Stan G. Sovern

STATUS AND TRENDS IN DEMOGRAPHY OF NORTHERN SPOTTED OWLS, 1985-2003

Abstract We analyzed demographic data from northern spotted owls (Strix occidentalis caurina) from 14 study areas in Washington, Oregon, and California for 1985–2003. The purpose of our analyses was to provide an assessment of the status and trends of northern spotted owl populations throughout most of their geographic range. The 14 study areas made up approximately 12% of the range of the subspecies and included federal, tribal, private, and mixed federal and private lands. The study areas also included all the major forest types that the subspecies inhabits. The analyses followed rigorous protocols that were developed a priori and were the result of extensive discussions and consensus among the authors. Our primary objectives were to estimate fecundity, apparent survival (φ), and annual rate of population change (λ) and to determine if there were any temporal trends in these population parameters. In addition to analyses of data from individual study areas, we conducted 2 meta-analyses on each demographic parameter. One meta-analysis was conducted on all 14 areas, and the other was restricted to the 8 areas that constituted the Effectiveness Monitoring Plan for northern spotted owls under the Northwest Forest Plan. The average number of years of reproductive data per study area was 14 (range = 5–19), and the average number of recapture occasions per study area was 13 (range = 4–18). Only 1 study area had <12 years of data. Our results were based on 32,054 captures and resightings of 11,432 banded individuals for estimation of survival and 10,902 instances in which we documented the number of young produced by territorial females. The number of young fledged (NYF) per territorial female was analyzed by testing a suite of a priori models that included (1) effects of age, (2) linear or quadratic time trends, (3) presence of barred owls (Strix varia) in spotted owl territories, and (4) an even-odd year effect. The NYF varied among years on most study areas with a biennial cycle of high reproduction in even-numbered years and low reproduction in odd-numbered years. These cyclic fluctuations did not occur on all study areas, and the even–odd year effect waned during the last 5 years of the study. Fecundity was highest for adults (x̄ = 0.372, SE = 0.029), lower for 2-year-olds (x̄ = 0.208, SE = 0.032), and very low for 1-year-olds (x̄ = 0.074, SE = 0.029). Fecundity was stable over time for 6 areas (Rainier, Olympic, Warm Springs, H. J. Andrews, Klamath, and Marin), declining for 6 areas (Wenatchee, Cle Elum, Oregon Coast Range, Southern Oregon Cascades, Northwest California, and Simpson), and slightly increasing for 2 areas (Tyee, Hoopa). We found little association between NYF and the proportion of northern spotted owl territories where barred owls were detected, although results were suggestive of a negative effect of barred owls on the Wenatchee and Olympic study areas. The meta-analysis on fecundity indicated substantial annual variability with no increasing or decreasing trends. Fecundity was highest in the mixed-conifer region of eastern Washington (x̄ = 0.560, SE = 0.041) and lowest in the Douglas-fir (Pseudotsuga menziesii) region of the Oregon coast (x̄ = 0.306, SE = 0.039). We used Cormack–Jolly–Seber open population models and Program MARK to estimate apparent survival rates of owls >1 year old. We found no differences in apparent survival rates between sexes except for 1 area (Marin), which had only 6 years of data. Estimates of apparent survival from individual study areas indicated that there were differences among age classes with adults generally having higher survival than 1- and 2-year-olds. Apparent survival rates ranged from 0.750 (SE = 0.026) to 0.886 (SE = 0.010) for adults, 0.626 (SE = 0.073) to 0.886 (SE = 0.010) for 2-year-olds, and 0.415 (SE = 0.111) to 0.860 (SE = 0.017) for 1-year-olds. These estimates were comparable to survival rates from previous studies on the subspecies. We found evidence for negative time trends in survival rates on 5 study areas (Wenatchee, Cle Elum, Rainier, Olympic, and Northwest California) and no trends in survival on the remaining areas. There was evidence for negative effects of barred owls on apparent survival on 3 study areas (Wenatchee, Cle Elum, and Olympic). Survival rates of adult owls on the 8 Monitoring Areas generally were high, ranging from 0.85 (SE = 0.009) to 0.89 (SE = 0.010), but were declining on the Cle Elum, Olympic, and Northwestern California study areas. The meta-analysis of apparent survival indicated differences among regions and changes over time with a downward trend in the mixed-conifer and Douglas-fir regions of Washington. The meta-analysis of apparent survival also indicated that there was a negative association between fecundity and survival the following year, suggesting a cost of reproduction on survival. This effect was limited to the Douglas-fir and mixed-conifer regions of Washington and the Douglas-fir region of the Oregon Cascade Mountains. We used the reparameterized Jolly–Seber method (λRJS) to estimate annual rate of population change of territorial owls in the study areas. This estimate answers the question, Are these territorial owls being replaced in this geographically open population? Point estimates of λRJS were <1.0 for 12 of 13 study areas. The analyses provided strong evidence that populations on the Wenatchee, Cle Elum, Rainier, Olympic, Warm Springs, H. J. Andrews, Oregon Coast Ranges, and Simpson study areas were declining during the study. The mean λ̂RJS for the 13 study areas was 0.963 (SE = 0.009), suggesting that populations over all the areas were declining about 3.7% per year during the study. The mean λ̂RJS for the 8 monitoring areas for the Northwest Forest Plan was 0.976 (SE = 0.007) compared to a mean of 0.942 (SE = 0.016) for the other study areas, a 2.4-versus-5.8% decline per year. This suggested that owl populations on federal lands had higher demographic rates than elsewhere; thus, the Northwest Forest Plan appeared to have a positive effect on demography of northern spotted owls. Populations were doing poorest in Washington, where apparent survival rates and populations were declining on all 4 study areas. Our estimates of λRJS were generally lower than those reported in a previous analysis (λ̂RJS = 0.997, SE = 0.003) for many of the same areas at an earlier date. The possible causes of population declines include but are not limited to habitat loss from timber harvest and fires, competition with barred owls, and weather patterns.

Diurnal behavior of the spotted owl in Washington

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Barred Owls and Landscape Attributes Influence Territory Occupancy of Northern Spotted Owls

We used multi-season occupancy analyses to model 2 fates of northern spotted owl territories in relation to habitat amount, habitat fragmentation, and the presence of barred owls in Washington State, USA, 1989–2005. Local colonization is the probability a territory unoccupied by a spotted owl in year i would be occupied in year i + 1, and local extinction is the probability a territory that was occupied by a spotted owl in year i would be unoccupied in year i + 1. We found a negative relationship between local extinction probability and amount of late-seral forest edge. We found a negative relationship between colonization probability and the number of late-seral forest patches (higher fragmentation), and a negative relationship between colonization probability and the amount of non-habitat within 600 m of a spotted owl territory center (Akaike weight = 0.59). The presence of barred owls was positively related to extinction probability and negatively related to detection probability of spotted owls. The negative relationship between presence of barred owls and detectability of spotted owls indicated that spotted owls could be modifying their calling behavior in the presence of barred owls. The positive relationship between barred owl detections and local extinction probability suggests that because of competition with barred owls, spotted owls are being displaced. Published 2014. This article is a U.S. Government work and is in the public domain in the USA.

NEST REUSE BY NORTHERN SPOTTED OWLS ON THE EAST SLOPE OF THE CASCADE RANGE, WASHINGTON

Abstract During a long-term demography study of Northern Spotted Owls (Strix occidentalis caurina) in the eastern Cascade Range of Washington State in 1989 to 2008, we documented 276 nests of Northern Spotted Owls at 73 different territories. Of these nests, 90.2% were on platforms, mostly in clumps of deformed limbs caused by dwarf mistletoe (primarily Arceuthobium douglasii), and 9.8% were in cavities in trees. Of the nests associated with dwarf mistletoe, 8.4% were nests built by other raptors and 91.6% were either natural accumulations of debris or debris accumulated by other birds or mammals. Owls switched nests between nesting attempts 81.2% of the time. The presence of a new male or female at a territory did not affect the odds of switching nests between nesting attempts. The odds an owl would reuse a nest were 6 times greater for owls that were successful in the previous nesting attempt compared to owls that were unsuccessful, given the same type of nest structure. The odds an owl would reuse a cavity nest were 4.7 times greater than the odds an owl would reuse a platform nest, given the same level of nest success the previous year. The estimated mean annual survival rate (ϕ) of nest structures was 0.98 (  =  0.006), suggesting that mean life expectancy of nests was 42 y. However, nests on dwarf mistletoe platforms may be more ephemeral than cavity nests or the nest trees themselves, and management for viable nest areas for Spotted Owls should include multiple trees with mistletoe brooms suitable for alternate nests. Our results, and results from other studies, indicate that Douglas-fir (Pseudotsuga menziesii) trees infected with dwarf mistletoe are an important habitat component for Spotted Owls and many other species of birds and arboreal mammals on the east slope of the Cascade Range in Washington.

HOME RANGE AND HABITAT SELECTION BY NORTHERN SPOTTED OWLS ON THE EASTERN SLOPE OF THE CASCADE MOUNTAINS, WASHINGTON

Abstract We used radiotelemetry to study space use and habitat selection of 16 Northern Spotted Owls (Strix occidentalis caurina) on the eastern slope of the Cascade Mountains, Washington, U.S.A., in 1989–1990. We used a geographical information system (GIS) and aerial photo interpretation of digital orthophotos to assign owl locations a value for vegetation type, topographic position, amount of edge, and distance to water. We compared owl relocations and random locations within 95% fixed kernel (FK) home ranges to determine each owls selection of cover types, using logistic regression and generalized estimating equations (GEE) to estimate an exponential resource selection function likelihood. Minimum convex polygon (MCP) home ranges (SE) averaged 2858 ha (712 ha) for males and 1883 ha (249 ha) for females. Individual 95% FK home ranges averaged 1980 ha (229 ha) for males and 1649 ha (163 ha) for females. Pair home ranges averaged 3419 ha (826 ha) for MCP and 2427 ha (243 ha) for 95% FK. Nonbreeding season home ranges averaged approximately 3.5 times larger than breeding season home ranges for both males and females. Our best habitat model indicated that owls selected closed-canopy forests with a component of large (≥50 cm dbh) trees for roosting and foraging. In a given cover type, owls foraged lower on the slope. Management circles centered on nest areas—commonly used as a surrogate for home ranges—can be relatively poor representations of actual ranges used by pairs. However, an alternative for managing Spotted Owl home ranges is not readily available. Maintaining sufficient closed-canopy forest to provide habitat for Spotted Owls in the dry, fire-prone forests on the eastern slope of the Washington Cascades will be a challenge because forestry methods used to reduce the risk or severity of fire generally reduce the prevalence of structural features that characterize good Spotted Owl habitat.

Variation in inbreeding rates across the range of Northern Spotted Owls (Strix occidentalis caurina): Insights from over 30 years of monitoring data

ABSTRACT Inbreeding has been difficult to quantify in wild populations because of incomplete parentage information. We applied and extended a recently developed framework for addressing this problem to infer inbreeding rates in Northern Spotted Owls (Strix occidentalis caurina) across the Pacific Northwest, USA. Using pedigrees from 14,187 Northern Spotted Owls, we inferred inbreeding rates for 14 types of matings among relatives that produce pedigree inbreeding coefficients of F = 0.25 or F = 0.125. Inbreeding was most common in the Washington Cascades, where an estimated 15% of individuals are inbred. Inbreeding was lowest in western Oregon (3.5%) and northern California (2.7%), and intermediate for the Olympic Peninsula of Washington (6.1%). Estimates from the Olympic Peninsula were likely underestimates because of small sample sizes and the presence of few pedigrees capable of resolving inbreeding events. Most inbreeding resulted from matings between full siblings or half siblings, although a high rate of inbreeding from mother–son pairs was identified in the Olympic Peninsula. Geographic variation in inbreeding rates may reflect population declines and bottlenecks that have been detected in prior investigations. We show that there is strong selection against inbred birds. Only 3 of 44 inbred birds were later identified as parents (6.8%), whereas 2,823 of 10,380 birds that represented a comparable cross section of the data were later seen as reproducing parents (27.2%). Habitat loss and competition with Barred Owls (S. varia) remain primary threats to Northern Spotted Owls. However, given the negative consequences of inbreeding, Spotted Owl populations in Washington with suitable habitat and manageable numbers of Barred Owls may benefit from translocations of individuals from Oregon and California to introduce new genetic variation and reduce future inbreeding events.