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Invasive species are a primary threat to biodiversity and are challenging to manage once populations become established in previously unoccupied areas. But removing them is further complicated when invasions occur in continental, mixed‐ownership systems. We demonstrate a rare conservation success: the regional‐scale removal of an invasive predator – the barred owl (Strix varia) – to benefit the spotted owl (Strix occidentalis) in California. Barred owl site occupancy declined sixfold, from 0.19 to 0.03, following 1 year of removals, and site extinction (0.92) far exceeded colonization (0.02). Spotted owls recolonized 56% of formerly occupied territories within 1 year, contrasting starkly with removals conducted after barred owls achieved high densities in the Pacific Northwest. Our study therefore averted the otherwise likely extirpation of California spotted owls (Strix occidentalis occidentalis) by barred owl competition. Collectively, leveraging technological advances in population monitoring, early intervention, targeting defensible biogeographic areas, and fostering public–private partnerships will reduce invasive species‐driven extinction of native fauna in continental systems.

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The great gray owl (Strix nebulosa) is listed by the state of California as endangered, with a population estimate of fewer than 300 individuals in the state. Nest‐site availability has been suggested as a limiting factor for population growth in California, but information on nest types and nesting habitat has been based on a small number of nests that may not fully represent the variety of conditions used by the species. We collated all known nesting records in the Sierra Nevada mountains of California since 1973 (n = 56) and then visited 47 of the nest sites to characterize habitat and compare them with paired reference sites. Great gray owls used a diversity of trees (8 species) and nest types. Although great gray owls in California are considered conifer‐forest specialists, 30% of nests were in oak trees and 21% were below 1,000 m, which loosely corresponds to the lower conifer‐zone limit. Across all elevations and tree species, degree of deterioration was the most important factor differentiating nest trees from paired reference trees at the same meadow, with nest trees being significantly more decayed. Nest trees (mean dbh = 100.5 ± SD 30.3 cm) were also significantly larger than reference trees. Canopy cover within 50 m of nest trees (x¯ = 85.1 ± 16.4%) was significantly greater at nest sites than at reference sites. At higher elevations, most nests were within 250 m of a meadow edge, but at lower elevations, 31% of nests were >750 m from the closest meadow. Based on these findings, we suggest that managers trying to promote great gray owl nesting maintain 4 or more large (100‐cm dbh) snags per hectare in dense forests, especially near meadows. We also recommend increasing great gray owl survey effort in habitats and areas that may have been inadequately surveyed in the past. Published 2015. This article is a U.S. Government work and is in the public domain in the USA.