Strix Varia Research Articles

Determining the Pharmacokinetic Properties of Two Different Doses of Meloxicam in Barred Owls (Strix varia).

Anthropogenic activities have negatively affected many birds, including owls. The Wildlife Hospital of Louisiana (WHL) has seen a 3.2-fold increase in barred owl (Strix varia) cases over the past eight years (2023, 134; 2015, 42). Because most of these animals present with traumatic injuries, analgesics should be considered in their treatment plan. To date, no study has measured the pharmacokinetics of an analgesic in barred owls. The goals of this study were to determine the harmonic means, times to maximum concentration, and elimination half-lives for single 1 mg/kg and 2 mg/kg intramuscular doses of meloxicam. Twelve barred owls (1 mg/kg, n = 6; 2 mg/kg, n = 6) admitted to the WHL and determined to be clinically normal based on examination and blood work were recruited for this study. Meloxicam was administered intramuscularly, and blood samples were collected intermittently over 12 h to measure plasma concentrations using high-performance liquid chromatography. Both doses had rapid elimination half-lives (1 mg/kg, 0.99 ± 0.1 h; 2 mg/kg, 1.07 ± 0.43 h) and were below the limits of quantification (0.1 µg/mL) by 6-12 h. Based on these results, 1 and 2 mg/kg doses of meloxicam were found to produce plasma concentrations below therapeutic concentrations for less than four hours, making current twice-daily recommended dosing intervals unlikely to provide desired analgesia.

Home ranges, habitat selection, and energy expenditure of Strix varia (Barred Owls): Understanding the full diel cycle matters for enhancing urban landscapes

Abstract We integrated GPS and accelerometer data to examine habitat selection and energy expenditure patterns across the diel cycle in Strix varia (Barred Owls), addressing a critical gap in wildlife research that often overlooks activity during the “inactive” phase. Owls in Baton Rouge, Louisiana, selected forests with tall canopies and open understories, particularly in affluent neighborhoods, supporting the “luxury effect” in urban biodiversity. Nocturnal home ranges were larger (31.8 ha) compared to diurnal home ranges (8.9 ha), indicating broader habitat use at night. The total area of preferred nocturnal habitat was 33% of the Baton Rouge study area, whereas preferred diurnal habitat comprised only 5%. Energy expenditure was inversely related to preference in nocturnal home ranges but increased with preference in diurnal home ranges. Our models were validated using independent data from Clemson, South Carolina, supporting the robustness of our analysis and revealing generalities in owl habitat selection across these regions. This research contributes to a deeper understanding of urban ecology, highlighting habitat components preferred by owls and possibly other forest-dwelling species. It emphasizes the difference in nocturnal and diurnal home range sizes, the scarcity of daytime refuges for S. varia in urban landscapes, and the variation in energy expenditure in preferred habitats. Our findings advocate for urban designs that accommodate wildlife activity throughout the day and night, and highlight the positive correlation between preferred owl habitat and affluent neighborhoods, underscoring the need for equitable distribution of green spaces to foster biodiversity across socioeconomic gradients. These insights will help develop strategies to enhance the ecological value of urban environments and the conservation of S. varia and associated forest-dwelling species in rapidly urbanizing areas.

Ultrasound-guided sciatic nerve block injections in barred owl (Strix varia) cadavers

ObjectiveTo describe an ultrasound-guided approach to the sciatic nerve and the distribution of nerve staining using two injectate volumes of dye in barred owls. Study designDescriptive, randomized, assessor-blinded, cadaveric study. AnimalsTwelve barred owl cadavers. MethodsThe sciatic nerve was visualized with a 13-6 MHz linear ultrasound probe placed on the medial aspect of the proximal femur in barred owl cadavers. A needle was inserted at the proximal caudal thigh, and cranially advanced in-plane until the tip was immediately adjacent and ventral to the sciatic nerve. Eleven owls were injected bilaterally with a 1:1 solution of 1% methylene blue and 0.5% ropivacaine, with high-volume (HV; 0.4 mL kg–1) and low-volume (LV; 0.2 mL kg–1) treatments. Nerve staining ≥ 1 cm circumferentially was determined by dissection 15 minutes post-injection. The coelom was then opened to inspect for aberrant dye spread. In one barred owl cadaver, a 1:1 solution of 1% methylene blue and 74% ioversol contrast was injected, and computed tomography was performed immediately before and 15 minutes after injection for evaluation of injectate spread. ResultsThe HV treatment (0.4 mL kg–1) resulted in successful nerve staining in 100% of injections, whereas the LV treatment (0.2 mL kg–1) resulted in successful nerve staining in 91% of injections. There was no leakage of injectate into undesired locations (e.g. coelomic cavity) in any cadaver. Conclusions and clinical relevanceIn barred owls, the sciatic nerve can be visualized with ultrasound and injecting a ropivacaine/dye solution under ultrasound guidance successfully stained the sciatic nerve in the majority of cases. This technique holds potential for providing analgesia distal to the stifle joint; however, further investigations are necessary to evaluate its practical application in a clinical setting.

Western screech-owl occupancy in the face of an invasive predator

Invasive predators can alter ecosystem function and be detrimental to native wildlife by direct predation and through exploitative and interference competition. Barred owls (Strix varia), a native species in eastern North America, have expanded their range to Pacific Northwest forests, threatening native owls and community dynamics. The western screech-owl (Megascops kennicottii) is a species of conservation concern, and the apparent population decline coincides with the arrival of barred owls. We collected data in 2021 under a broad scale passive acoustic monitoring program at 2482 stations in nine study areas to evaluate western screech-owl distribution and potential threats to population persistence in the Pacific Northwest. We fitted single-species occupancy models to estimate barred owl and western screech-owl landscape use and identify important features of habitat. We quantified effects of barred owls on western screech-owl occupancy and detection probability using co-occurrence models. Barred owls used approximately 0.79 (95 % confidence interval: 0.72, 0.85) of our sampling locations. Western screech-owl occupancy and weekly detection varied by study area from 0.24 (0.15, 0.34) to 0.92 (0.89, 0.95) and 0.12 (0.08, 0.16) to 0.71 (0.69, 0.74), respectively. Western screech-owl detection dropped slightly from 0.36 (0.31, 0.41) to 0.29 (0.26, 0.33) where barred owls were present. Western screech-owls frequently occupied sites used by barred owls but occurred less in older forests where barred owls were present, suggesting they may experience higher predation risk in older forests. Western screech-owls may be unable to avoid habitat used by ubiquitous barred owls, but they may adapt behaviorally to avoid this invasive predator.

Determination of Coagulation Parameters by Whole Blood Dynamic Viscoelastic Coagulometry in Strigiformes.

No reference values are available in Strigiformes to evaluate blood coagulation using dynamic viscoelastic coagulometry (DVC) with the Sonoclot (Sienco, Boulder, CO, USA) analyzer. The objectives of this study were 1) to assess the feasibility of DVC in Strigiformes, 2) to calculate the index of individuality of each coagulation parameter, and 3) to assess interspecies variability and establish reference intervals, if relevant, based on the index of individuality. Fresh whole blood samples were obtained from healthy Strigiformes, including 13 barred owls (Strix varia), 10 great horned owls (Bubo virginianus), 6 snowy owls (Bubo scandiacus), and 7 eastern screech owls (Megascops asio), and analyzed with DVC with glass bead (gb) and kaolin clay (k) coagulation activators. Activated clotting time (ACT), clot rate (CR), and platelet function were determined immediately after collection using fresh native whole blood. Intraindividual variability was assessed with a second fresh native whole blood sample from 5 barred owls. Interindividual variability was assessed using a Kruskall-Wallis test. For the parameters gbACT (n = 35), gbCR (n = 34), and kACT (n = 27), no significant differences were detected between species (all P ≥ 0.05). Based on low index of individuality, global Strigiformes reference intervals were determined for gbACT (32.3-852.5 seconds; n = 35), gbCR (0-20.1 units/min; n = 29), and kACT (0-1570.3 seconds; n = 27). In conclusion, DVC can be used in Strigiformes and the gb coagulation activator would be more appropriate when basal individual values are not available in a tested individual.

Estimating population size for California spotted owls and barred owls across the Sierra Nevada ecosystem with bioacoustics

Monitoring population size at ecosystem scales is difficult for most species of conservation concern. While assessing site occupancy at broad scales has proven feasible, rigorous tracking of changes in population size over time has not – even though it can provide a stronger basis for assessing population status and conservation-decision making. Therefore, we demonstrate how relatively low-intensity, ecosystem-scale passive acoustic monitoring (PAM) can be linked to local-density monitoring to estimate the population size of native California spotted owls (Strix occidentalis occidentalis) and invasive barred owls (S. varia) across the western Sierra Nevada, California. Based on a PAM sampling grid with 400 ha cells (the approximate home range size of these species), we estimated site occupancy to be between 0.42 (SE = 0.02) and 0.30 (SE = 0.02) for California spotted owls using relatively liberal and strict criteria, respectively, for considering a cell occupied. PAM-based site occupancy estimates within local-scale density monitoring study areas (range = 0.41–0.78 and 0.28–0.76 for liberal and strict criteria, respectively) were strongly and positively correlated with local density (range = 0.08–0.31 owl/km2) for this species. In contrast, ecosystem-wide site occupancy of barred owls was very low based on PAM (0.034, SE < 0.01), as were densities within local monitoring studies (range = 0–0.005 owls/km2). By scaling ecosystem-wide site occupancy estimates to densities estimated with local monitoring studies, we estimated that, depending on occupancy criteria, 2,218 (SE = 278) or 2,328 (SE = 489) California spotted owls occurred in the Sierra Nevada ecosystem in 2021. Thus, while California spotted owls are a rare subspecies, they were well-distributed across the Sierra Nevada. Because there were so few barred owl detections, we could not estimate ecosystem-scale abundance, which reflects the success of prior experimental removals in the region. In conclusion, our study provides a generalizable framework for estimating population size for territorial species with PAM at ecosystem scales when local-scale estimates of density are available. Thus, we demonstrate that this approach can provide novel and valuable insights into monitoring populations to aid species conservation.

APPARENT PREVALENCE, DIVERSITY, AND ASSOCIATED LESIONS OF PERIORBITAL NEMATODES IN A POPULATION OF BARRED OWLS (STRIX VARIA) FROM NORTHERN CALIFORNIA, USA.

Over the last four decades, Barred Owls (Strix varia) have expanded their range to include much of western North America, including California. This expansion is suspected to have contributed to declining populations of a closely related species, the federally threatened Northern Spotted Owl (Strix occidentalis caurina). As a result, understanding potential health threats to Barred Owls has implications for Spotted Owl health and recovery. From 2016 to 2020, 69 Barred Owls were collected to determine the apparent prevalence of periorbital nematode infection to identify the parasite species present and to investigate the potential pathologic effects on their hosts. The nematodes were morphologically identified as Oxyspirura and Aprocta spp. On the basis of phylogenetic analyses, they were clearly divergent from published sequences of other species within these genera. Overall, 34 (49%) Barred Owls were infected with periorbital nematodes, with Oxyspirura sp. infections being much more common (94%) than Aprocta sp. (18%). Histopathology revealed varying severity of conjunctivitis in infected owls. Despite the frequency of infection and subsequent inflammation, parasite burden was not associated with reduced body weight in these owls. As a result, the potential health effect of these nematodes is unclear. Further taxonomic characterization is needed to determine potential novelty of these nematodes.

Assessing plastic ingestion in birds of prey from British Columbia, Canada

Since first being introduced for public use in the 1960s, plastic has become one of the most pervasive and ubiquitous forms of pollution globally. The potential fate and effects of plastic pollution on birds is a rapidly growing area of research, but knowledge of terrestrial and freshwater species is limited. Birds of prey have been particularly understudied, with no published data on plastic ingestion in raptors in Canada to date, and very few studies globally. To assess the ingestion of plastic in raptors, we analysed the contents of the upper gastrointestinal tracts from a total of 234 individuals across 15 raptor species, collected between 2013 and 2021. Upper gastrointestinal tracts were assessed for plastics and anthropogenic particles > 2 mm in size. Of the 234 specimens examined, only five individuals across two species had evidence of retained anthropogenic particles in the upper gastrointestinal tract. Two of 33 bald eagles (Haliaeetus leucocephalus, 6.1%) had retained plastics in the gizzard, while three of 108 barred owls (Strix varia, 2.8%) had retained plastic and non-plastic anthropogenic litter. The remaining 13 species were negative for particles > 2 mm in size (N = 1–25). These results suggest that most hunting raptor species do not appear to ingest and retain larger anthropogenic particles, though foraging guild and habitat may influence risk. We recommend that future research investigate microplastic accumulation in raptors, in order to gain a more holistic understanding of plastic ingestion in these species. Future work should also focus on increasing sample sizes across all species to improve the ability to assess landscape- and species-level factors that influence vulnerability and susceptibility of plastic pollution ingestion.

Passive acoustic monitoring indicates Barred Owls are established in northern coastal California and management intervention is warranted

Abstract Barred Owls (Strix varia) have recently expanded westward from eastern North America, contributing to substantial declines in Northern Spotted Owls (Strix occidentalis caurina). Passive acoustic monitoring (PAM) represents a potentially powerful approach for tracking range expansions like the Barred Owl’s, but further methods development is needed to ensure that PAM-informed occupancy models meaningfully reflect population processes. Focusing on the leading edge of the Barred Owl range expansion in coastal California, we used a combination of PAM data, GPS-tagging, and active surveys to (1) estimate breeding home range size, (2) identify patterns of vocal activity that reflect resident occupancy, and (3) estimate resident occupancy rates. Mean breeding season home range size (452 ha) was reasonably consistent with the size of cells (400 ha) sampled with autonomous recording units (ARUs). Nevertheless, false-positive acoustic detections of Barred Owls frequently occurred within cells not containing an activity center such that site occupancy estimates derived using all detected vocalizations (0.61) were unlikely to be representative of resident occupancy. However, the proportion of survey nights with confirmed vocalizations (VN) and the number of ARUs within a sampling cell with confirmed vocalizations (VU) were indicative of Barred Owl residency. Moreover, the false positive error rate could be reduced for occupancy analyses by establishing thresholds of VN and VU to define detections, although doing so increased false negative error rates in some cases. Using different thresholds of VN and VU, we estimated resident occupancy to be 0.29–0.44, which indicates that Barred Owls have become established in the region but also that timely lethal removals could still help prevent the extirpation of Northern Spotted Owls. Our findings provide a scalable framework for monitoring Barred Owl populations throughout their expanded range and, more broadly, a basis for converting site occupancy to resident occupancy in PAM programs.

Adult female Barred Owl (Strix varia) eating small caterpillars (family Geometridae) while guarding a recently fledged chick in Winnipeg, Manitoba

Adult female Barred Owl (Strix varia) eating small caterpillars (family Geometridae) while guarding a recently fledged chick in Winnipeg, Manitoba

IN VIVO CONFOCAL MICROSCOPY FOR CHARACTERIZATION OF MYCOTIC KERATITIS IN OWLS (BUBO SCANDIACUS, STRIX VARIA) AND A WOODCOCK (SCOLOPAX MINOR): THREE CASES.

This case series describes the use of in vivo confocal microscopy in the diagnosis and treatment of mycotic keratitis in two owls (one Bubo scandiacus, one Strix varia) and one woodcock (Scolopax minor). Each bird was at increased risk of fungal infection due to recent injury or stress. Ophthalmic findings in all birds included blepharospasm, ocular discharge, ulcerative keratitis, white or yellow corneal plaques, and anterior uveitis. Fungal hyphae were identified in corneal samples from all three eyes examined cytologically and in all three eyes by using in vivo confocal microscopy. Aspergillus fumigatus was isolated from a corneal culture in one bird. Despite medical treatment, progressive ocular disease prompted enucleation in two birds. Fungal hyphae were detected by histopathology in one of the two enucleated eyes. In vivo confocal microscopy aided the diagnosis of fungal keratitis in all birds and was the only diagnostic method that allowed immediate, real-time quantification of the extent (area and depth) and severity of mycotic keratitis.

Estimating population dynamics trajectories of raptors from a multi-species hierarchical distance sampling model

Community dynamics are essential for ecosystems as they support the functioning of food webs and maintain biodiversity. However, the fluctuations of species abundances are rarely analyzed simultaneously despite their interconnectedness across space and time. We developed a Bayesian multi-species model that estimates community- and taxon-level parameters related to dynamics, abundance, detection probability, and environmental stochasticity. From this model, we analyze the variability of populations of a community of raptors composed of accipiters (sharp-shinned hawk [Accipiter striatus], and Cooper's hawk [A. cooperii]), buteos (red-tailed hawk [Buteo jamaicensis], and red-shouldered hawk [B. lineatus]) and owls (great horned owl [Bubo virginianus], and barred owl [Strix varia]). We show that migration pulses of accipiters, buteos, and dispersal of fledging owls drive variation in raptor abundances, indicating a defined production and dispersal period. Also, large-scale climatic processes such as the North Atlantic Oscillation (NAO) influenced the variation in abundances of raptors. When the NAO was in a positive phase, the abundance of raptors decreased, and inversely. Local weather did not affect raptor population abundances. Our approach facilitated the modelling of species-specific effects of environmental variation and guild-level dynamics that could be used for ecosystem-based conservation measures.

Using passive acoustic monitoring to estimate northern spotted owl landscape use and pair occupancy

AbstractManaging forests for biodiversity conservation while maintaining economic output is a major challenge globally and requires accurate and timely monitoring of imperiled species. In the Pacific Northwest, USA, forest management is heavily influenced by the status of northern spotted owls (Strix occidentalis caurina), which have been in continued population decline for the past four decades. The monitoring program for northern spotted owls is transitioning from mark–resight surveys to a passive acoustic framework, requiring development of alternative analysis approaches. To maintain relevance for conservation and management, these analyses must accurately track underlying population changes, identify responses to disturbance, and estimate occupancy of owl pairs. We randomly selected and surveyed 5‐km2 hexagons for 6 weeks using passive acoustic monitoring in the Olympic Peninsula of Washington and the Oregon Coast Range during the 2018 spotted owl breeding season. We used a convolutional neural network to identify spotted owl calls, followed by logistic regression to determine the sex of vocalizing owls to assign pair status. We implemented multistate occupancy models to estimate probabilities of detection, species‐level landscape use, and pair occupancy of spotted owls. We also quantified detections of barred owls (Strix varia), a congeneric competitor and important driver of spotted owl population declines. The overall rate of hexagon use by spotted owls was estimated at 0.21 (SD 0.04) after adjusting for imperfect detection, and pair occupancy was 0.07 (SD 0.02). The probability of detecting a pair (i.e., both female and male) during a weekly occasion was relatively low (0.03, SD 0.01), indicating that true pair occupancy was between 1.3 and 4.1 times greater than the proportion of hexagons with observed pair detections. Barred owls were ubiquitous, with a naïve occupancy rate of 0.97. The intensity of calling by barred owls had a weak, negative effect on the probability of spotted owls being paired when present but had little measurable effect on their detectability. This work establishes a framework that may be effective for spotted owl population monitoring and illustrates that pairs have very low detection probability, which—combined with increasingly low numbers of spotted owl pairs—is an important consideration for conservation and management.

Estimating fat content in barred owls (Strix varia) with predictive models developed from direct measures of proximate body composition.

Body condition indices and related metrics can help assess habitat quality and other ecological processes, and ideally, these metrics are based on measures of lipids directly extracted from the species of interest. In recent decades, barred owls (Strix varia) have become a species of conservation concern as they invaded older forests of the US Pacific Northwest, and caused population declines of the closely related and federally threatened northern spotted owl (Strix occidentalis caurina). A simple and effective measure of barred owl body condition could help to understand how habitat quality varies within their new range, which in turn can inform their management and other aspects of their ecology. Using 77 barred owl carcasses collected during experimental removals in Washington and Oregon, USA, we measured the amount of lipid in each specimen with proximate body composition analysis. We then fit and compared (with adjusted R2 values) alternative linear regression models to estimate the percent lipids in dry mass of the owls based on morphometric body condition indices, a qualitative fat score of subcutaneous breast fat, sex and the time of year females were collected (relative to egg production). Adjusted R2 values for all models ranged from 0.49 to 0.87, with the best model including mass divided by foot-pad length, fat score, sex and the time of year a female was collected. Most models generated comparable estimates of percent lipids at a population level and we provided correction factors to apply these models when used with live barred owls, allowing for site-specific comparisons of body condition among individuals inhabiting a diversity of environmental conditions.

Long-term monitoring in transition: Resolving spatial mismatch and integrating multistate occupancy data

The success of long-term wildlife monitoring programs can be influenced by many factors and study designs often represent compromises between spatial scales and costs. Adaptive monitoring programs can iteratively manage this tension by adopting new cost-efficient technologies, which can provide projects the opportunity to reallocate costs to address new hypotheses, adapt to changing ecological conditions, or adjust sampling scale or resolution. If there is interest in longer time series of monitoring data, methodological transitions may necessitate integrated models to link newer data with historical data. However, data integration can be difficult if spatial or temporal scales are mismatched. Here, we develop an integrated multistate site-occupancy model and resolve sample unit spatial mismatch to link datasets from two northern spotted owl (Strix occidentalis caurina) monitoring schemes that broadly overlapped during a methodological transition. The first dataset was obtained from a decades-long spotted owl monitoring program using call-playback and mark-resight surveys on historical territories of varying size and shape. This monitoring program has recently transitioned to passive acoustic monitoring of randomly selected 5-km2 hexagons over larger spatial extents. Both monitoring datasets overlapped with areas in which barred owl (Strix varia), an invasive competitor that has played an important role in northern spotted owl declines, were being removed experimentally. Reconciling spatial mismatch substantially increased the representation of the call-playback dataset and integrating the two datasets increased precision of spotted owl use and paired occupancy estimates relative to single dataset estimates. Estimates of spotted owl pair occupancy across the study area were lower than previous territory-based estimates based on call-playback surveys. Our integrated model further showed that a concurrent barred owl removal experiment increased landscape use and site occupancy by pairs of spotted owls. Our empirical application of an integrated modelling approach demonstrates a useful analytical framework for long-term monitoring efforts undergoing methodological transitions (e.g. mark-recapture to non-invasive population monitoring). This framework allows monitoring programs to maintain continuity of monitoring objectives across methodological transitions, rigorously incorporate previous findings, and adaptively respond to changing ecological conditions.

Scavenging by a Barred Owl in Northern Georgia

Scavenging by owls is rarely documented, and fidelity to a scavenged carcass is even less common. Herein we describe multiple visits by presumably the same Strix varia (Barred Owl) to a carcass over 4 consecutive nights during the breeding season. Our observations demonstrate higher site-fidelity than is typically reported for owls and suggest that opportunistic scavenging may occasionally represent a low-cost means of meeting energetic demands with potentially important consequences for individual survival during nesting.

BARRED OWL PREDATION OF WESTERN SPOTTED SKUNKS

The potential for trophic cascades triggered by recent range expansion of the Barred Owl (Strix varia) to the Pacific Northwest has caused concern among conservationists and managers. Barred Owl predation of small forest carnivores is a particular concern because these carnivores typically have low population growth rates relative to their body size owing to long interbirth intervals, which may result in sensitivity to increased mortality. The Western Spotted Skunk (Spilogale gracilis) is a common small carnivore in forests of the Pacific Northwest that may be a prey item for Barred Owls, and previous research suggests that avian predation can be a primary cause of mortality for congeneric spotted skunks (Spilogale spp.). We report a confirmed predation event of a Western Spotted Skunk by a Barred Owl and 3 additional predation events that we suspect were due to Barred Owls based on circumstantial evidence. During a Western Spotted Skunk research study, we recovered the radio collar of an adult male skunk from the top of a tall snag and located intestines and avian feces at the base of this snag. DNA metabarcoding revealed that the avian feces contained Western Spotted Skunk and Barred Owl DNA. Barred Owls are a novel predator of the Western Spotted Skunk in forests of the Pacific Northwest and may have both direct and indirect negative impacts on Western Spotted Skunk populations.

Forest Management, Barred Owls, and Wildfire in Northern Spotted Owl Territories

The Northern Spotted Owl (Strix occidentalis caurina) (NSO) was listed as federally threatened in 1992 due to widespread logging of its old-growth forest habitat. The NSO recovery plan in 2011 elevated competition with Barred Owls (Strix varia) (BO) and wildfires as primary NSO threats based partly on the assumption that severely burned forests were no longer NSO nesting and roosting habitat. We quantified amount of logging before and/or after wildfire and opportunistic detections of BOs within two home range scales (0.8 and 2.09 km) at 105 NSO sites that experienced severe wildfire from 2000–2017. Logging affected 87% of severely burned NSO sites, with BO recorded at 22% of burned-and-logged sites. Most (60%) severely burned NSO sites had evidence of logging both before and after fires while only 12% of severely burned sites had no logging or BO detections, indicating rarity of NSO territories subjected to severe fire without the compounding stressors of logging and invasive BOs. We recommend changes to NSO habitat modeling that assume nesting and roosting habitat is no longer viable if severely burned, and to the U.S. Fish and Wildlife Service’s practice of granting incidental take permits for NSOs in logging operations within severely burned owl sites.

DNA metabarcoding reveals the threat of rapidly expanding barred owl populations to native wildlife in western North America

Invasive predators can have detrimental impacts on native species and biological communities through direct consumptive effects and indirect effects on trophic interactions, yet these relations are often poorly understood through the initial stages of predator expansion. Leveraging early, lethal collection efforts to study invasive barred owls (Strix varia) at the leading edge of their range expansion in northeastern California, we conducted DNA metabarcoding of intestinal samples to assess the diet of this invasive predator. Through the development of customized primers and this novel approach to observing owl diet, we screened the intestinal contents of 124 barred owls and detected a broad diet of 78 unique prey types (48 vertebrates and 30 invertebrates), including prey types undetected in previous methodologies. Mammals were the most consumed vertebrate class (frequency of occurrence = 65 %), followed by amphibians (32 %), birds (22 %), and reptiles (19 %). Diets differed regionally but were similar among ages and sexes and exhibited limited variation in response to local environmental conditions. Our work highlights the generalist predatory strategy of invasive barred owls, identifies numerous native species potentially threatened by their range expansion, and indicates that they will not serve as ecological replacements for congeneric spotted owls (Strix occidentalis) they displace. These findings suggest that expanding currently limited barred owl removals could benefit native species and communities in western North America. More broadly, we demonstrate DNA metabarcoding, combined with early intervention, provides a powerful tool for conducting detailed assessments of species consumed by invasive predators, potentially incentivizing conservation actions and improving outcomes.

Anticoagulant Rodenticide Contamination of Terrestrial Birds of Prey from Western Canada: Patterns and Trends, 1988-2018.

As the dominant means for control of pest rodent populations globally, anticoagulant rodenticides (ARs), particularly the second‐generation compounds (SGARs), have widely contaminated nontarget organisms. We present data on hepatic residues of ARs in 741 raptorial birds found dead or brought into rehabilitation centers in British Columbia, Canada, over a 30‐year period from 1988 to 2018. Exposure varied by species, by proximity to residential areas, and over time, with at least one SGAR residue detected in 74% of individuals and multiple residues in 50% of individuals. By comparison, we detected first‐generation compounds in <5% of the raptors. Highest rates of exposure were in barred owls (Strix varia), 96%, and great horned owls (Bubo virginianus), 81%, species with diverse diets, including rats (Rattus norvegicus and Rattus rattus), and inhabiting suburban and intensive agricultural habitats. Barn owls (Tyto alba), mainly a vole (Microtus) eater, had a lower incidence of exposure of 65%. Putatively, bird‐eating raptors also had a relatively high incidence of exposure, with 75% of Cooper's hawks (Accipiter cooperii) and 60% of sharp‐shinned hawks (Accipiter striatus) exposed. Concentrations of SGARs varied greatly, for example, in barred owls, the geometric mean ∑SGAR = 0.13, ranging from <0.005 to 1.81 μg/g wet weight (n = 208). Barred owls had significantly higher ∑SGAR concentrations than all other species, driven by significantly higher bromadiolone concentrations, which was predicted by the proportion of residential land within their home ranges. Preliminary indications that risk mitigation measures implemented in 2013 are having an influence on exposure include a decrease in mean concentrations of brodifacoum and difethialone in barred and great horned owls and an increase in bromodialone around that inflection point. Environ Toxicol Chem 2022;41:1903–1917. © 2022 Her Majesty the Queen in Right of Canada. Environmental Toxicology and Chemistry published by Wiley Periodicals LLC on behalf of SETAC. Reproduced with the permission of the Minister of Environment and Climate Change Canada.