Review of the Science Directly Related to the Effects of Barred Owls on Spotted Owls
This blog post was based on a report prepared for the Western Klamath Restoration Partnership (WKRP) and can be downloaded here. Addiitonally, this blog post served as the basis for a paper we published in the Journal of Wildlife Management.
Barred Owls (Strix varia) and Spotted Owls (Strix occidentalis) are both large, forest-dwelling predators. Although both Barred and Spotted Owls rely on forested landscapes, the Barred Owl exhibits a more diverse diet – consuming mammals, birds, fish, amphibians, and invertebrates (Mazur and James 2000) – compared to Spotted Owls that specialize on mammalian prey (Gutiérrez et al. 1995, Hamer et al. 2001, Wiens et al. 2014). Unlike the Barred Owl, the Spotted Owl is emblematic of controversy surrounding their dwindling numbers, protected status, and reliance on economically-valuable timberlands (Keane 2017). Because Barred Owls exert a disproportionately negative influence on Spotted Owl fitness where the two species co-occur, the continuing expansion of Barred Owls into the range of the Northern and California Spotted Owl has presented emergent and difficult challenges for wildlife managers.
In this blog, we aimed to summarize the existing published research on Barred Owls within the range of the Spotted Owl as well as identify future research priorities. Broadly, we provide synthesized information for researchers and managers to help develop strategies focused on mitigating the deleterious effects of Barred Owl expansion in western North America.
Westward Expansion of the Barred Owl
Before the second half of the last century, the ranges of the Spotted Owl and Barred Owl did not overlap. The Spotted Owl occupied the western edge of North America (Gutiérrez et al. 1995) and the Barred Owl occupied the eastern edge (Mazur and James 2000), where the Great Plains divided the two species. Barred Owls began to expand their range across the midwestern United States and central Canada in the early part of the last century (Livezey 2009a). There are several non-mutually exclusive hypotheses regarding what led to this westerly expansion, including: climate change resulting in warmer weather (Johnson 1994, Monahan and Hijmans 2007), changes in the Great Plains such as increased woody development in the form of tree plantings for shelterbelts and fire control (Knopf 1994, Livezey 2009a, 2009b), and other habitat manipulations (Root and Weckstein 1994).
After crossing central North America, the Barred Owl expanded its range into the Pacific Northwest where the first individual was observed in Alberta in 1912 (Boxall and Stepney 1982), British Columbia in 1943 (Grant 1966), eastern Washington 1966 (Rogers 1966), western Washington 1986 (Sharp 1989), Oregon 1974 (Taylor and Forsman 1976), northwestern California 1982 (Evens and LeValley 1982), and as far south as Marin County in California by 2002 (Jennings et al. 2011) and the Sierra Nevada Mountains in eastern California in 1991 (Dark et al. 1998). The western Barred Owl range now overlaps the range of the Northern Spotted Owl (S. o. caurina) (Livezey 2009a) and the California Spotted Owl (S. o. occidentalis).
The Barred Owl has yet to expand into the range of the California Spotted Owl in the densities seen in the Northern Spotted Owl range (Keane 2017). As of 2013, 51 Barred Owls and 27 hybrids had been detected in the Sierra Nevada range, though none in the coastal or southern California parts of their range. Barred Owl numbers remain relatively low in the California Spotted Owl range when compared to the Northern Spotted Owl range, though contemporary survey efforts for Barred Owls remain incidental (Keane 2017).
Barred Owl influence on Spotted Owl habitat use and selection
As Barred Owls expanded their range across western North America, their newly established populations in British Columbia, Washington, Oregon, and northern California continued to grow (Dark et al. 1998, Davis et al. 2016, Jones and Kroll 2016, Kelly et al. 2003, Pearson and Livezey 2003). Pervasive expansion and population growth continued until only a few isolated areas in the range of the Northern Spotted Owl remained without Barred Owls (Kroll et al. 2016). Barred Owl expansion eventually reached a critical threshold in many parts of the northern Spotted Owl range: the number of territorial Barred Owls surpassed Spotted Owl territories (Fig. 1, Lesmeister et al. 2016).
Figure 1. Proportion of spotted owl sites in which barred owls and spotted owls were detected on the Oregon Coast , 1990–2015. From Lesmeister et al. (2016).
To date, few studies have tried to estimate Barred Owl population size. In one such case, Kelly (2001) estimated there were a total of 706 Barred Owl territories in Oregon by 1998, slightly more than 20 years after the first Barred Owl was recorded in Oregon in 1974. However, Gutiérrez et al. (2004) stated that detection methods which reported cumulative detections may lead to over estimates of Barred Owl populations. On the other hand, Gutiérrez et al. (2004) also stated that most Barred Owl sightings are reported incidentally during Spotted Owl surveys, suggesting that they may be more abundant than estimated. A new modelling framework developed by Zipkin et al. (2017) has leveraged these detection-nondetection data along with count data to estimate population dynamics, abundance, and individual detection probabilities from sampling Barred Owls in the Oregon Coast Ranges. They estimated that the mean site-specific number of 0.13 territorial Barred Owls in 1995 increased to 7.5 owls in 2016, with survival probabilities of 0.86-0.93 and an increased colonization rate of 0.14 in 1996 to 0.90 in 2016. Developing and implementing Barred Owl surveys represents an important step towards fully understanding how environmental variation affects Barred Owl population growth.
[endif]--Both Barred and Spotted owls utilize old-growth forests. While four researchers found no differences between species’ use of different age classes and composition of forests (Buchanan et al. 2004, Pearson and Livezey 2007, Singleton 2015, Singleton et al. 2010), others found Spotted Owls use older forests at greater frequencies than Barred Owls. For example, Hamer et al. (2007) reported that the home ranges of Spotted Owls in Washington were negatively influenced by the lack of old forest, that is, home ranges with less old forest were larger, indicating Spotted Owls may increase their range to increase amount of old forest in their territory. This was only slightly true for Barred Owls. Also in Washington, Herter and Hicks (2000) found that Spotted Owl territories contained more mature coniferous forests than Barred Owl territories and Pearson and Livezey (2003) found the mean age of forest stand at a site-center was significantly greater for Spotted Owls (254.7 ± 76.5 yr) than for Barred Owls (228.3 ± 101.5 yr). In Oregon, Wiens et al. (2014) found that both species used patches of old (>120 yr) coniferous forest in proportions two to three times greater than available in the study area (Table 1). In northern California, Weisel (2015) researched the owls’ use of the understory components of the coastal redwood ecosystem and found that both species selected habitats with understory vegetation, hardwood trees, and close to their nest sites; however, Barred Owls were more likely to select foraging habitat that contained a greater percentage of hardwoods than Spotted Owls only up to a certain volume, when the stand becomes too dense to support successful foraging, suggesting that Spotted Owls are better able to utilize dense understory for foraging. Similarly, Irwin et al. (2017) found that Barred Owls selectively hunted for prey near streams at low elevations, often within hardwood-dominated dominated stands, but use decreased with increasing densities of small-diameter trees.
Table 1. Mean values of environmental conditions measured at foraging and roosting locations used by individual Northern Spotted Owls or Barred Owls as compared to a set of random locations plotted in the western Oregon study area, USA, 2007–2009. Forest types are expressed as the mean percentage of total foraging, roosting, or random locations. We show sample sizes (number of individual owls or random points) in parentheses. From Wiens et al. (2014).
Habitat around nest sites in the Cascades were found to be similar between the species (Buchanan et al. 2004, Pearson and Livezey 2003). However, Buchanan et al. (2004) found differences in nest trees and nest configurations selected by each species. In Washington, Spotted Owls almost always placed nests in Douglas-firs (Psuedotsuga menziesii) (9 of 10 nests), while Barred Owls used five different tree species, including three black cottonwoods (Populus trichocarpa), three Douglas-firs, and two grand firs (Abies grandis) (Buchanan et al. 2004). Most of the trees used by Spotted Owls were alive and intact (7 of 10) while eight trees used by Barred Owls had broken boles (six alive, two dead). Also, Spotted Owls primarily used platform nests consisting of clumps of branches or goshawk nests (8 of 10), while Barred Owls used cavities or sites with chimney-like structures. Allen (1987) noted that Barred Owls in the east required large, decadent trees with cavities for nesting, though Livezey (2007) found 25% used other locations such as hawk nests, tops of hollow trees, and squirrel nests throughout their range.
Barred Owls appear to have a negative effect on the Spotted Owl’s ability to utilize preferred habitat. For example, Davis et al. (2016) modeled habitat suitability in the Tyee density study area in Oregon and found a strong negative correlation (r = −0.894) between increasing trend of Spotted Owl territories with Barred Owls and the average Habitat Suitability Index at annual Spotted Owl locations. The 2013 index was significantly lower than it was in 1990 when the study began with Barred Owls occurring in low numbers. Pearson and Livezey (2003) found that Late Successional Reserves (LSRs) in Washington were getting more use by Barred Owls than Spotted Owls for whom they were set aside. They found 34% more Barred Owl sites than Spotted Owl sites in LSRs, while there were 33% more Spotted Owl than Barred Owl sites in non-reserve lands.
The greatest difference in habitat use between Spotted and Barred Owls is an apparent dissimilar use of topographies by the two species: Spotted Owls used steeper, higher-elevation sites while Barred Owls used flatter, low-elevation sites – sometimes along streams (Hamer et al. 2007, Herter and Hicks 2000, Pearson and Livezey 2003, 2007, Wiens et al. 2014) (Table 1). Pearson and Livezey (2007) found that elevation and slope were important factors in explaining densities of Spotted Owls in Washington, when combined with measures of forest quality, forest age, distance to water, and abundance and availability of prey. They posited that the persistence and higher numbers of Spotted Owls in one LSR, in spite of the invasion of Barred Owls in other LSRs, may indicate that there are local environmental factors such as elevation and slope that favor Spotted Owls over Barred Owls, and that a natural balance had been achieved in their study area which allowed the coexistence of these two species.
Influence of Barred Owls on Spotted Owl Occupancy, Survival, and Fecundity
As Barred Owl populations increase, they can either form new territories in Spotted Owl territories without displacing Spotted Owls (Wiens et al. 2014), actively displace resident Spotted Owls from their territories (Dugger et al. 2011, Kelly et al. 2003, Pearson and Livezey 2003, Sharp 1989), or reduce Spotted Owl occupancy rates (Kelly et al. 2003, Kroll et al. 2010, Pearson and Livezey 2003). When Barred Owls establish territories inside Spotted Owl territories, some Barred Owl territories may be contained entirely within Spotted Owl territories , since the estimated size of Spotted Owl territories can be more than six times the size of Barred Owl territories (Table 2) (Hamer et al. 2007, Wiens 2012, Wiens et al. 2014). Wiens et al. (2014) found that each individual Spotted Owl shared a portion of its home range, usually foraging areas, with 0-8 Barred Owls in adjacent territories (average = 2.4 Barred Owls per Spotted Owl). Spotted Owls were less likely to use an area if it was within or near a core-use area of a Barred Owl (Wiens et al. 2014). By contrast, in coastal northern California, Weisel (2015) found no significant difference in average home range size between the two species in either the breeding or non-breeding seasons, however Spotted Owl territories tended to be slightly larger (Table 2).
Table 2. Home range sizes (mean ha ± SE) for Northern Spotted Owls and Barred Owls in Washington, Oregon, and northern California. Hamer et al. (2007) estimates were calculated using 95% Adaptive Kernel Method, all others used 95% Fixed Kernel Method.
One of the earliest incidents of “displacement” of a Spotted Owl by a Barred Owl was recorded in Washington state by Sharp (1989). In 1985, Sharp recorded the first Barred Owls on the Olympic Peninsula. The following year, two Barred Owl pairs had apparently displaced two Spotted Owl pairs from their territories.
The presence of Barred Owls yields a significant effect on Spotted Owl colonization and extinction rates. Five of the six studies that estimated colonization probabilities found a negative effect of Barred Owls on colonization, though it was sometimes weak (Dugger et al. 2011, 2016; Kroll et al. 2010; Olson et al. 2005; Yackulic et al. 2014). Sovern et al. (2014) found no effects of Barred Owls on colonization rates. Dugger et al. (2016) found the presence of Barred Owls resulted in a lower local colonization rate in five of eleven study areas in the Northern Spotted Owl range. Also, Yackulic et al. (2014) found that two of their top four models for the Tyee study area suggested that Spotted Owls were less likely to colonize an area that was already occupied by Barred Owls, while, conversely, Barred Owls were more likely to colonize areas already occupied by Spotted Owls. Dugger et al. (2011) found that Barred Owls displaced Spotted Owls from historical breeding territories in southern Oregon, as Spotted Owl site occupancy was lower where Barred Owls were detected as compared to sites where Barred Owls were not detected (Fig. 2). Similarly, Kelly et al. (2003) found that mean annual Spotted Owl occupancy declined after Barred Owls were detected within 0.80 km of territory centers in Oregon and Washington, as compared to Barred Owl-absent territories. Pearson and Livezey (2003) noted about 20% of 129 Spotted Owl sites surveyed from 1996-2001 were unoccupied by Spotted Owls by 2001 in southwestern Washington; they determined there were significantly more Barred Owl site-centers in unoccupied than occupied Spotted Owl home range circles. Kroll et al. (2010) documented lower occupancy probabilities when Barred Owls were presences throughout their study sites in eastern Washington.
Figure 2. Estimates of mean annual site occupancy generated across all Northern Spotted Owl territories in southern Oregon from 1991 to 2006. From Dugger et al. (2011).
Spotted Owl extinction probabilities increased when Barred Owls were present in all six studies for which they were estimated (Dugger et al. 2011, 2016; Kroll et al. 2010; Olson et al. 2005; Sovern et al. 2014, Yackulic et al. 2014). Dugger et al. (2016) found higher extinction rates for the Northern Spotted Owl in all study areas in its range when Barred Owls were present. Olson et al. (2005) noted that the increase in local extinction probabilities affected occupancy probabilities, leading to occupancy rate declines of up to 15% for Spotted Owls. When the two species were modeled together, Yackulic et al. (2014) found that extinction probabilities increased for each species when the other was present, suggesting a strong role of competition in occupancy dynamics.
Simulations have yielded concordant results. For example. Yackulic et al. (2014) found a strong role of intraspecific competition in structuring occupancy dynamics in their modelling. Their simulations suggested competition has a much more substantial impact on the equilibrium occupancy values of Spotted Owls than that of Barred Owls. These simulations also suggest that competition at the patch scale led to increased rates of local extinction for Spotted Owls and, though this would probably not directly drive competitive exclusion from territories, it would result in reduced equilibrium occupancies. Dugger et al. (2011) noted that the strong negative effect of the Barred Owl on occupancy dynamics of Northern Spotted Owls provided evidence of interference competition. By contrast, Bailey et al. (2009) modeled the co-occurrence of Spotted and Barred owls using a two-species occupancy model and found no evidence that Barred Owls excluded Spotted Owls from territories. However, the two species co-occurred less often than expected. Multiple field studies coupled with simulations demonstrate that interspecific competition occurs between Barred and Spotted Owls which has lead to reduced colonization coupled with heightened extinction probabilities of Spotted Owls.
The effect of Barred Owls on Northern Spotted Owl fecundity and recruitment has been mixed. Three of six studies showed no effect of the presence of Barred Owls on Spotted Owl fecundity (measured as number of young fledged per female per year; Anthony et al. 2006, Dugger et al. 2016, Iverson 2004). The other three studies showed a negative, though not always strong, effect (Glenn et al. 2010, 2011b; Olson et al. 2004). Glenn et al. (2010) found that Spotted Owl recruitment rates were diminished in the presence of Barred Owls in four of six study areas. This mixed result may point to other factors influencing fecundity, for instance Wiens (2012) found that the number of young fledged by Spotted Owls increased linearly with increasing distance from the nearest Barred Owl nest or territory center. When Spotted Owl young fledged, Barred Owl detections had no direct relationship with natal or settling locations or dispersal distances (Hollenbeck et al. 2018). They also found that net dispersal distances varied by ecoregions (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/yr).
Spotted Owl survival was mostly negatively affected by the presence of Barred Owls. Of six studies that analyzed apparent or actual survival, two showed definite negative effects of Barred Owl presence on Northern Spotted Owl survival estimates (Dugger et al. 2016, Glenn et al. 2010), three showed weak negative effects (Anthony et al. 2006, Glenn et al. 2011a, Wiens 2012), and one showed no effect (Schilling et al. 2013).
Spotted and Barred owl diets overlap to some degree in all regions. Hamer et al. (2001) and Wiens et al. (2014) used the Pianka Index to estimate a dietary overlap of 76% in the western Cascades Range in Washington and 44.6% in the central Coast Range in Oregon, respectively. In both studies, Spotted Owl diets were heavily dependent on mammals, which were about 95% of the prey items consumed, especially Northern Flying Squirrels (Glaucomys sabrinus); and while more than half of Barred Owl diets depended on mammals including the Northern Flying Squirrel, they also included a greater percentage of smaller mammals like shrews (Sorex and Neurotrichus spp.) and moles (Scapanus spp.), as well as birds, amphibians, and arthropods (Table 3).
Table 3. Select Barred Owl and Spotted Owl prey items found in regurgitated pellets from three studies in Washington and Oregon. Percent of total number of items identified in owl pellets are listed, with number of pellets sampled in parentheses. No sample size was reported from Hamer et al. (2001).
Barred Owl diets also varies between regions, demonstrating a broad range in prey. Graham (2012) examined Barred Owl pellets from three study areas: Olympic National Forest and the eastern Cascades in Washington, and the Central Coast Range in Oregon. He found that their diet was considerably different between the two western mountain range study areas and the eastern Cascade Range, which is considerably drier and hotter (Table 3). In the eastern mountains, Barred Owls did not depend as much on mammals which was only 26.5% of their diet, but instead relied more on arthropods (47%), as well as amphibians and reptiles (11.1%). Livezey et al. (2008) found that Barred Owls will also eat soft-bodied prey such as earthworms and slugs, which would not be easily detected in regurgitated pellets, upon which these studies depended.
Barred Owls are known to be predators of other owl species, including Strix species on rare occasions (Graham 2012, Wiens 2012). Graham found the remains of two unidentified Strix species near Barred Owl roost trees or nests. Wiens found the remains of two Spotted Owls cached beneath fallen logs with wounds consistent with those inflicted by a large raptor. He was unable to rule out they were not killed by Barred Owls, since Great Horned Owls (Bubo virginianus) were also in the vicinity. Leskiw and Gutiérrez (1998) found a freshly dead Spotted Owl that may have been killed by a Barred Owl. The Barred Owl that flew in next to the authors in response to a Spotted Owl call had feathers similar to those of a Spotted Owl in its talons.
When Spotted and Barred owls interact, the Barred Owl most frequently assumes the dominant role (Van Lanen et al. 2011). Van Lanen et al. conducted experiments using playback tapes and mounted owl taxidermy mounts for both species. They found that male Barred Owls gave more aggressive calls and were more likely to attack the Spotted Owl mount, while male Spotted Owls were less likely to give aggressive calls or attack the Barred Owl mount. Gutiérrez et al. (2004) reported instances where Barred Owls have attacked Spotted Owls, as well as surveyors imitating Spotted Owl calls. Wiens (2012) reported regular interspecific territorial interactions between newly colonizing Barred Owls within the breeding home ranges of Spotted Owls. Interactions included agitated vocalizations by both species near nest sites and Barred Owls chasing Spotted Owls out of shared core-use areas (but not the opposite). In California, Jennings et al. (2011) reported a Barred Owl chasing a female Spotted Owl.
Conversely, there are few observations of Spotted Owl aggressions towards Barred Owls, with the exception of a few reports of nesting Spotted Owls defending a nest or a family group (Gutiérrez et al. 2004) and Jennings et al.’s (2011) report of a pair of Spotted Owls charging and diving at a Barred Owl and an “aerial clash” between a Spotted and Barred Owl (though they did not report how that interaction started).
Crozier et al. (2006) reported that both California and Northern Spotted Owls responded less frequently to Spotted Owl calls after exposure to Barred Owl calls and Northern Spotted Owls responded less frequently in areas having higher numbers of Barred Owls. Other researchers have also noted that the presence of Barred Owls adversely affected Spotted Owl detectability (Bailey et al. 2009, Crozier et al. 2006, Kroll et al. 2010, Olson et al. 2005, Sovern et al. 2014). Olson et al. (2005) found the presence of Barred Owls was important for modeling detectability among years in all occupancy analyses, having a negative effect on Spotted Owl detectability. However, Gutiérrez et al. (2004) noted that even if this were the case, there does not appear to be a decline in Spotted Owl recapture rates, which he stated would be expected if Barred Owls were causing significant behavioral interference.
In consideration of the potential for modified Spotted Owl behavior in the presence of Barred Owls, Wasser et al. (2012) demonstrated an alternate method for surveying both owl species. Their method uses trained “detection dogs” to search out accumulated owl pellets under roost sites, then analyzing the mitochondrial DNA in the pellets to confirm species. The researchers found that detection probabilities using this method were significantly higher for both species than with standard vocalization surveys.
Hybridization between Spotted and Barred Owls
The first Spotted Owl x Barred Owl hybrids were reported in 1987 (Kelly 2001, Kelly and Forsman 2004). Since then, at least 50 hybrids have been reported in the Northern Spotted Owl’s range (Hamer et al 1994, Mazur and James 2000, Pearson and Livezey 2003, Seamans et al. 2004). Kelly and Forsman (2004) reported the very low rate of interspecific matings, thereby suggesting that the rate of hybridization will likely not be a serious threat to the Spotted Owl. However, they also stated, it is possible that hybridization is more common than reported since hybrid backcrosses are hard to identify in all cases. Funk et al. (2007) found that of 12 owls identified as hybrids by plumage in the field, five (almost half) were either Barred (3) or Spotted (2) owls by genetic testing. Hanna et al. (2018) suggested that if hybridization does become more frequent, genetic swamping of the Northern Spotted Owl may occur leading to genetic introgression, which could reduce fitness; however he found no evidence of introgression. Forensic genetic investigation shows that the two species show extensive evolutionary divergence, and that the hybrids are primarily crosses between male Spotted Owls and female Barred Owls (Haig et al. 2004). This makes sense when considering that male owls often present females with food during courtship: male Barred Owls presenting non-mammalian prey to female Spotted Owls would not result in a successful courtship.
As a recent but closely-related invader to the west coast, Barred Owls have the potential to bring novel, harmful pathogens and parasites from east coast populations which could be transmitted to Spotted Owls. As such, Lewicki et al. (2015) examined the Haemoproteus blood parasite assemblages of Barred Owls in both their native and invasive ranges and Northern Spotted Owls. They found that Northern Spotted Owls had a slightly lower prevalence of Haemoproteus infection than both populations of Barred Owls, but mean infection intensity was almost 100 times greater than that of western Barred Owls. They noted their results suggested that Haemoproteus in Spotted Owls are not solely influenced by Barred Owls. They did not directly evaluate if and to what extent parasite infection may have influenced fitness but noted that parasites can become pathogenic with additional stressors, such as competition with Barred Owls.
The future of Spotted Owl and Barred Owl populations
Gutiérrez et al. (2004) outlined and discussed the uncertainty of the ultimate outcome of the invasion of the Barred Owl on the future of these two species. They listed nine potential futures for the Northern Spotted Owl, listed in order of their outcome from most serious to least serious effect:
1. Barred Owls will replace the Northern Spotted Owl throughout its range (behavioral and competitive dominance hypothesis).
2. Barred Owls will replace the Northern Spotted Owl in the northern, more mesic areas of its range (moisture-dependent hypothesis).
3. Barred Owls will replace Northern Spotted Owls over much of its range, but the Spotted Owl could persist in some areas with management intervention (management hypothesis).
4. Barred Owls will replace Northern Spotted Owls over much of its range, but the Spotted Owl will persist in refugia (refugia hypothesis).
5. Barred Owls will replace Northern Spotted Owls in the northern part of its range but the Spotted Owl will maintain a competitive advantage in habitats where its prey is abundant and diverse (specialist vs. generalist hypothesis).
6. Barred Owls will replace Spotted Owls only where weather and habitat change have placed Spotted Owls at a competitive disadvantage (synergistic effects hypothesis).
7. Barred Owls will replace Northern Spotted Owls in some habitats but not in others (habitat hypothesis based on structural elements of forest, which confer a maneuverability advantage to the smaller Spotted Owl).
8. Barred Owls and Spotted Owls will compete, with the outcome being an equilibrium favoring Barred Owls over Spotted Owls in most but not all of the present NSO habitat range (interference competition hypothesis).
9. Barred Owls will increase to a peak number, then decline or stabilize at a lower density, which will permit the continuation of Spotted Owls (dynamics hypothesis). cellspacing="0" width="100%"
A two-day workshop in 2005 was convened to discuss approaches to combat the threat of the Barred Owl invasion to Northern Spotted Owls (Buchanan et al. 2007). Suggestions ranged from no action, to removing Barred Owls using lethal methods. The final recommendation was that emphasis should be on removal experiments and on intensive field studies on aspects of Barred Owl life history and interspecific interactions. Removal of Barred Owls has also been suggested by others (Courtney and Franklin 2004, Gutiérrez et al. 2007), noting that it would have the added benefit of ascertaining the effect of Barred Owls on Spotted Owls.
Diller et al. (2014) began removing Barred Owls on Green Diamond Resources lands in northern California in 2009 as a pilot study to develop methods and study design and to evaluate the feasibility of a removal program. They determined that it was a relatively quick, effective, and low-cost program. Following removal, Spotted Owl occupancy made a slow recovery in areas where Barred Owls were removed compared to untreated areas where occupancy declined, while Barred Owl occupancy declined (Diller et al. 2016). Spotted Owl fecundity rates did not change in the treated areas, however the greater number of pairs in treated areas resulted in greater overall population productivity; there was also an increase in estimated survival and population change, and a decrease in extinction rate. Barred Owl removals continued through 2013, resulting in an increased rate of population change as well as realized increased population change (Dugger et al. 2016). In 2015, Wiens et al. (2017) began a 5-year Barred Owl removal study in established study areas in Washington and Oregon. After removal, they noted a decline in the probability of use by territorial pairs of Barred Owls on both the control and treatment areas between 2015 and 2016, which they felt might indicate a decline not directly attributable to experimental removal activities.
The question remains if such an approach would actually be successful and feasible throughout the Northern Spotted Owl range. Bodine and Capaldi (2017) used mathematical modelling to determine if removing Barred Owls could ultimately save the Northern Spotted Owl population in the Oregon Coast Range. Their models showed that Barred and Northern Spotted owls could not coexist in the long run, suggesting that Barred Owls would need to be eliminated. They determined that by 2030, less than 5% of historical Spotted Owl sites would contain Spotted Owls. However, removal of Barred Owls to eliminate them would require clearing about 50% of the Barred Owl sites per year to eliminate them in over 100 years. To eliminate them in 10 years, it would take removing over 90% of Barred Owl sites yearly. Perlman (2017) examined the effects of various Barred Owl removal rates in four existing demographic Northern Spotted Owl study areas using a two-species individual-based model. She found that viable, long-term recovery was observed for only two of the four areas, and was dependent on high removal intensities. No model showed a return to pre-invasion population numbers. However, her models agree with Bodine and Capaldi, in that with no removal action taken, the Northern Spotted Owl population would go extinct within 100 years. Baumbusch (2016) investigated the effect of size of patch on removals with modelling and found that large contiguous removal areas maintained a lower Barred Owl occupancy and required removal of fewer owls compared to small fragmented removal areas covering the same acreage.
By contrast, Yackulic et al. (2014) noted that complete eradication of Barred Owls is unlikely, but that it might be more feasible to maintain their occupancy at a low level (0.2), benefitting Spotted Owls while decreasing cost. They suggested that initial removal effort be focused on patches with high quality habitat, where Spotted Owls have typically had high reproductive rates or are currently or have recently been observed to be occupied by Spotted Owls. Holm et al. (2016) suggested that removal efforts would be most effective in areas with low Barred Owl populations and where it would be defensible against Barred Owl colonization or have high-quality Spotted Owl habitat. Yackulic et al. (2012) noted that understanding the influence of regional occupancy on local colonization and extinction rates of Barred Owls will be important to predict the impacts of Barred Owl removal as a management tool.
Diller et al. (2014) found the cost for such removals not to be overly burdensome, costing up to $150/owl in direct costs. On the other hand, Livezey (2010) estimated cost to be about $1 million annually for the entire Northern Spotted Owl range, including additional indirect costs, breaking down to about $700/owl in the first year and $2800/owl in subsequent years. Ultimately, there are also questions about interfering with a natural process of competition and how long any removal program could last, since there would still likely be immigration from the east, if not from the much closer Barred Owl populations in Canada to the north (Bodine and Capaldi 2017, Cornwall 2014). Public acceptance of such methods may also be difficult to overcome (Lute and Attari 2017). Other authors noted that habitat management may aid the Spotted Owl or that a natural balance between Barred and Spotted Owls has or can be achieved at least in certain areas, allowing for coexistence between the species (Buchanan et al. 2007, Pearson and Livezey 2007).
Important information gaps
• Effects of disturbance on Barred Owls. The influence of fuel treatments and thinning on Spotted Owl occupancy has been repeatedly explored. Determining how variation in such treatments affects Barred Owl colonization, and their subsequent effects on Spotted Owls, represents an important line of inquiry.
• Interactive effects of Barred Owls and fire on Spotted Owls. Barred Owls can use a wider range of habitats and food resources than Spotted Owls. Such behavioral flexibility may make Barred Owls relatively more successful in burned landscapes than Spotted Owls. To our knowledge this conjecture remains unverified.
• Recolonization rates of Barred Owls after their removal. The positive response of Spotted Owls to Barred Owl removal has been encouraging. However, the rate at which Barred Owl recovery occurs post-removal is unknown and would greatly inform planning for future removal efforts. cellspacing="0" width="100%"
Key Messages for Managers
• Barred Owls reduce Spotted Owl occupancy, survival, productivity, fecundity, and population growth.
• Barred Owls exhibit a wide dietary breadth and are likely influencing food webs in unforeseen ways.
• Be cautious in assuming that recently arrived Barred Owls will remain in low numbers in perpetuity. Barred Owls have repeatedly shown that they occur in low numbers prior to dramatically increasing in abundance.
• Barred Owls appear to suppress detection probabilities of Spotted Owls. Consider alternative techniques, such as scat sniffing dogs, or increasing survey effort to detect Spotted Owls.
• Experimental removal of Barred Owls has proven effective in improving Spotted Owl population growth.
• Maximize the value of Barred Owl removals by focusing on those habitats most valuable for Spotted Owls with the least number of Barred Owls, prior to removals in poor habitat with high numbers of Barred Owls.
Annotated Bibliography of Cited References
Anthony, R.G., E.D. Forsman, A.B. Franklin, D.R. Anderson, K.P. Burnham, G.C. White, C.J. Schwarz, J. Nichols, J.E. Hines, G.S. Olson, S.H. Ackers, S. Andrews, B.L. Biswell, P.C. Carlson, L.V. Diller, K.M. Dugger, K.E. Fehring, T.L. Fleming, R.P. Gerhardt, S.A. Gremel, R.J. Gutiérrez, P.J. Happe, D.R. Herter, J.M. Higley, R.B. Horn, L.L. Irwin, P.J. Loschl, J.A. Reid, and S.G. Sovern. 2006. Status and trends in demography of Northern Spotted Owls, 1985–2003. Wildlife Monographs 163:1-48.
As part of a meta-analysis of the demography of the Northern Spotted Owl across its range, the authors included the effect of Barred Owls on demographic parameters such as recapture probabilities, apparent survival, and fecundity. They hypothesized that the overall apparent lack of effect on fecundity or survival that they found was due to Barred Owls potentially having a greater effect on Spotted Owl occupancy of territories.
Bailey, L.L. J.A. Reid, E.D. Forsman, and J.D. Nichols. 2009. Modeling co-occurrence of Northern Spotted and Barred owls: Accounting for detection probability differences. Biological Conservation 142(12):2983-2989.
Authors investigated the co-occurrence of Northern Spotted and Barred owls using detection probabilities and spatial patterns with a two-species model. They found no evidence that Barred Owls excluded Spotted Owls from territories or that the two species co-occurred less often than expected. Detection probabilities for the two species were substantially different, with the Spotted Owls having a higher detection probabilities for both day and night surveys. Detection probabilities were mostly independent; however daytime surveys were less so – that is, that if you detected one species, you were unlikely to detect the other species.
Baumbusch, R.C. 2016. A model to evaluate Barred Owl removal strategies for the conservation of Northern Spotted Owls. M.S. Thesis, Humboldt State University, Arcata, CA. 68 p.
Using individual-based modelling, the author compared alternative spatial strategies for removing Barred Owls in a localized area. Using an array of patches of identical size, he compared the effectiveness of large, continuous areas to several designs of progressively smaller, fragmented areas with different distribution. Effectiveness was evaluated using occupancy rates within the removal areas, while efficiency was measured by the number of owls removed annually. Larger, continuous removal areas maintained lower Barred Owl occupancy rates than smaller, fragmented ones, and while the number of owls removed was greater, the number of owls removed per patch per year was lower than for other patch arrays.
Bodine, E.N. 2017. Can culling Barred Owls save a declining Northern Spotted Owl population? Natural Resources Modelling 30(3). DOI: 10.1111/nrm.12131.
The authors modeled competition between Spotted and Barred owls of the Oregon Coast region using information theory. Their analysis shows that even with culling Barred Owls, no model contained a coexistence equilibrium given the current growth and competition parameters. Therefore, the authors conclude, complete elimination of the Barred Owl population is required for conservation of the Spotted Owl population, with a culling rate around 90% of the sites per year to eliminate the Barred Owl population within 10 years. However, they noted their analysis does not take into account the continual influx of Barred Owls from the east coast.
Buchanan, J.B., T.L. Fleming, and L.I. Irwin. 2004. A comparison of Barred Owl and Spotted Owl nest-site characteristics in the eastern Cascade Mountains, Washington. Journal of Raptor Research 38:231–237.
Ten Barred Owls nests found in the course of timber-sale evaluations in the eastern Cascade mountains from 1988-1994 are described, including the nest, nest tree, and surrounding vegetation. These were compared with a sample of Spotted Owl nests from a previous investigation. Among the findings were that Barred Owl nests were generally in areas with low relief, while Spotted Owls nested on significantly steeper terrain. Elevation of Barred Owl nest locations was not significantly different from Spotted Owl nests. There was little difference in the vegetation surrounding the nest trees.
Buchanan, J.B., R.J. Gutiérrez, R.G. Anthony, T. Cullinan, L.V. Diller, E.D. Forsman, and A.B. Franklin. 2007. A synopsis of suggested approaches to address potential competitive interactions between Barred Owls (Strix varia) and Spotted Owls (S. occidentalis). Biological Invasions 9:679–691.
The authors present a synthesis of ideas developed and expressed at a workshop to consider how to manage the potential threat of Barred Owls to the viability of the Spotted Owl, and summarizes the suggested management actions and potential studies to address key management questions. Suggestions include no action, more studies, removal experiments of immature Spotted Owls, culling Barred Owls, habitat management, supplemental feeding, and disrupting Barred Owl reproduction.
Courtney, S. and A. Franklin. 2004. Chapter twelve: Information needs. Pages 12-1–12-18 in S. Courtney, ed. Scientific evaluation of the status of the Northern Spotted Owl. Sustainable Ecosystems Institute, Portland, OR.
As part of the five-year status review report on the Northern Spotted Owl, past and present information needs for management of the Northern Spotted Owl are reviewed, including information needs on competition with Barred Owls. These include basic natural history of Barred Owls in the Pacific Northwest, development of an appropriate Barred Owl covariate, Barred Owl use of lands set aside in the Northwest Forest Plan for Northern Spotted Owls, and understanding the magnitude and mechanisms of competition between the two species.
Crozier, M.L., M.E. Seamans, R.J. Gutiérrez, P.J. Loschl, R.B. Horn, S.G. Sovern, and E.D. Forsman. 2006. Does the presence of Barred Owls suppress the calling behavior of Spotted Owls? Condor 18:760-769.
The authors examined whether the response frequency of naive and experienced Spotted Owls to simulated Spotted Owl calls differed after exposure to simulated Barred Owl calls. Summarizing an experiment using taped Spotted and Barred Owl calls, they found that both subspecies of Spotted Owl responded less to Spotted Owl calls after exposure to Barred Owl calls and Northern Spotted Owls responded less frequently in areas having higher numbers of Barred Owls. They inferred that the simulated presence of a Barred Owl might negatively affect Spotted Owl response.
Dark, S.J., R.J. Gutiérrez, G.I. Gould, Jr. 1998. The Barred Owl (Strix varia) invasion in California. Auk 115:50–56.
Review of sightings of Barred Owls in California from 1981 to 1996. Sightings, or “sites”, were placed into 3 time periods corresponding to (1) the establishment of statewide surveys by the U.S. Forest Service, (2) beginning of demographic studies in northwestern California and the Sierra Nevada, and (3) start of intensive surveys for Spotted Owls mandated by the federal government. The authors noted that Barred Owl territories were significantly lower in elevation than Spotted Owl territories, both for the authors’ studies at Redwood National Park and analysis of observations from the California Department of Fish and Game regional database. Two Northern Spotted Owl pairs were displaced by Barred Owl pairs in their study area.
Davis, R.J., B. Allen, J. Hollen, J.E. Gower and D. Keenum. 2016. Status and trends of Northern Spotted Owl habitats. Gen. Tech. Rpt. PNW-GTR-929. USDA Forest Service, Pacific Northwest Research Station, Portland, OR. 54 p.
The authors analyzed the effect of Barred Owls on Northern Spotted Owl habitat selection in the course of evaluating habitat status trends for the Northern Spotted Owls. They found a strong negative correlation between the increasing trend of Spotted Owl territories with Barred Owl detections and average Habitat Suitability at Spotted Owl locations in the Tyee, Oregon, study area. Over 23 years, Habitat Suitability was significantly lower if there were Barred Owls detected, suggesting that Spotted Owls were choosing habitat of a lower quality.
Diller, L.V., J.P. Dumbacher, R.P. Bosch, R.R. Brown, and R.J. Gutiérrez. 2014. Removing Barred Owls from local areas: Techniques and feasibility. Wildlife Society Bulletin 38:211–216.
Reviews the results of the first Barred Owl removal study, in which the feasibility and cost of removing Barred Owls using lethal methods within the context of a removal experiment in northern California was examined. The authors determined that lethal removal of Barred Owls was rapid, technically feasible, and cost-effective. They review the ease of locating and identifying Barred Owls and noted that new, colonizing Barred Owls replaced those that were removed.
Diller, L.V., K.A. Hamm, D.A. Early, D.W. Lamphear, K.M. Dugger, C.B. Yackulic, C.J. Schwarz, P.C. Carlson, and T.L. McDonald. 2016. Demographic response of Northern Spotted Owls to Barred Owl removal. Journal of Wildlife Management 80(4):691–707. DOI: 10.1002/jwmg.1046.
Summary of the results of a Barred Owl 2009-2010 culling experiment in northern California. The treated areas did not result in an increased Spotted Owl fecundity, but did result in a decrease in Spotted Owl extinction rates to one similar to areas without Barred Owls, leading to in an increase in apparent survival and annual rate of population change.
Dugger, K.M., R.G. Anthony, and L.S. Andrews. 2011. Transient dynamics of invasive competition: Barred Owls, Spotted Owls, habitat, and the demons of competition present. Ecological Applications 21(7):2459–2468.
The authors used the recent expansion of Barred Owls into the range of the endemic Northern Spotted Owl to directly explore competition processes using single-species, multi-season occupancy models, quantifying Barred Owl detections and habitat characteristics to model extinction and colonization rates of Spotted Owl pairs in southern Oregon. There was strong support for an association between detection of Barred Owls and increased extinction and decreased colonization rates of Spotted Owl pairs. The analyses provided evidence of interference competition between the species. Presence of Barred Owls resulted in lower occupancy rates where they were detected, lower colonization rates, and higher extinction rates.
Dugger, K.M., E.D. Forsman, A.B. Franklin, R.J. Davis, G.C. White, C.J. Schwarz, K.P. Burnham, J.D. Nichols, J.E. Hines, C.B. Yackulic, P.F. Doherty, Jr., L. Bailey, D.A. Clark, S.H. Ackers, L.S. Andrews, B. Augustine, B.L. Biswell, J. Blakesley, P.C. Carlson, M.J. Clement, L.V. Diller, E.M. Glenn, A. Green, S.A. Gremel, D.R. Herter, J.M. Higley, J. Hobson, R.B. Horn, K.P. Huyvaert, C. McCafferty, T. McDonald, K. McDonnell, G.S. Olson, J.A. Reid, J. Rockweit, V. Ruiz, J. Saenz, and S.G. Sovern. 2016. The effects of habitat, climate, and Barred Owls on long-term demography of Northern Spotted Owls. Condor 118:57–116.
The authors evaluated population processes of the Northern Spotted Owl in 11 study areas in Washington, Oregon, and northern California from 1985-2013, including competition with Barred Owls. They found that Barred Owl presence was associated with increased local extinction rates of Spotted Owl pairs and had a negative effect on apparent survival of Spotted Owls most study areas. They suggested that Barred Owl densities may now be high enough across the range of the Northern Spotted Owl that, despite the continued management and conservation of suitable owl habitat on federal lands, the long-term prognosis for the persistence of Northern Spotted Owls may be in question without additional management intervention.
Funk, W.C., T.D. Mullins, E.D. Forsman, and S.M. Haig. 2007. Microsatellite loci for distinguishing Spotted Owls (Strix occidentalis), Barred Owls (Strix varia), and their hybrids. Mol. Ecol. Notes 7: 284–286.
The authors identified four microsatellite loci that correctly distinguishes between Spotted Owls, Barred Owls, F1 hybrids, and backcrosses. They tested 90 owls that were initially identified in the field using plumage characteristics. Those specimens field identified to species were confirmed as correctly identified using these loci. Four owls confidently identified as F1 hybrids were also correctly identified. However, of 8 field-identified as potential F1 hybrids and backcrosses, five (more than half) were misidentified in the field and were either Spotted or Barred owls, not hybrids.
Glenn, E.M., R.G. Anthony, and E.D. Forsman. 2010. Population trends in Northern Spotted Owls: Associations with climate in the Pacific Northwest. Biological Conservation 143:2543-2552.
In the course of modeling population trends with climate for Northern Spotted Owl, the authors found that declines in recruitment at four of six sites were also associated with increased presence of Barred Owls. At two sites, they also found annual survival was negatively associated with the proportion of Spotted Owl territories containing Barred Owl detections. Only at one site did the presence of Barred Owls have a negative effect of numbers of young fledged per year.
Glenn, E.M., R.G. Anthony, E.D. Forsman, and G.S. Olson. 2011a. Local weather, regional climate, and annual survival of the Northern Spotted Owl. Condor 113:159–176.
Survival was negatively associated with Barred Owls at one area but not in five other areas. In a regional meta-analysis, the authors found that Barred Owls accounted for only slight amounts of temporal variation in survival. However, the parameter estimates indicated that Barred Owls were negatively associated with survival at all six areas.
Glenn, E.M., R.G. Anthony, E.D. Forsman, and G.S. Olson. 2011b. Reproduction of Northern Spotted Owls: The role of local weather and regional climate. Journal of Wildlife Management 75:1279-1294.
The authors analyzed the relationships between reproduction and several climate variables at varied spatial and temporal scales. Though Barred Owl influence on reproduction was not the primary focus, they also included the presence of the species in some analyses. They observed a negative association between reproduction and the presence of Barred Owls only at one of four sites; however in a meta-analysis there was a negative association at most sites.
Graham, Scott A. 2012. Diet composition, niche and geographic characteristics, and prey size preference of Barred Owls (Strix varia) in the Pacific Northwest. M.S. Thesis, Boise State University, Boise, ID. 149 p.
The author describes the dietary composition, niche, geographic characteristics, and prey size preference of the Barred Owl in the Pacific Northwest. His primary study site was in Western Oregon in 2007-2009. He analyzed regurgitated pellets and found their diets comprised 64.8% mammals, followed by birds, reptiles, amphibians, and others. Diets were generally dominated by northern flying squirrels (Glaucomys sabrinus). Barred Owls had a high taxonomic richness in its diet, indicating the versatility and range of prey they consumed. He expanded his study to other study sites in Oregon and Northwest Washington and found diets varied between sites, with the central Coast Ranges and Olympic National park having similar diets, dominated by forest mammals, and the eastern Cascades had diets where insects were the most numerous taxa. In controlled lab experiments, both Northern Spotted Owls and Barred Owls preferred smaller mice over larger rats.
Gutiérrez, R.J., M. Cody, S. Courtney, A.B. Franklin. 2007. The invasion of Barred Owls and its potential effect on the Spotted Owl: A conservation conundrum. Biological Invasions 9:181-196. DOI 10.1007/s10530-006-9025-5.
The authors evaluated the potential negative impact of Barred Owls on Spotted Owls by reviewing relevant literature of studies of sympatric owls, from which they concluded that there should be strong competition between the two species making a stable coexistence unlikely. They also reviewed results of recent Spotted Owl studies that included Barred Owls, noting that because Barred Owls have similar sizes of broods, smaller territories, and share similar habitat preferences, then they will have some degree of competition. This could be ameliorated if the Spotted Owl occupies habitat outside of the preferred habitat of Barred Owls. They also noted that these species have similar dietary preferences, though the Barred Owl is a food generalist, so by competing for the same prey the prey population that is shared between the two species could be sufficiently reduced such that the Spotted Owl may not be able to find sufficient food.
Gutiérrez, R.J., M. Cody, S. Courtney, and D. Kennedy. 2004. Chapter seven: Assessment of the potential threat of the Northern Barred Owl. Pages 7-1 to 7-51 in: Courtney, S.P., J.A. Blakesley, R.E. Bigley, M.L. Cody, J.P. Dumbacher, R.C. Fleischer, A.B. Franklin, J.F. Franklin, R.J. Gutiérrez, J.M. Marzluff, and L. Sztukowski. Scientific evaluation of the status of the Northern Spotted Owl. Sustainable Ecosystems Institute, Portland, OR. 507 p.
As part of the five-year review for the Northern Spotted Owl, the authors review current information about the Barred Owl, noting that most data on the Barred Owl in the Pacific Northwest was recorded incidental to Northern Spotted Owl surveys. The authors also discuss the differences and similarities between the two species in body size, habitat use, and prey, and compare the current situation to that of Strix owls coexisting sympatrically in other areas. They note that two subspecies of the Spotted and Barred owls already coexist in northern Mexico: Mexican Spotted Owl [S. o. lucida] and Mexican Barred Owl [S. v. sartorii]. The Mexican Barred Owl is be estimated at twice the mass as the Mexican Spotted Owl allowing for coexistence. However, the Northern Spotted Owl and Northern Barred Owl are much more similar in mass. They outline nine possible hypotheses regarding the potential consequences of the Barred Owl invading the range of the Northern Spotted Owl.
Gutiérrez, R.J., A.B. Franklin and W.S. Lahaye. 1995. Spotted Owl (Strix occidentalis), The Birds of North America (P.G. Rodewald, Ed.). Ithaca: Cornell Lab of Ornithology; Retrieved from the Birds of North America: https://birdsna.org/Species-Account/bna/species/spoowl. DOI: 10.2173/bna.179
The authors describe the Spotted Owl’s appearance, systematics, distribution, habitat, general biology, behavior, subspecies, conservation and management. Spotted Owls are associated with late seral stage conifer forests in the western North America and Mexico. The Northern Spotted Owl forages almost exclusively on small and medium-sized mammals, the other two subspecies also include a fair amount of birds and invertebrates. They forage primarily at night. They lay one clutch per breeding season of 1 to 4 eggs using existing structures such as cavities, broken-top trees, and abandoned nests.
Haig, S.M., T.D. Mullins, E.D. Forsman, P.W. Trail, and L. Wennerberg. 2004. Genetic identification of Spotted Owls, Barred Owls, and their hybrids: Legal implications of hybrid identity. Conservation Biology 18(5):1347-1357.
The authors used mitochondrial gene analysis to distinguish between Northern Spotted Owls and Barred Owls, and found them to be in two distinct clades with no signs of previous introgression, as well as extensive divergence between the two species. Hybrids had unique genetic combinations and were most frequently due to crosses between female Barred Owls and male Spotted Owls.
Hamer, T.E., D.L. Hays, C.M. Senger, and E.D. Forsman. 2001. Diets of Northern Barred Owls and Northern Spotted Owls in an area of sympatry. Journal of Raptor Research 35:221–227.
The authors collected regurgitated pellets from Spotted and Barred owl territories in Washington state from 1985-1989 during the spring and summer. They found Spotted Owl diet consisted almost entirely of mammals (98.6% biomass), while Barred Owls were more general (74.5% mammal biomass). There was a 76% overlap in diets. The most important prey in diets of Spotted Owls and Barred Owls were northern flying squirrels (Glaucomyssa brinus) and snowshoe hares, respectively. Barred Owls ate more diurnal species and terrestrial species, while Spotted Owls ate more arboreal species. Overall, Barred Owls had a more diverse diet than Spotted Owls.
Hamer, T.E., E.D. Forsman, A.D. Fuchs, and M.L. Walters. 1994. Hybridization between Barred and Spotted owls. Auk 111:487–492.
The authors list and describe 3 adult and 1 yearling hybrids found in Oregon and Washington from 1989-1991. All were identified by unique plumage characteristics, vocalizations, and measurements. The body mass of all adults was as great as or greater than average values for the Barred Owl and greater than the upper limits for Spotted Owls. However, the wing chord and tail lengths were intermediate between the two parent species. They also describe the differences in the calls compared to the two species.
Hamer, T.E., E.D. Forsman, and E.M. Glenn. 2007. Home range attributes and habitat selection of Barred Owls and Spotted Owls in an area of sympatry. Condor 109:750–768.
Researchers radio-tagged Spotted and Barred Owls in the northern Cascades Range in Washington state. Spotted Owls had home ranges about 2-5 times larger than those of Barred Owls. The greatest differences were shown by females, with a home range averaging 6.7 times larger than female Barred Owls. Barred Owl winter home ranges were larger and tended to be expansions of the summer range, similar to some Spotted Owls. Both species used old forest habitat, though this was more significant for Spotted Owls. Spotted Owls were found at higher elevations on average than Barred Owls and selected areas near perennial streams in contrast to Barred Owls who showed no preference.
Hanna, Z.R., J.P. Dumbacker, R.C.K. Bowie, J.B. Henderson, and J.D. Wall. 2018. Whole-genome analysis of introgression between the spotted owl and barred owl (Strix occidentalis and Strix varia, respectively; Aves: Strigidae) in western North America. bioRxiv, doi: http://dx.doi.org/10.1101/343202.
The authors examined genome sequence data from 51 Barred and Spotted owls to investigate introgression between the species. They did confirm genetically that these species do hybridize and backcross, but found no evidence of widespread introgression which could lead to genetic swamping of the Spotted Owl and erase their genome. All non-hybrid individuals were either pure Spotted Owls or pure Barred Owls. They also found no significant difference in ancestry between the eastern and western Barred Owls they tested.
Herter, D.R. and L.L. Hicks. 2000. Barred Owl and Spotted Owl populations and habitat in the central Cascade Range of Washington. Journal of Raptor Research 34:279–286.
The authors surveyed for Spotted and Barred owls in western Washington using taped calls and vocal imitations from 1991-1993. They found Barred Owls at greatest densities on the wetter, western portions of the Cascade Range. On the drier, eastern portions of the range, Barred Owls were usually found along major river and stream corridors, in the vicinity of forested wetlands, or at higher elevations receiving increased precipitation. Spotted Owls used sites with greater amounts of mature coniferous forest than did Barred Owls within 0.8 km of site centers. They found no evidence of mixed-species pairing or hybrids.
Hollenbeck, J.P., S.M. Haig, E.D. Forsman, and J.D. Wiens. 2018. Geographic variation in natal dispersal of Northern Spotted Owls over 28 years. Condor 120: 230-542. DOI: 10.1650/CONDOR-17-164.1.
Net natal dispersal distances varied by four ecoregions, but declined similarly over time (~1 km/yr). Dispersal direction also varied by ecoregion: bimodal (north-south) in the Oregon Coast Range, south-southwest in the Oregon and California Cascades, and little directionality in the remaining regions. Distances also varied by ecoregion (mean 62.3-99.5 km), with the longest dispersals (up to 177 km) originating in the southern-most ecoregions. There was no direct relationship between Barred Owl detections near natal or selling locations and dispersal distance.
Holm, S.R., B.R. Noon, J.D. Wiens, and W.J. Ripple. 2016. Potential trophic cascades triggered by the Barred Owl range expansion. Wildlife Society Bulletin 40(4):615-624.
The authors identify possible direct and indirect ecological consequences following invasion of the Barred Owl into forest ecosystems of the west coast. They reviewed and evaluated current information on potential widespread ecological changes triggered by an increase in abundance of the Barred Owl as a new nocturnal avian predator, focusing on potential behavioral and adaptive changes in prey populations. They explored how Barred Owls at high density may affect ecosystem functions via indirect changes to food-web dynamics and predator-prey relationships. They discussed how this could lead to niche displacement or competitive exclusion of the Spotted Owl by the Barred Owl. They hypothesized that expanding Barred Owl populations will result in increased predation pressure on traditional and naïve prey species within the range of the Northern Spotted Owl. Initially, the largest declines would be in the prey species utilized by both owl species, such as the northern flying squirrel, red tree vole, and lagomorphs. In turn, this could lead to potential declines in sensitive species, effects on the predator communities, and indirect effects on ecosystem processes.
Irwin, L.L., D.F. Rock, and S.C. Rock. 2017. Barred owl habitat selection in west coast forests. Journal of Wildlife Management 82(1):202–216. https://doi.org/10.1002/jwmg.21339.
Barred Owls were radio-tagged at Chehalis, Washington, Springfield, Oregon, and Arcata, California. Data suggested that Barred Owls selectively hunted in lower-slope positions on southern aspects, often near streams at low elevations near nests. They also appeared to avoid clearcuts and young plantations. At Arcata, high densities of Douglas-fir (Pseudotsuga menziesii) trees and increasing basal area of tanoak (Notholithocarpus densiflorus) were negatively associated with barred owl habitat selection.
Iverson, Wayne F. 2004. Reproductive success of Spotted Owls sympatric with Barred Owls in western Washington. Journal of Raptor Research 38(1):88-91.
The author compared reproductive success for Spotted Owls nesting in western Washington for sites with Barred Owls within 0.8 km of the nest, within 2.5 km, and not within 2.5 km. He found no significant difference in reproductive success (percent fledged) between these three groups. Nor was there significant difference in mean amount of older forest available or the mean elevation of these sites. The author speculated that the success of Spotted Owls despite the presence of Barred Owls may be due to differences in their diets on the west slopes of the Cascades, as described by other researchers.
Jennings, S., R.L. Cormier, T.G. Gardali, D.T. Press, and W.W. Merkle. 2011. Status and distribution of the Barred Owl in Marin County, California. Western Birds 42:103–110.
The authors summarize and discuss detections of Barred Owls in Marin County, California, at the furthest southern extent of the Barred Owl’s current coastal range, gathering citizen science data and other incidental data to augment data from the Northern Spotted Owl monitoring programs by federal and county authorities from 2002-2010. The first Barred Owl was detected in 2002. Since then, 107 detections of Barred Owls have been recorded. Barred Owls successfully raised young in 2007 and 2008 in Muir Woods, but otherwise the population appears to primarily consist of adults. Three aggressive interactions between Spotted and Barred owls were recorded. They also note observations of Barred Owls foraging in streams hunting for crayfish (Pacifastacus spp.) and foraging while walking on the ground.
Johnson, N.K. 1994. Pioneering and natural expansion of breeding distributions in western North American birds. Studies in Avian Biology 15:27-44.
The author discusses the possible causes for the range expansions of a number of species, the Barred Owl among them. The author speculates that this species’ expansion may reflect a natural ebb and flow at the range margins and thus reclaiming ground occupied in the past. Another factor may be related to climate change, in that this species in the east is typified by wet summers and high humidity, and the western United States has become warmer and wetter when comparing 1921-1954 to 1956-1977 and from the mid-1970s to mid-1980s.
Jones, J.E. and A.J. Kroll. 2016. A joint model of species interaction dynamics for multistate processes incorporating imperfect detection. Ecosphere 7(10):e01477.10.1002/ecs2.1477.
The authors present a model of across-season species interaction dynamics with multistate processes for dominant and subordinate species while incorporating imperfect species detection, which had been a concern with earlier modelling efforts. They demonstrate this model using data from a long-term study in Oregon, evaluating interactions between the Northern Spotted Owl and Barred Owl, with the latter species considered to be dominant for the model. Their results suggest that Barred Owls are associated with declines in Spotted Owl forming pairs, but not necessarily occupancy. Estimated counts of occupied Barred Owl territories doubled over the 10-year span, while Spotted Owl pair territories declined by 1.3 territories per year. Though there appears to be a rise in unoccupied and single-occupied Spotted Owl territories, the posterior credible intervals contained zero, indicating uncertainty in the trend direction.
Keane, J.J. 2014. California Spotted Owl. Pages 437–467 in: J.W. Long, L. Quinn-Davidson, and C.N. Skinner, eds. Science synthesis to promote resilience of social-ecological systems in the Sierra Nevada and southern Cascades. Gen. Tech. Rep. PSW-GTR-247. U.S.D.A. Forest Service, Pacific Southwest Research Station, Albany, CA.
The author describes the threat of Barred Owls as an “Additional Ecological Stressor” for the California Spotted Owl, due to its expansion into the range of that subspecies. He reviews the literature of its impact on the Northern Spotted Owl as it invaded their range over the last 40 years. He notes that to date a total of 73 Barred Owls and hybrids have been detected in the Sierra Nevada. He believes that Barred Owl numbers are likely higher than documented in the Sierras, as there has been no systematic surveys for them to date and species-specific survey methods are required to account for differences between Spotted and Barred owls.
Keane, J.J. 2017. Chapter 7: Threats to the viability of California Spotted Owls. Pages 185-238 in: R.J. Gutiérrez, P.N. Manley, and P.A. Stine, tech. eds. The California Spotted Owl: current state of knowledge. Gen. Tech. Rep. PSW-GTR-254. U.S.D.A. Forest Service, Pacific Southwest Research Station, Albany, CA.
After reviewing the threats currently listed in the California Spotted Owl Technical Assessment, the author outlines several new threats that have emerged since that 1992 assessment including the invasion of the Barred Owl. He notes that the Barred Owl is considered a primary threat to the Northern Spotted Owl and they are currently expanding their range into the Sierra Nevada range. He reviews recent literature describing the impact of the invader on the Northern Spotted Owl. No formal surveys had been conducted for Barred Owls in the range of the California Spotted Owl. He reviews incidental sightings of a total of 51 Barred Owls and 27 hybrids that had been detected in the Sierra Nevada between 1989 and 2013. He suggests that without control efforts, Barred Owls can potentially become a primary threat to the California Spotted Owl in the Sierra Nevada.
Kelly, E.G. 2001. The range expansion of the Northern Barred Owl: An evaluation of the impact on Spotted Owls. M.S. Thesis, Oregon State University, Corvallis, Oregon. 78 p.
The author characterizes Barred Owl distribution and population increase in Oregon, investigates Spotted Owl territory performance before and after Barred Owl detection, and documents cases of hybridization between Barred Owls and Spotted Owls. When Barred Owls were detected within 0.80 km of a Spotted Owl territory center, 85% of Spotted Owls moved >0.80 km or disappeared completely. In comparison, at territories without Barred Owls, 32% of Spotted Owls moved >0.80 km or disappeared.
Kelly, E.G., and E.D. Forsman. 2004. Recent records of hybridization between Barred Owls (Strix varia) and Northern Spotted Owls (S. occidentalis caurina). Auk 121:806–810.
The authors summarized records of hybridization between Barred Owls and Northern Spotted Owls in Washington and Oregon through 1999, from surveys and >9,000 banding records in 1970-1999. The authors report on both F1 and F2 individuals and their parentage, if known. They found hybridization at 28 locations. A total of 47 hybrids were recorded. They conclude that there is little evidence that hybridization will be a serious threat to either species, given the low number of hybrids compared to the high numbers surveyed and banded.
Kelly, E.G., E.D. Forsman, and R.G. Anthony. 2003. Are Barred Owls displacing Spotted Owls? Condor 105:45–53.
By 1998, there were an estimated 706 Barred Owl territories in Oregon state, while between 1987 and 1998, Spotted Owl demographic study areas in Oregon and Washington showed significant increases in the percentage of Spotted Owl territories with Barred Owl detections. The authors found that for territories with Barred Owls close to the territory center (within 0.8 km), Spotted Owl occupancy scores declined. By contrast, if Barred Owls were further away, occupancy scores were only slightly less than those where no Barred Owls were detected. The authors noted that even though occupancy scores went down when Barred Owls were close to Spotted Owl territory centers, it did not necessarily lead to displacement, as some pairs of Spotted Owls persisted (or new ones arrived) at territories where Barred Owls had been detected near the territory center.
Knopf, F.C. 1994. Avian assemblages on altered grasslands. Pages 247-254 in J.R. Jehl and N.K. Johnson, Eds. A century of avifaunal change in western North America. Studies in Avian Biology 15.
The author discusses changes in habitat across the grasslands that may have enabled the Barred Owl to expand to the west coast, such as an increase in tree plantings and ecological invasions. This consists of fire control which enabled woody vegetation on the northern grasslands and planting of shelterbelts to reduce wind erosion. There has also been development of streamside forests of alien and exotic tree species across the western landscape due to water management practices favoring woody colonization and secondary succession.
Kroll, A.J., T.L. Fleming, and L.L. Irwin. 2010. Site occupancy dynamics of Northern Spotted Owls in the eastern cascades, Washington, USA, 1990–2003. Journal of Wildlife Management 76:1264–1274.