McPeek 1996). The Northern Spotted Owl (Strix occidentalis caurina) is a federally threatened species associated with mature conifer forest habitats in the Pacific Northwest of North America. Timber harvest and other land conversions over the last century have substantially reduced the forests used by Northern Spotted Owls. Motivated in part by the status of the Northern Spotted Owl, the U.S. government implemented the Northwest Forest Plan (USDA Forest Service and USDI Bureau of Land Management 1994) to provide habitat for the Northern Spotted Owl and other ‘‘old-growth’’ forest-dependent species, while also providing opportunities for timber harvest. Northern Spotted Owls evolved in relatively abundant and regionally connected habitat with infrequent disturbance (mostly large fires), which favored short natal dispersal distances with random directionality (Lamberson et al. 1992, 1994, Davis et al. 2011). Historically, the primary driver of dispersal behavior may have been interactions with territorial adults (Lamberson et al. 1994, Forsman et al. 2002). Recent changes in regional habitat, such as fragmentation from timber harvest and negative impacts of invasive Barred Owls (Strix varia), may impede dispersing juvenile Northern Spotted Owls and negatively affect population persistence (Lamberson et al. 1992, 1994, Miller et al. 1997, Kelly et al. 2003, Hamer et al. 2007, Sovern et al. 2014). Indeed, Northern Spotted Owls have declined substantially in recent years (Dugger et al. 2016). Given the importance of landscape composition and structure to dispersing Northern Spotted Owls (Sovern et al. 2015), we predict that the negative demographic

Natal dispersal of Northern Spotted Owls

531

impacts identified by Dugger et al. (2016) will be reflected in patterns of natal dispersal. The most recent comprehensive estimates of natal dispersal by Northern Spotted Owls were reported in 2002 and were limited in their ability to determine trends of natal dispersal over time or geography (Forsman et al. 2002). Decades of Northern Spotted Owl research in the Pacific Northwest have resulted in a substantial dataset, including locations of owls banded at natal sites and their subsequent relocations as territorial adults. Using these data, we expand on previous analyses of natal dispersal (Forsman et al. 2002) with an additional 16 yr of regionwide data collected from 1997 to 2013 (28 yr total) to determine current and robust natal dispersal distance estimates, as well as temporal and geographic trends of natal dispersal, that can be used by managers to inform conservation efforts. Our specific objectives in this study were to: (1) determine net natal dispersal distances relative to geographic region and time (year); (2) determine directional movements (i.e. azimuths) of natal dispersal relative to geographic region, year, and sex; and (3) describe patterns of natal dispersal relative to Barred Owl presence. METHODS Study Area We analyzed natal dispersal of Northern Spotted Owls in the Pacific Northwest region of the United States. Most of the owls analyzed originated from 7 long-term demographic study areas (Dugger et al. 2016; Figure 1)—the Olympic Peninsula (OLY; ~257,000 ha), Cle Elum (CLE; ~178,000 ha), Oregon Cascades (CAS; ~237,000 ha), H.J. Andrews Experimental Forest (HJA; ~160,000 ha), Tyee (TYE; ~102,000 ha), Oregon Coast Ranges (COA; ~340,000 ha), and Klamath Mountains (KLA; ~138,000 ha)—that were located in 5 ecophysiographic provinces (hereafter, ecoregions) in Oregon and Washington (Davis et al. 2016, Dugger et al. 2016). These ecoregions (Figure 1) were the Washington Coast and Cascades (WCC), Washington Eastern Cascades (WEC), Oregon Coast Range (OCR), Oregon and California Cascades (OCC), and southern Oregon and California Klamath (OCK; Davis et al. 2016). Forest type and species composition varied among ecoregions. In WCC, forests were dominated by western hemlock (Tsuga heterophylla), western redcedar (Thuja plicata), and Sitka spruce (Picea sitchensis) in coastal areas, and Douglas-fir (Pseudotsuga menziesii) in drier inland areas. In OCR, Douglas-fir and western hemlock were dominant. Forests in WEC included a mixture of Douglas-fir, ponderosa pine (Pinus ponderosa), and western larch (Larix occidentalis). The OCC ecoregion was predominantly Douglas-fir forest, and forests in OCK were diverse, including mixed stands of Douglas-fir,

The Condor: Ornithological Applications 120:530–542, Q 2018 American Ornithological Society

532 Natal dispersal of Northern Spotted Owls

FIGURE 1. Locations of long-term demographic study areas and Pacific Northwest (USA) ecoregions used in analyses of Northern Spotted Owl natal dispersal, 1983–2012. Demographic study areas: CAS ¼ Oregon Cascades, Oregon; CLE ¼ Cle Elum, Washington; COA ¼ Oregon Coast Ranges, Oregon; HJA ¼ H.J. Andrews Experimental Forest, Oregon; KLA ¼ Klamath Mountains, Oregon; OLY ¼ Olympic Peninsula, Washington; and TYE ¼ Tyee, Oregon. Ecoregions: OCC ¼ Oregon and California Cascades; OCK ¼ Oregon and California Klamath; OCR ¼ Oregon Coast Range; WCC ¼ Washington Coast and Cascades; and WEC ¼ Washington Eastern Cascades.

western hemlock, and tanoak (Notholithocarpus densiflorus) near the coast and mixed-conifer forests of Douglasfir, ponderosa pine, sugar pine (P. lambertiana), grand fir (Abies grandis), Oregon white oak (Quercus garryana), California black oak (Q. kelloggii), canyon live oak (Q. chrysolepis), and giant chinquapin (Chrysolepis chrysophylla) in the drier interior regions. All ecoregions included a mixture of public and private lands. Banding and Relocation From March through August of 1985 to 2012, and in March of 2013, we conducted surveys to locate owls and nests, band unmarked owls, confirm the bands of previously marked owls, and determine the number of young produced by territorial pairs of Northern Spotted Owls (Forsman et al. 2002, 2011, Anthony et al. 2006). Standardized survey methods using unique color-marked


individuals and repeated observations of nest and roost locations, pair status, and movements among well-defined nesting territories allowed us to identify territorial residents and dispersing owls (Franklin et al. 1996, Forsman et al. 1996, 2002). The sex of each owl banded as a juvenile was determined from postjuvenile vocalizations and behavior (Franklin et al. 1996) or blood samples (Fleming et al. 1996). We also compiled data using the same methods from public and private sources outside demographic study areas. Thus, sampling effort to band juvenile owls primarily occurred within demographic study areas, but postdispersal relocations also came from beyond demographic study area borders. Previous researchers have discussed the difficulty of determining dispersal endpoints when study areas are finite (e.g., Koenig et al. 2000, Lahaye et al. 2001, Zimmerman et al. 2007). Our analyses were based on observed dispersal of owls banded as juveniles from finite demographic study areas, which likely led us to underestimate dispersal distance by an unknown amount (Barrowclough 1978). However, we made the assumption that we captured the majority of dispersal endpoints for birds banded as juveniles because of the intensive region-wide surveys that included public and private sector monitoring efforts throughout the range of the Northern Spotted Owl (Lint et al. 1999, Davis et al. 2011, USFWS 2011). Because we limited our study to individuals banded as juveniles that were subsequently relocated, we were unable to evaluate the fates of individuals that were not relocated. We did not attempt to estimate the survival or emigration rates of natal dispersers, which have been shown to be sensitive to finite area limitations (Zimmerman et al. 2007). We defined the natal location as the center of the territory of parent birds, the natal nest, or the averaged location of fledged young with adults (Dugger et al. 2016). Settling locations were defined as the territory centroids of dispersed owls. Juvenile owls were considered to have dispersed (settled) when we observed a given individual defending the same territory 3 times (Forsman et al. 2002). Dispersal distances were defined as straight-line distances between natal and settling locations (net dispersal). Natal Dispersal Distance Because dispersal distance distributions are typically skewed (Sutherland et al. 2000, Paradis et al. 2002, Kesler et al. 2010, Graves et al. 2014), we used skew-normal t-tests to compare dispersal distances between males, females, and natal ecoregions, adjusted for multiple comparisons as appropriate (Benjamini and Hochberg 1995). In addition, we report median as well as mean distances. We modeled changes in dispersal distance over time (years) using least-squares regression on log-transformed dispersal distances to meet assumptions of normality. The



model included natal ecoregion as well as the interaction of year and natal ecoregion as a test for dissimilar trends in each natal ecoregion. A ‘‘parallel lines’’ model would show similar trends in dispersal distance over time among natal ecoregions. For all analyses of trends through time (years), we excluded data from the first and last few study years (1983–1985 and 2011–2013) because these observations did not have complete dispersal events associated with them (i.e. both natal and subsequent settling location for given birds) or because these years had insufficient observations. Direction of Dispersal We analyzed net dispersal directions as azimuths between natal and settling locations. We analyzed directionality as a function of sex and natal ecoregion (OCC, OCR, OCK, WEC, and WCC). We determined directionality using Rayleigh’s and Watson’s U2 tests (Batschelet 1981). Rayleigh’s tests are robust for unimodal directionality and Watson’s U2 tests are robust for data that exhibit multimodal directionality (Batschelet 1981, Li and Hoffman-Kim 2008). Simultaneous interpretation of these tests thus detects and identifies directional patterns (e.g., unimodal, bimodal): A significant Rayleigh’s and insignificant Watson’s test indicate a unimodal distribution, and a significant Watson’s and insignificant Rayleigh’s test indicate a bimodal distribution (see Li and Hoffman-Kim 2008). We examined circular histograms to confirm directionality and modality. Long-distance Dispersal We defined long-distance dispersers as individuals that traveled 50 km. This distance is equal to that necessary to cross a major geographic (regional-scale) barrier to natal dispersal within our study area, the midsection of the Willamette Valley in western Oregon. Using this criterion did not imply that long-distance dispersers directly crossed major barriers, but rather that the dispersal distances involved were sufficient to bridge potentially isolated groups or populations. We analyzed observed long-distance dispersal distances and directions as a function of sex, natal ecoregion, and time (years). However, we note that the potential for sampling bias increases with dispersal distance (Barrowclough 1978) and, even with the intensive survey and monitoring effort expended throughout the study region, long-distance events were infrequent and likely underestimated. We report observed long-distance dispersal events with the assumption that sampling bias was similar throughout the study region (Washington and Oregon). Barred Owls We did not explicitly survey Barred Owls in the demographic study areas, but recorded their presence when seen or heard from 1990 to 2013 (Wiens et al. 2011).


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We linked Northern Spotted Owl locations with detections of Barred Owls within 0.8 km of natal and settling locations and compared net dispersal distances with and without Barred Owl presence (Kelly et al. 2003, Sovern et al. 2014). Because of imperfect detection of Barred Owl presence, we made comparisons under 2 assumed conditions: (1) Barred Owl presence varied by year (blinked on and off ); or (2) Barred Owl presence was constant after the first year of detection. We were unable to evaluate locations outside demographic study areas because we lacked suitable data. We also considered the general pattern of Barred Owl invasion and, as a surrogate for region-wide presence, developed a function of Barred Owl presence across latitude and time by using first observations of Barred Owls in demographic study areas. We then used a least-squares regression of time (years), latitude, and their interaction on log-transformed net dispersal distances to evaluate regional patterns. RESULTS Of 1,534 owls banded as juveniles that were subsequently relocated as territorial adults (Table 1), nearly all (1,521 owls; 99%) had attained territorial status by their first relocation. Few owls were relocated 2 (12 individuals; 1%) or 3 times (1 bird; ,0.01%) before attaining territorial status. Efforts focused on banding juvenile owls increased from 1983 to 1986 but thereafter remained consistent throughout the study period. The number of owls banded as juveniles (and their relocations) declined in the mid-tolate 2000s, which reflected declines in productivity and territory occupancy (Forsman et al. 2011, Dugger et al. 2016). Because our study period ended in early 2013, there were no relocations of owls banded in 2012. Dispersal Distance Natal dispersal distances ranged from 0.5 to 177.3 km (mean ¼ 23.8 km; median ¼ 18.2 km; Table 2). On average, females dispersed farther (mean ¼ 29.8 6 21.5 SD km) than males (mean ¼ 18.5 6 15 SD km; t1233 ¼ 11.7, P , 0.001; Table 2). Distance distributions varied by natal ecoregion (Table 2, Figure 2). On average, birds from OCC dispersed the farthest, followed by owls from OCR, WCC, WEC, and OCK. Owls banded in OCK dispersed, on average, the shortest distances among southern ecoregions, but not significantly shorter distances than birds from the 2 northern-most ecoregions (WCC and WEC; Table 2). Sex-specific dispersal distances within natal ecoregions showed a pattern of females dispersing farther than males in every ecoregion (OCC: t ¼ 5.73, P , 0.001; OCK: t ¼ 5.13, P , 0.001; OCR: t ¼ 8.17, P , 0.001; WEC: t ¼ 4.55, P ¼ 0.002; WCC: t ¼ 3.18, P , 0.001). Region-wide, annual net dispersal distance declined over time (1986– 2011) at a rate of nearly 1 km year1 (Figure 3), which was




TABLE 1. Number of fledgling Northern Spotted Owls banded at the nest (1985–2011) and relocated as territorial adults, by natal ecoregion and sex (male [M], female [F], and unknown [U]). See Figure 1 for ecoregion codes and locations. Relocated Natal ecoregion OCC OCR OCK WEC WCC Total

M

F

U

Total

167 361 204 53 37 822

147 313 163 57 26 706

3 3 0 0 0 6

317 677 367 110 63 1,534

similar for all ecoregions (year 3 ecoregion interactions: all P 0.23). Dispersal Direction Male owls generally dispersed in a southwesterly direction (2398 azimuth) more often than random, and females generally dispersed in a north–south pattern (Table 3). Owls in the OCC ecoregion dispersed to the southwest (2288 azimuth), and owls in OCR dispersed bimodally (north–south). Juvenile owls in the remaining ecoregions did not show directionality (Table 3). Sex-specific directionality within ecoregion was only significant for females in OCC and males in OCR, with unimodal and bimodal directional distributions, respectively (Table 3). Long-distance Dispersal We observed 124 (8% of total dispersal observations) longdistance dispersal events (.50 km straight-line net dispersal distance). In general, we observed few longdistance dispersals in most ecoregions (Table 4). Among the observed long-distance events, male and female dispersal distance was similar (mean distance: Male ¼ 71.4 km, Female ¼ 72.6 km; t48.9 ¼ 0.26, P ¼ 0.80), and there was no relationship between distance and time (years; t23 ¼ 1.03, P ¼ 0.32). Most observed long-distance dispersal events originated in the southern ecoregions, with only 9 long-distance dispersals originating from the northern ecoregions (WEC and WCC; Table 4, Figure 4). In general, the dispersal

directions of long-distance dispersers followed the coarsescale distribution of suitable habitat, that is, forest dominated by mature conifers within mountain ranges (e.g., the Cascade and Coast Ranges; Figure 4), whereby owls from the OCR and OCC ecoregions showed a bimodal, north–south dispersal pattern and dispersers from the OCK ecoregion showed no directionality. Barred Owls We found no evidence for an effect of Barred Owl presence, at either natal or settling locations, on net dispersal distance within demographic study areas (WelchSatterthwaite t-test for unequal variance; Table 5). We found a significant trend in the first year of observation of Barred Owls in demographic study areas as a function of latitude (Figure 5). However, we found no evidence that net dispersal distance was related to this surrogate measure (latitude 3 time) of invasion (t1530 ¼ 0.312, P ¼ 0.76). DISCUSSION We found variation in natal dispersal distances and directions of Northern Spotted Owls over 28 yr of observations. Natal dispersal in northern ecoregions consisted of mostly short-distance dispersal and few long-distance dispersal events. Meanwhile, we found that owls in southern ecoregions dispersed longer distances (with the exception of OCK). Dispersal directions among dispersing juvenile Northern Spotted Owls also varied by ecoregion, with northern ecoregions showing no trends but southern ecoregions having significant directional trends toward the southwest or approximately north– south patterns, often coinciding with the distribution of forested areas along mountain ranges. Net dispersal distances declined over time (years) at a similar rate (~1 km yr1) among ecoregions. We confirmed the previously reported north-to-south invasion of Barred Owls over the duration of our study (Kelly et al. 2003, Anthony et al. 2006). However, we did not find direct evidence of a relationship between detections of Barred Owls at natal locations and Northern Spotted Owl natal dispersal in any ecoregion

TABLE 2. Mean and median dispersal distances (km) of juvenile Northern Spotted Owls by sex (male [M], female [F], and unknown [U]) and natal ecoregion (1985–2012). Ecoregions that share the same superscript letters are not significantly different (P , 0.05). See Figure 1 for ecoregion codes and locations. OCC

Median Mean SD n

abde

OCR

abde

OCK

cde

WEC

abcde

WCC

abcde

Overall

F

M

U

M

F

M

F

M

F

M

F

M

F

18.2 23.8 19.2 1,534

24.3 29.8 21.5 706

14.4 18.5 15.0 822

24.2 33.6 19.3 6

15.21 20.39 16.88 167

26.57 32.57 21.12 147

15.12 19.88 14.91 361

27.83 31.67 21.85 313

12.90 15.81 14.85 204

19.22 24.59 22.56 163

12.21 14.54 8.52 53

23.06 28.01 17.89 57

13.11 16.99 12.88 37

24.14 28.26 15.12 26




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FIGURE 2. Natal dispersal distance distributions for Northern Spotted Owls in the Pacific Northwest, USA, during 1986–2012: (A) Oregon Coast Range (OCR), (B) Washington Eastern Cascades (WEC), (C) Oregon and California Klamath (OCK), (D) Washington Coast and Cascades (WCC), and (E) Oregon and California Cascades (OCC). The solid blue line shows the fitted distribution, and the dashed vertical line indicates the threshold distance for long-distance dispersal (50 km).

(Table 5). Spatially and temporally suitable data that explicitly relate Barred Owl presence and Northern Spotted Owl natal dispersal (Livezey and Fleming 2007, Wiens et al. 2011) may be required to determine such relationships.

Dispersal Distance The natal dispersal distances that we documented were similar to those reported by Forsman et al. (2002), with females dispersing farther than males. We found that dispersal distances varied with natal ecoregion (Table 2),



FIGURE 3. Annual mean (filled circles) and median (open circles) natal dispersal distances for Northern Spotted Owls in the Pacific Northwest, USA, 1983–2012. Values are shown for years for which adequate data were available (1986–2010). The negative trend indicates a decline of ~1 km yr1 in mean dispersal distance over the study period.

but declined over time at a similar rate (~1 km yr1; Figure 3) in all ecoregions. The reason for this is unclear. Juveniles appeared to be dispersing shorter distances before attaining territorial adult status, perhaps because of the availability of nearby territories released by a declining adult population, the tighter packing of occupied territories (possibly by both Barred and Northern Spotted owls), or the ability to detect conspecifics as a cue to suitable settling areas (e.g., Seamans and Gutiérrez 2006). Declining dispersal distances may reflect changing territory dynamics in the presence of Barred Owls, whereby adult Northern Spotted Owls tolerate increased territory overlap (among conspecifics) as Barred Owls move into and occupy an area (Loschl 2008). Among Oregon ecoregions, Glenn et al. (2004) noted that territories in the OCR ecoregion were smaller than expected, which may reflect these dynamics. With lower and more variable juvenile survival, in particular among northern ecoregions (Loschl 2008), individuals that engage in prolonged prospecting (wandering) may be less likely to survive to become


territorial adults at farther distances from the natal territory. Juvenile owls moving through low-quality habitat (young forest and/or unforested habitats) are reported to have lower survival (Miller et al. 1997, Forsman et al. 2002), and presence of Barred Owls may further negatively affect survival during prospecting through competition for limited prey and nesting resources (McCarthy 1999). Increasing fragmentation of forested habitats is likely to influence these processes. Turchin (1998) suggested that first-year dispersing juvenile Northern Spotted Owls may use a hierarchical, multiscale movement strategy, in which owls maintain temporary home ranges (,3 km2) and make repeated exploratory (prospecting) forays from these temporary home ranges (3 to 6 km) before making determined dispersal movements (.6 km) that constitute actual net dispersal. We found that most individuals dispersed ,20 km (net distance) before settling as territorial adults, suggesting that many owls made only a few deliberate dispersal movements (steps) from the natal territory before settling. Because of the spatial variation associated with relocating individuals (i.e. spatial error associated with estimated activity centers), we were unable to fully evaluate the role of natal philopatry (Doerr and Doerr 2005) in short-distance dispersal. However, many dispersers settled in territories within ~13 km of their natal territory, suggesting that many juveniles may benefit from remaining in known conditions similar to those in the natal territory, and corroborating the important role of exploratory forays adjacent to the natal territory (e.g., Turchin 1998, Forsman et al. 2002). Genetic analyses by Funk et al. (2010) found evidence for significant population bottlenecks in Northern Spotted Owls, suggesting that decreasing dispersal distances may further restrict gene flow and contribute to genetic drift in this subspecies. While the phenotypic result of lower gene flow has not been identified, we think that the trend is noteworthy, and it may be useful to distinguish the relative roles that short- and long-distance dispersal play in the movement of genetic material in the context of Northern Spotted Owl conservation. Dispersal Direction We found directional patterns of natal dispersal that often coincided with the regional distribution of forested habitat and potential dispersal barriers (e.g., the Willamette Valley; Forsman et al. 2002, Davis et al. 2011). Previous analyses using combined data from fewer ecoregions (Forsman et al. 2002) did not find directional trends in natal dispersal. Our analysis of 5 ecoregions and 28 yr of observations suggested that owls in the Oregon Coast Range and Oregon and California Cascades dispersed bimodally (north and south), following forested habitat along these ranges (Sovern et al. 2014), and that owls in the OCK




537

TABLE 3. Circular statistics testing directionality (against random uniform) of Northern Spotted Owl natal dispersal in Washington and Oregon, USA, 1985–2011. See Figure 1 for location codes and locations.

Male Female * OCC OCK OCR * WCC WEC OCC Male Female OCR Male * Female OCK Male Female WEC Male Female WCC Male Female

Mean azimuth *

Azimuth SD

Rayleigh’s z-statistic

Rayleigh’s P-value

Watson’s statistic

Watson’s P-value

238.67 — 228.26 — — — —

2.27 — 2.08 — — — —

0.076 0.041 0.115 0.066 0.043 0.159 0.095

0.009 0.31 0.02 0.21 0.29 0.20 0.37

0.295 0.236 0.388 0.136 0.294 0.118 0.072

P P P P P P P

— 219.0

— 2.0

0.093 0.147

0.24 0.04

0.162 0.287

P , 0.10 P , 0.01

— —

— —

0.077 0.045

0.12 0.52

0.254 0.147

P , 0.03 P . 0.10

— —

— —

0.104 0.124

0.11 0.08

0.179 0.153

P , 0.10 P , 0.10

— —

— —

0.074 0.128

0.75 0.39

0.067 0.083

P . 0.10 P . 0.10

— —

— —

0.144 0.180

0.47 0.44

0.115 0.071

P . 0.10 P . 0.10

, , , . , . .

0.01 0.03 0.01 0.10 0.01 0.10 0.10

* Mean azimuth not interpretable for bimodal distribution of dispersal directions.

ecoregion, where regional forested habitat features were less defined, dispersed randomly, similarly to patterns reported for dispersal in northern California (Gutiérrez and Carey 1985). Thus, owls may respond to regional habitat configuration, supporting coarse-scale, habitatbased approaches to facilitating natal dispersal, in particular among disparate populations (Lamberson et al. 1994, Schumaker 1995).

Long-distance Dispersal Long-distance dispersal (i.e. net dispersal distance 50 km) was uncommon (8% of all dispersal events) and was observed more frequently in southern ecoregions (Figure 4). Observed dispersal distances ranged up to 177 km, showing long-distance dispersal potential similar to that reported for Northern Spotted Owls in northern California (up to 156 km; Gutiérrez and Carey 1985) and greater than

that reported for Mexican Spotted Owls (Strix occidentalis lucida) in Arizona (72 km; Ganey et al. 1998). Net dispersal segments (Figure 4) may suggest dispersal across potential barriers (i.e. the Willamette, Umpqua, and Rogue river valleys). However, we note that individuals may have circumnavigated these barriers via multiple movements, or alternatively may have traveled through narrow corridors of suitable habitat (e.g., Forsman et al. 2002). Long-distance dispersers in the OCR and OCC ecoregions dispersed in north–south directions, following the orientation of their respective mountain ranges. There were too few long-distance dispersers in northern ecoregions to determine directional trends. Pairwise comparisons of genetic differentiation (FST values) and geographic distances for Northern Spotted Owl populations throughout Washington and Oregon suggested that only the Oregon Klamath Mountains were in equilibrium with respect to gene flow and genetic drift

TABLE 4. Long-distance Northern Spotted Owl natal disperal distances (km) by sex and natal ecoregion in the Pacific Northwest, USA, 1985–2011. See Figure 1 for ecoregion codes and locations. OCC

Median Mean SD n

OCR

OCK

WEC

WCC

Male

Female

Male

Female

Male

Female

Male

Female

Male

Female

72.8 79.1 25.2 7

59.9 67.9 17.0 26

63.5 64.5 10.8 18

67.3 70.5 18.5 49

86.1 99.5 34.5 4

71.8 88.6 40.1 11

— — — 0

67.3 65.7 7.8 6

65.0 65.0 NA 1

62.3 62.3 15.6 2




FIGURE 5. Latitude (degrees North) and year in which Barred Owls were first observed in demographic study areas (see Figure 1) in the Pacific Northwest (Washington and Oregon), USA. The linear trend suggests a rate of invasion of 14 km yr1: Latitude ¼ 293.4 – 0.12(Year).

FIGURE 4. Long-distance dispersal segments (50 km) for juvenile Northern Spotted Owls in Washington and Oregon, USA, 1986–2012. Gray-shaded regions are relatively unforested areas (major river valleys) that present potential regional dispersal barriers.

(Haig et al. 2001). Conversely, genetic population structure in Washington populations showed recent genetic bottlenecks and increased vulnerability to inbreeding (Funk et al. 2010). We noted some long-distance dispersal into northern California from the OCK ecoregion (Figure 4), which may have implications for the stability of the hybridization zone between Northern and California Spotted Owls and potential introgression. Concurrent evaluation of California Spotted Owl dispersal is of interest as studies suggest more northerly movement into Oregon (Haig et al. 2004, Barrowclough et al. 2005, Funk et al. 2008). Long-distance dispersal is a risky and relatively uncommon event that has disproportionately important implications for (meta)population persistence (Harrison 1989, Trakhtenbrot et al. 2005). Genetic exchange between separated populations facilitates rescue of isolated, inbred populations, as well as colonization and recolonization of available habitat (Mills and Allendorf 1996). For Northern

Spotted Owls, understanding long-distance dispersal in the context of Barred Owl presence may be crucial as the interplay between these competitors may influence the dispersal strategies of both species and, consequently, population persistence (see Muller-Landau et al. 2003, Trakhtenbrot et al. 2005). However, because long-distance dispersal is an infrequent event that typically involves great distances, it may be difficult to collect adequate and appropriate data. Long-term, coarse-scale datasets, such as the demographic monitoring effort used in this study, may provide a starting point for such efforts, but explicit sampling schemes that address potential bias (e.g., Barrowclough 1978) are needed. Barred Owls We expected Barred Owls to influence natal dispersal, particularly in areas with high densities of Barred Owls (e.g., Yackulic et al. 2012), through negative interactions between the 2 species as juvenile Northern Spotted Owls explored habitat adjacent to the natal territory (Wiens et al. 2016, 2017), but we were unable to determine the existence of such an influence. Regardless, the negative impacts of Barred Owls on territorial adult Northern Spotted Owls (e.g., demographic processes and territory occupancy; Olson et al. 2005, Gutiérrez et al. 2007, Hamer et al. 2007, Pearson and Livezey 2007, Dugger et al. 2011,




539

TABLE 5. Comparison of Northern Spotted Owl natal dispersal distances (km) with and without Barred Owl presence within 0.8 km of natal or settling location in the Pacific Northwest, USA, 1985–2011. Time-varying ¼ Barred Owl presence varied by year. Persistent ¼ Barred Owl presence assumed to continue after first year detected. Ecoregion names and locations can be found in Figure 1. Time-varying

Persistent

Location

nwith

nwithout

t

P

nwith

nwithout

t

P

Regionwide Natal Regionwide Settling OCR Natal OCR Settling OCC Natal OCC Settling OCK Natal OCK Settling WCC Natal WCC Settling WEC Natal WEC Settling

73 164 49 96 8 29 5 20 4 3 7 16

1,174 1,009 507 419 242 237 307 245 33 36 85 72

1.08 0.61 1.08 1.50 0.14 0.82 1.39 0.03 0.60 1.04 1.42 0.42

0.29 0.55 0.48 0.68 0.90 0.70 0.48 0.98 0.71 0.70 0.48 0.85

288 391 161 201 45 82 33 47 8 11 41 50

959 782 395 314 205 184 279 218 29 28 51 38

0.08 0.83 1.10 1.42 1.50 1.30 1.53 1.11 0.16 0.89 1.23 0.47

0.94 0.41 0.34 0.45 0.34 0.45 0.34 0.45 0.88 0.48 0.34 0.64

Sovern et al. 2014, Diller et al. 2016), including local extinction and colonization processes (Yackulic et al. 2012), warrant further study of Barred Owl impacts on the natal dispersal process (Livezey and Fleming 2007). In general, the coarse-scale geographic configuration of suitable habitat (coniferous forests associated with mountain ranges) appeared to influence dispersal directions and dispersal distances of Northern Spotted Owls. Decreasing natal dispersal distances over the past 2 decades coincided with reported negative demographic changes occurring in Northern Spotted Owl populations. The precise cause of this decline is difficult to determine. Although we were unable to demonstrate a correlation, Northern Spotted Owl dispersal dynamics may have been related to the invasion of Barred Owls, because there is growing evidence of their documented negative effects on territorial adult Northern Spotted Owls. Therefore, an explicit understanding of the relationships between Barred Owls, habitat loss and fragmentation, and long-distance dispersal may better inform management actions for the conservation of the Northern Spotted Owl. ACKNOWLEDGMENTS We are especially indebted to Drs. Chuck Meslow and Len Ruggerio, who helped to initiate the long-term demographic studies of Spotted Owls in Oregon and Washington, and Drs. Robert Anthony and Katie Dugger, who continued these studies after the old-timers retired. We thank J. Matthew Johnson for helping to organize .25 yr of dispersal data. We also thank Elizabeth Glenn and Carrie Phillips for comments on the paper. This study would not have been possible without the help of many dedicated field biologists in Washington and Oregon. Robert Anthony, Rich Fredrickson, Patti Happe, Joe Lint, Chuck Meslow, Bruce Moorhead, Len Ruggerio, and Erran Seaman were instrumental in getting many of the studies started. Other key field people include: Steve Ackers,

Steve Adams, Meg Amos, Steve Andrews, Doug Barrett, Greg Bennett, Brian Biswell, Rita Claremont, Summer Cross, Roli Espinosa, Ernie Fliegel, Ray Forson, Chris Foster, Laura Friar, Ken Fuckuda, Scott Graham, Scott Gremel, Dale Herter, Rob Horn, Tom Kaufmann, Debaran Kelso, Chris Larson, Richard Leach, Pete Loschl, Rich Lowell, Travis Mackie, Chris McAfferty, Brian Meiering, Gary Miller, Jason Mowdy, Matt Nixon, Mary Oleri, Frank Oliver, Ivy Otto, Tom Phillips, Tim Plawman, Marlin Pose, Amy Price, Janice Reid, Melanie Roan, Paula Shaklee, Kristian Skybak, Steve Small, Alexis Smoluk, Tom Snetsinger, Stan Sovern, Denise Strejc, Keith Swindle, Jim Swingle, Margy Taylor, Jim Thrailkill, Trinity Tippin, Sheila Turner-Hane, Frank Wagner, Kari Williamson, Heather Wise, and Joe Witt. Any use of trade, product, or firm names is for descriptive purposes only and does not imply endorsement by the U.S. Government. Funding statement: Funding for this project was provided by Sierra Pacific Industries, the USDI (U.S. Department of the Interior) USGS (U.S. Geological Survey) Forest and Rangeland Ecosystem Science Center, USDA (U.S. Department of Agriculture) Forest Service, and USDI Bureau of Land Management. No approval from funders was required for the submission of this manuscript. Ethics statement: This research presented here was conducted in compliance with the Guidelines to the Use of Wild Birds in Research. Author contributions: J.P.H. and S.M.H. developed questions and analytical approach; S.M.H. supervised analyses and manuscript preparation; J.D.W. and E.D.F. supervised data collection; J.D.W. assisted with manuscript preparation; and J.P.H. analyzed data and assisted with manuscript preparation.

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