Documenting the breeding habitat and distribution of migratory songbirds is essential for accurately assessing their conservation status. The “sagebrush” subspecies of the Brewer’s Sparrow (Spizella breweri breweri) breeds in greatest abundance in sagebrush-dominated (Artemisia spp.) shrublands of western North America, and the “timberline” subspecies (S. b. taverneri) breeds in shrubs or krummholz at or above treeline in mountain ranges from Alaska to Montana. Brewer’s Sparrows have also been reported at alpine sites in summer in mountain ranges across the western United States, but their taxonomic affiliation and breeding status are unknown. We reviewed monitoring, citizen science, and specimen data for Colorado and identified 186 historical summer observations of Brewer’s Sparrows at 59 alpine sites (3334–4288 m elevation). We surveyed 39 alpine sites in June-July 2021–2023, detected a total of 100 adults (mostly singing males) at 26 alpine sites (3395–3754 m elevation), and confirmed breeding at three sites. Males occupied mixed-species willow patches 0.9–1.8 m tall, often intermixed with sparse conifer krummholz. We recorded songs and captured, measured, photographed, and collected blood and feather samples from birds at a subset of alpine sites and nearby, lower-elevation sagebrush sites in May-July 2021. Vegetation associations and timing of breeding at alpine sites closely matched those of taverneri, but short songs, external morphology, coloration, and genetics of alpine birds overlapped with sagebrush birds and more closely matched those of range-wide breweri. Our results indicate that western Colorado supports a widely-distributed, but poorly-documented population of alpine breweri breeding in willows with sparse conifer krummholz within a relatively narrow elevation band at or above treeline. Whether alpine birds are itinerant breeders that first nested in sagebrush remains unknown. Our results complicate interpretation of differences in breeding habitat and breeding phenology as supporting criteria for subspecific identification and taxonomic delineation in this species.

Native to eastern North America, barred owls invaded and occupied the range of northern and California spotted owls in recent times. The invasion of competitively dominant barred owls poses an existential threat to spotted owls and is likely adversely affecting many other native species. Lethal removals are currently the most effective tool for curbing barred owl populations, but whether they can be implemented effectively to recover spotted owl populations across the State of California is uncertain. To test the feasibility of state-wide barred owl management, we have initiated a removal study that encompasses all national forests and national parks in the Sierra Nevada and many landownerships from the San Francisco Bay Area to the Oregon border. Here we report on the number and locations of barred owl removals across the state and preliminary findings of recolonization patterns by spotted owls. We also discuss challenges and opportunities confronting the implementation of barred owl management in California. Finally, we describe a suite of other questions related to the rapid invasion of this generalist predator that we propose to address using biological samples collected from lethal removals.

The consequences of environmental change on animal communities can be difficult to predict and measure because individual ecological stressors can interact in complex ways. Here, we assessed the relative and interactive effects of severe wildfire and invasive barred owls (Strix varia) on native forest owls in the northern Sierra Nevada, California, USA, using occupancy data collected via regional-scale passive acoustic surveys under a quasi-experimental design from 2018 to 2023. Our findings suggest severe wildfire reduced occupancy rates for flammulated owls (Psiloscops flammeolus) but not for great horned owls (Bubo virginianus) or northern pygmy owls (Glaucidium californicum). By contrast, barred owl removals did not increase occupancy rates for any of the three owl species. Our research demonstrates the pronounced impact of severe wildfires on some native owls, while the initial invasion of barred owls at low densities seems to have no adverse effects on them, underscoring the effectiveness of barred owl removals in protecting native owl populations. Using a rare Before-After Control-Impact design, our study highlights the importance of experimental methods in diagnosing causes of species endangerment and understanding ecosystem responses, while also elucidating the complexities of multifactorial studies at regional spatial scales.

Fire-adapted species have evolved to exploit foraging opportunities in heterogenous landscapes. However, the widespread disruption of historical fire regimes may threaten the ability of fire-adapted species to acquire sufficient energy to survive and reproduce. The California spotted owl is a forest species that benefits from forest heterogeneity for hunting. While fires can have long lasting negative effects on spotted owl populations, we have yet to identify a mechanistic explanation for this species’ response to novel megafires. Here, we used a combination of fine-scale GPS tagging and camera monitoring in the Sierra Nevada and San Bernardino Mountains, California, to examine the impact of fire on the foraging behaviors and energetic consumption of breeding spotted owls. We found that individuals spent more energy for no energetic benefits in severely burned landscapes. In landscapes with higher pyrodiversity, individuals spent more energy obtaining resources but delivered prey with higher energetic potential to nestlings. At a fine-scale, successful prey captures were less likely where there was more high-severity fire, while lower-severity fire and a moderate level of pyrodiversity facilitated prey capture for some individuals. Thus, fire-driven heterogeneity may create hunting opportunities that promote energetic investments towards reproduction. However, as the climate warms and fires become increasingly large and contiguously severe, the energy spotted owls spend to forage may offset any energetic benefits of fire-driven heterogeneity. Ultimately, identifying forest restoration strategies that limit high-severity fire while promoting structural heterogeneity will be necessary to conserve trophic interactions in this and other forested ecosystems.

Concern for potential effects on California spotted owls can constrain forest restoration projects intended to reduce large, severe wildfires and drought-related tree mortality in the Sierra Nevada. We developed an acoustically assisted survey design that could increase the efficiency and effectiveness of project-level surveys for spotted owls, allowing surveys to be completed in a single year. To do so, we deployed 126 autonomous recording units (ARUs) and identified spotted owl vocalizations using BirdNET. We evaluated spatio-temporal patterns in vocalizations near occupied territories and a survey crew’s ability to locate owls based on these detections. After 3 weeks of acoustic surveys, ≥1 ARU within 750 m of all 17 occupied territories obtained spotted owl detections across ≥2 nights. Surveyors naïve to territory occupancy and location located owls in 93%-100% of occupied territories with ≤3 active (broadcast calling) surveys near ARUs with detections. We also developed a statistical model to identify and prioritize areas across the Sierra Nevada for different survey methods (active only/acoustically assisted/no surveys) based on the expected probability of occupancy. Collectively, these findings can help managers streamline the survey process and thus increase the pace of forest restoration while minimizing potential near term adverse effects to California spotted owls.

Robust monitoring of imperiled species is critical for developing effective forest management strategies and conserving biodiversity. The California spotted owl is a subspecies of conservation concern due to a combination of past land-use practices and emerging threats (e.g., large, high-severity fire), and is at the center of forest planning and restoration efforts. In 2021, we initiated a large-scale bioacoustics monitoring program—significantly expanding upon surveys conducted since 2017—by deploying ~1,600 autonomous recording units annually across the Sierra Nevada. We then leveraged a novel bioinformatics pipeline and machine learning algorithm (BirdNET) to detect spotted owl calls in >1 million hours of audio recordings. Here, we provide a summary of the first two years of the expanded monitoring program, with a focus on estimating spotted owl occupancy and evaluating the effects of high severity fire. Results from such monitoring and occupancy modeling efforts will be used to better inform spotted owl conservation and forest management practices, and can also be extended to derive estimates of population size. Overall, our research program demonstrates the feasibility of monitoring rare, vocalizing species over a large scale with bioacoustics, and in the long-term will enable us to study avian community responses to global change.

Effective wildlife management requires an understanding of how site, patch, and landscape-scale habitat features interact to shape the distribution and abundance of species. Diverse forest management practices and disturbance events within the Sierra Nevada, California have created a spatially complex landscape where multiscale processes and patch dynamics play a central role in species interactions. In this system, the dusky-footed woodrat (Neotoma fuscipes) serves as an important prey species for many forest predators, including California spotted owls (Strix occidentalis occidentalis). While site-level features used by woodrats have been previously investigated, the significance of patch dynamics and landscape composition have received considerably less attention. We live-trapped woodrats and modeled occupancy relative to habitat features at all three spatial scales, including a number of a priori interactions. Occupancy probability increased with understory cover and hardwoods, although the relative importance of each was greater within mature forest. Further, occupancy was greatest in patches of younger forest and, within mature forest, increased with proximity to younger forest. Our findings highlight the benefits of multiscale approaches and demonstrate that promoting landscape heterogeneity, as well as resource availability and protective cover, may benefit woodrat populations and enhance spotted owl conservation in Sierra Nevada forests.