Wildfire has been a fundamental component of ecosystem dynamics for millennia. However, climate change, fire suppression, and land use practices have produced larger, more severe, and more frequent fires that threaten a range of wildlife species. Land managers in some fire-prone forest systems use fuels reduction treatments, including prescribed burning and mechanical thinning, to reduce vegetation density and the risk of large, high-severity fires. The strategy is to create forest landscapes that are more resilient to wildfire, which in turn will lessen the negative impact of uncharacteristically severe wildfire on wildlife. Yet there is uncertainty regarding the extent to which fire-adapted species are threatened by novel wildfire characteristics. The spotted owl (Strix occidentalis) has become a focal point for this controversy owing to its use of fire-suppressed forests that have been and will be vulnerable to changing fire regimes in the future. In this study, we attempted to address the question of how spotted owls respond to high-severity fire in California, USA, by leveraging 1) over 3 decades of detection-non-detection surveys in 1,514 historical spotted owl territories and 2) global positioning system (GPS) movement data from 171 individual spotted owls. Our study area included 7 national forests, 3 national parks, and extensive privately owned forests in California. Dynamic occupancy analysis indicated that spotted owl territories were more likely to become vacant and less likely to be colonized if they experienced recent and extensive high-severity fire. Consistent with the occupancy analysis, our movement analysis indicated that individual spotted owls avoided using severely burned forest. Although spotted owl territory persistence was higher in pyrodiverse territories (i.e., those with a mixture of low, moderate, and high severity burned areas), foraging owls showed mixed responses to local pyrodiversity, with some evidence that use of severely burned forest increased when pyrodiversity was high. We conclude that large and severe wildfires threaten the persistence of spotted owls in California. That said, forest restoration and fuels reduction treatments that reduce the risk of large and uncharacteristically severe fires while promoting multi-scale heterogeneity and pyrodiversity will likely benefit spotted owls and other species that occupy mixed conifer forests in California.

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.

In the first sentence of the fifth paragraph of the Discussion, the authors incorrectly stated the relationship between logging and territory occupancy. The sentence should read instead: "Logging was associated with occupancy in only one study area (Eldorado), where it unexpectedly had a negative association with territory extinction."

Restoration of western dry forests in the USA often focuses on reducing fuel loads. In the range of the spotted owl, these treatments may reduce canopy cover and tree density, which could reduce preferred habitat conditions for the owl and other sensitive species. In particular, high canopy cover (>= 70%) has been widely reported to be an important feature of spotted owl habitat, but averages of stand-level forest cover do not provide important information on foliage height and gap structure. To provide better quantification of canopy structure, we used airborne LiDAR imagery to identify canopy cover in different height strata and the size and frequency of gaps that were associated with owl nest sites, protected activity centers (PACs), and territories within four study areas and 316 owl territories. Although total canopy cover was high in nest stands and PAC areas, the cover in tall ( > 48 m) trees was the canopy structure most highly selected for, while cover in lower strata (2-16 m) was avoided compared to availability in the surrounding landscape. Tall tree cover gradually decreased and lower strata cover increased as distance increased from the nest. Large ( > 1000 m(2)) gaps were not found near nests, but otherwise there was no difference in gap frequencies and sizes between PACs and territories and the surrounding landscape. Using cluster analysis we classified canopy conditions into 5 structural classes and 4 levels of canopy cover to assess the relationship between total canopy cover and tree size within nest sites, PACs, and territories. High canopy cover (>= 70%) mostly occurs when large tree cover is high, indicating the two variables are often confounded. Our results suggest that the cover of tall trees may be a better predictor of owl habitat than total canopy cover because the latter can include cover in the 2-16 m strata - conditions that owls actually avoid. Management strategies designed to preserve and facilitate the growth of tall trees while reducing the cover and density of understory trees may improve forest resilience to drought and wildfire while also maintaining or promoting the characteristics of owl habitat.

We assessed the occupancy dynamics of 275 California Spotted Owl (Strix occidentalis occidentalis) territories in 4 study areas in the Sierra Nevada, California, USA, from 1993 to 2011. We used Landsat data to develop maps of canopy cover for each study area, which we then used to quantify annual territory-specific habitat covariates. We modeled the relationships between territory extinction and colonization using predictor variables of habitat, disturbance (logging, fire), climate, and elevation. We found that forests with medium (40-69%) and high (>= 70%) canopy cover were the most important predictors of territory occupancy in all study areas, and that both canopy cover categories were positively correlated with occupancy. We used analysis of deviance to estimate the amount of variation explained by the habitat covariates (primarily medium and high canopy cover) and found that these covariates explained from 35% to 67% of the variation in occupancy. Climatic covariates were not correlated with occupancy dynamics and explained little of the variation in occupancy. We also conducted a post hoc analysis in which we partitioned canopy cover into 10% classes, because our original partitioning into 3 classes may have lacked sufficient resolution to identify canopy cover levels where occupancy changed abruptly. In this post hoc analysis, occupancy declined sharply when territories contained more area with <40% canopy cover, and the amount of 50-59% and 60-69% canopy cover had a more positive association with occupancy than did 40-49% canopy cover. Our results suggest that some fuels treatments intended to reduce fire risk and improve forest resilience could be located within Spotted Owl territories without adversely impacting territory occupancy if such treatments do not consistently reduce canopy cover below 50%. We suggest that future work quantify components of forest structure (e.g., large tree density, vertical complexity) known to be selected by owls and relate these characteristics to occupancy and fitness metrics.

Monitoring studies often use marked animals to estimate population abundance at small spatial scales. However, at smaller scales, occupancy sampling, which uses detection/nondetection data, may be useful where sites are approximately territories, and occupancy dynamics should be strongly correlated with population dynamics. Occupancy monitoring has advantages in that it is less expensive and invasive, and marked animals are not needed. Here, we used empirical data to determine whether and when change in occupancy is a good proxy for population change for a territorial species. As part of this overall goal, we also compared maximum-likelihood estimates using a model-averaging approach with a Bayesian MCMC approach. We used field data collected from 1993 to 2013 on three study areas for California spotted owls (Strix occidentalis occidentalis), a territorial species. Although correlations for trajectories of realized population change (Delta(t)) between territory occupancy and Pradel models were moderate to high for Bayesian MCMC-based estimates and high for model-averaged estimates, magnitudes of the trajectories were different with the Pradel model reporting greater magnitudes of change. For the two areas showing a decline, Delta(t) for the Pradel model was approximately 20-30% lower than for the occupancy model, and 25% higher in the area showing an increase. These differences can arise because the occupancy model is less sensitive, in that if two owls share a territory, the loss of one may be reflected in survival and, consequently in Delta(t) by the Pradel model, but because the territory remains occupied it is not reflected by the occupancy model. Bayesian MCMC-based and model-averaged estimates of Delta(t) were in close agreement in pattern (correlation >= 0.74) and magnitude (relative differences of last Delta(t) were <= 5%) for both occupancy and mark-resight models. Results from the Pradel model may lead to conservation actions necessary to avoid high extinction or extirpation risk for small populations, while results from the territory occupancy model may result in status quo management. We found both Bayesian MCMC-based and model-averaged estimates of Delta(t) robust approaches to evaluate population trends. However, we recommend the Bayesian MCMC approach for estimating risk (e.g., probability of declines) for retrospective analyses.

ABSTRACTThe annual rate of population change (λt) is a good metric for evaluating population performance because it summarizes survival and recruitment rates and can be used for open populations. Another measure of population performance, realized population change (Δt) is an encompassing metric of population trend over a period of time; it is the ratio of population size at an end time period relative to the initial population size. Our first goal was to compare mean λ and Δt as summaries of population change over time. Our second goal was to evaluate different methods for estimating these parameters; specifically we wished to compare the value of estimates from fixed effects models, random effects estimates from mixed effects models, and Bayesian Markov chain Monte Carlo (MCMC) methods. Our final goal was to evaluate the use of the posterior distribution of Δt as a means for estimating the probability of population decline retrospectively. To meet these goals, we used California spotted owl (Strix occidentalis occidentalis) data collected on 3 study areas from 1990 to 2011 as a case study. The estimated MCMC median λs for 2 of the study areas were 0.986 and 0.993, indicating declining populations, whereas median λ was 1.014 for the third study area, indicating an increasing population. For 2 of the study areas, estimated MCMC median Δts over the 18‐year monitoring period were 0.78 and 0.89, suggesting 21% and 11% declines in population size, whereas the third study area was 1.22 suggesting a 22% increase. Results from Δt analyses highlight that small differences in mean λ from 1.0 (stationary) can result in large differences in population size over a longer time period; these temporal effects are better depicted by Δt. Fixed effects, random effects, and MCMC estimates of mean and median λ and of Δt were similar (≤9% relative difference). The estimate of temporal process variance was larger for MCMC than the random effects estimates. Results from a Bayesian approach using MCMC simulations indicated that the probabilities of a ≥15% decline over 18 years were 0.69, 0.40, and 0.04 for the 3 study areas, whereas the probabilities the populations were stationary or increasing were 0.07, 0.22, and 0.82. For retrospective analyses of monitored populations, using Bayesian MCMC methods to generate a posterior distribution of Δt is a valuable conservation and management tool for robustly estimating probabilities of specified declines of interest. © 2013 The Wildlife Society.

ABSTRACT The California spotted owl (Strix occidentalis occidentalis) is the only spotted owl subspecies not listed as threatened or endangered under the United States Endangered Species Act despite petitions to list it as threatened. We conducted a meta‐analysis of population data for 4 populations in the southem Cascades and Sierra Nevada, California, USA, from 1990 to 2005 to assist a listing evaluation by the United States Fish and Wildlife Service. Our study areas (from N to S) were on the Lassen National Forest (LAS), Eldorado National Forest (ELD), Sierra National Forest (SIE), and Sequoia and Kings Canyon National Parks (SKC). These study areas represented a broad spectrum of habitat and management conditions in these mountain ranges.We estimated apparent survival probability, reproductive output, and rate of population change for spotted owls on individual study areas and for all study areas combined (meta‐analysis) using model selection or model‐averaging based on maximum‐likelihood estimation. We followed a formal protocol to conduct this analysis that was similar to other spotted owl meta‐analyses. Consistency of field and analytical methods among our studies reduced confounding methodological effects when evaluating results. We used 991 marked spotted owls in the analysis of apparent survival. Apparent survival probability was higher for adult than for subadult owls. There was little difference in apparent survival between male and female owls. Model‐averaged mean estimates of apparent survival probability of adult owls varied from 0.811 ± 0.021 for females at LAS to 0.890 ± 0.016 for males at SKC. Apparent survival increased over time for owls of all age classes at LAS and SIE, for adults at ELD, and for second‐year subadults and adults at SKC. The meta‐analysis of apparent survival, which included only adult owls, confirmed an increasing trend in survival over time. Survival rates were higher for owls on SKC than on the other study areas.We analyzed data from 1,865 observations of reproductive outcomes for female spotted owls. The proportion of subadult females among all territorial females of known age ranged from 0.00 to 0.25 among study areas and years. The proportion of subadults among female spotted owls was negatively related to reproductive output (no. of young fledged/territorial F owl) for ELD and SIE. Eldorado study area and LAS showed an alternate‐year trend in reproductive output, with higher output in even‐numbered years. Mean annual reproductive output was 0.988 ± 0.154 for ELD, 0.624 ± 0.140 for LAS, 0.478 ± 0.106 for SIE, and 0.555 ± 0.110 for SKC. Eldorado Study Area exhibited a declining trend and the greatest variation in reproductive output over time, whereas SIE and SKC, which had the lowest reproductive output, had the lowest temporal variation. Meta‐analysis confirmed that reproductive output varied among study areas. Reproductive output was highest for adults, followed by second‐year subadults, and then by first‐year subadults.We used 842 marked subadult and adult owls to estimate population rate of change. Modeling indicated that Λ t (Λ t is the finite rate of population change estimated using the reparameterized Jolly–Seber estimator [Pradel 1996]) was either stationary (LAS and SIE) or increasing after an initial decrease (ELD and SKC). Mean estimated Λ t for the 4 study areas was 1.007 (95% CI = 0.952–1.066) for ELD; 0.973 (95% CI = 0.946–1.001) for LAS; 0.992 (95% CI = 0.966–1.018) for SIE; and 1.006 (95% CI = 0.947–1.068) for SKC. The best meta‐analysis model of population trend indicated that Λ varied across time but was similar in trend among the study areas. Our estimates of realized population change (Δ t ; Franklin et al. 2004), which we estimated as the product 1 λ3, were based on estimates of Λ t from individual study areas and did not require estimating annual population size for each study area. Trends represented the proportion of the population size in the first year that remained in each subsequent year. Similar to λ4 on which they were based, these λk‐1 showed evidence of decline over the study period for LAS and SIE. The best model indicated recruitment of male and female adult and subadults varied from 0.10 to 0.31 new territorial individuals at time t/number of territorial individuals at time t–1 and similarly among areas. We also conducted a population viability analysis (PVA) based on results of our meta‐analysis. This PVA was of limited utility for ELD and SKC study areas because 95% confidence intervals on the probability of decline or increase spanned the interval [0, 1] within 5–10 years. When we restricted inferences to 7 years, estimated probability of a >10% decline for SIE was 0.41 (95% CI = 0.09–0.78); for LAS the probability was 0.64 (95% CI = 0.27–0.94). In contrast, estimated probability of a >10% increase in 7 years for SIE was 0.23 (95% CI = 0.01–0.55) and for LAS was 0.10 (95% CI = 0.00–0.34). For comparisons, we simulated a PVA for a hypothetical population with mean Λ = 1.0 and the same temporal variation as observed in our owl populations. Our PVA suggested that both the SIE and LAS populations had higher probabilities of declining in a 7‐year period than increasing but that it would be difficult to determine if a population was in a slight gradual decline. Our analysis and the repository of information on our 4 study populations provide a data‐rich template for managers to monitor impacts of future management actions on the owl. Specifically, our data can be used to evaluate the effect of management strategies on spotted owls that are being implemented by the United States Forest Service to reduce the risk of wildfire in the Sierra Nevada ecosystem. Our information also provides baseline information for evaluating the status of the owl for potential listing as a threatened species by the United States Fish and Wildlife Service.RESUMEN El búho californiano manchado (Strix occidentalis occidentalis) es la única subespecie de búhos manchados que no está listada como amenazada o en peligro de extinción en el Acta de E.E.U.U. para las Especies en Peligro de Extinción a pesar de las peticiones para que sea incluida en la lista como una especie amenazada. Nosotros realizamos un meta‐análisis de los datos de la población de 4 poblaciones del sur de Cascades y de la Sierra Nevada, California desde 1990 hasta 2005 como ayuda a una evaluación de listado hecha por el U.S Fish and Wildlife Service. Nuestras áreas de estudio (de norte a sur) estuvieron localizadas en el Bosque Nacional Lassen (LAS), en el Bosque Nacional Eldorado (ELD), en el Bosque Nacional Sierra (SIE) y en los Parques Nacionales Sequoia y Kings Canyon (SKC). Estas áreas de estudio representaron un amplio espectro del hábitat y de las condiciones de manejo en estas cadenas de montañas.Nosotros calculamos la probabilidad de supervivencia aparente, el volumen de reproducción y el cambio en la tasa de población de los búhos manchados en áreas de estudio individuales y para todas las áreas de estudio combinadas (meta‐análisis) utilizando selección de modelos o promediando modelos basados en la estimación de máxima probabilidad. Seguimos un protocolo formal para realizar este análisis que fuera similar a otros meta‐análisis con búhos manchados. La consistencia del campo y los métodos analíiticos en nuestros estudios redujeron la confusión de efectos metodológicos al evaluar los resultados. Utilizamos 991 búhos manchados marcados en el análisis de supervivencia aparente. La probabilidad de supervivencia aparente fue más alta para búhos adultos que para subadultos. Hubo poca diferencia en la supervivencia aparente entre hembras y machos. Para los modelos promediados, los cálculos de la media de la probabilidad de supervivencia aparente para búhos adultos tuvo una variación de 0.811 ± 0.021 para hembras en LAS a 0.890 ± 0.016 para machos en SKA. La supervivencia aparente aumentó con el tiempo para los búhos de todos los grupos de edad en LAS y SIE, para adultos en ELD, y para subadultos del segundo año y para adultos en SKC. El meta‐análisis de supervivencia aparente, que incluyó únicamente a búhos adultos, confirmó una tendencia al aumento en la supervivencia con el tiempo. Las tasas de supervivencia fueron más altas para los búhos en SKC que en las otras áreas de estudio.Analizamos información de 1.865 observaciones de resultados de reproducciones para búhos manchados hembra. La proporción de hembras subadultas entre todas las hembras territoriales de edad conocida fluctuó de 0.00 a 0.25 a través de las áreas de estudio y de los años. La proporción de subadultos entre los búhos manchados hembra estuvo relacionada negativamente con el volumen de reproducción (número de pichones emplumados por búho hembra territorial) para ELD y SIE. ELD y LAS mostraron una tendencia anual alternada en el volumen de reproducción, con un volumen mayor en los años pares. La media del volumen de reproducción anual fue 0.988 ± 0.154 para ELD, 0.624 ± 0.140 para LAS, 0.478 ± 0.106 para SIE y 0.555 ± 0.154 para SKC. ELD exhibió una tendencia a disminuir y la variación más alta en el volumen de reproducción a través del tiempo; mientras que SIE y SKC, que tuvieron el más bajo volumen de reproducción, tuvieron la menor variación temporal. El meta‐análisis confirmó que el volumen de reproducción varió entre las áreas de estudio. El volumen de reproducción fue más alto para adultos, seguido por subadultos del segundo año, y luego por subadultos del primer año.Nosotros utilizamos 842 búhos marcados, adultos y subadultos, para calcular el índice de cambio de la población. La selección de modelos indicó que Λ t era, o relativamente fija (LAS y SIE) o aumentaba después de una disminución inicial (ELD y SKC). La media calculada Λ t para las cuatro áreas de estudio fue: 1.007 (95% CI = 0.952–1.066) para ELD; 0.973 (95% CI = 0.946–1.001) para LAS; 0.992 (95% CI = 0.966–1.018) para SIE; y 1.006 (95% CI = 0.947–1.068) para SKC. El mejor modelo de meta‐análisis de la tendencia de población indicó que Λ variaba con el tiempo pero que era una tendencia similar entre las áreas de estudio. Nuestros cálculos sobre el cambio de población realizado (Δ t ) se basaron en los cálculos de Λ t de las áreas de estudio individuales y no requirieron calcular el tamaño de la población anual para cada área de estudio. Las tendencias representaron la proporción del tamaño de la población en el primer año que permaneció en cada año subsiguiente. De manera similar a λt, en la que se basaron, éstas Δt mostraron evidencia de disminución durante el período de estudio para LAS y SIE. El mejor modelo de reclutamiento indicado, el reclutamiento de búhos machos y hembras, adultos y subadultos, varió de 0.10 a 0.31 individuos territoriales nuevos al tiempo t por el número de individuos territoriales al tiempo t–1 y de manera similar entre las otras áreas. También realizamos un análisis de viabilidad de población (PVA) basado en los resultados de nuestro meta‐análisis. Este análisis PVA fue de limitada utilidad para las áreas de estudio ELD y SKC porque el 95% de intervalos de confiabilidad en la probabilidad de disminución o aumento extendió el intervalo [0, 1] de 5–10 años. Cuando restringimos las inferencias a 7 años, la probabilidad estimada de a >10% de disminución para SIE fue 0.41 (95% CI = 0.09–0.78); para LAS la probabilidad fue 0.64 (95% CI = 0.27–0.94). En contraste, la probabilidad estimada de un >10% de aumento en 7 años para SIE fue 0.23 (95% CI = 0.01–0.55) y para LAS fue 0.10 (95% CI = 0.00–0.34). Para comparar, simulamos un PVA para una población hipotética con una media Λ = 1.0, y con la misma variación temporal observada en nuestras poblaciones de búhos. Nuestro PVA sugirió que ambas poblaciones SIE y LAS tenían, en un período de 7 años, mayores probabilidades de disminución que de aumento, pero que sería muy difícil determinar si alguna de las poblaciones estaba en una ligera disminución gradual. El depósito de información de nuestras 4 áreas de estudio provee una plantilla rica en información para que los administradores monitoreen los impactos de acciones futuras en el manejo de los búhos (por ejemplo, nuevas estrategias de manejo del Plan de Sierra Nevada Forest). También provee evidencia importante para evaluar el estatus del búho para su potencial inclusión en el listado de especies amenazadas.RÉSUMÉ Le hibou tacheté californien (Strix occidentalis occidentalis) est la seule sous‐espèce de hibou tacheté ne figurant pas sur la liste des animaux menacés ou vulnérables sous la Loi des Espèces en Danger des Etats‐Unis malgré des pétitions pour l'inscrire sur cette liste en tant que sous‐espèce menacée. Nous avons effectué une méta‐analyse des données de population pour 4 populations dans le sud des Cascades et dans la Sierra Nevada, en Californie de 1990 à 2005 pour aider une évaluation de leur statut établie par les Services des Eaux et Forêts des Etats‐Unis. Nos aires d'étude (du nord au sud) étaient dans la forêt nationale Lassen (LAS), la forêt nationale Eldorado (ELD), la forêt nationale Sierra (SIE), et les parcs nationaux Sequoia et Kings Canyon (SKC). Ces aires d'étude représentaient un large échantillon des conditions de l'habitat et de la gestion dans ces chaînes de montagnes.Nous avons estimé la probabilité de survie apparente, le succès de reproduction, et le taux de changement de la population pour les hiboux tachetés dans chaque aire d'étude individuelle et dans toutes les aires réunies (méta–analyse) en utilisant la sélection de modèles ou le calcul de la moyenne des modèles basé sur une estimation du maximum de vraisemblance. Pour effectuer cette analyse nous avons suivi un protocole rigoureux similaire à d'autres méta‐analyses de hiboux tachetés. La cohérence des observations de terrain et des méthodes analytiques entre ces études a réduit les effets méthodologiques confondants lors des évaluations des résultats. Nous avons utilisé 991 hiboux tachetés marqués dans l'analyse de survie apparente. La probabilité de survie apparente a été plus élevée pour les hiboux adultes que pour les sous‐adultes. Il y a eu peu de différence pour ce qui est de la survie apparente entre les hiboux mâles et femelles. La moyenne des estimations de la probabilité de survie apparente des hiboux adultes basée sur la moyenne des modèles a varié entre 0,811 ± 0,021 pour les hiboux femelles à LAS et 0,890 ± 0,016 pour les hiboux mâles à SKC. La survie apparente a augmenté avec le temps pour les hiboux de toutes les classes d'âge à LAS et SIE, pour les adultes à ELD, et pour les sous‐adultes de deux ans et les adultes à SKC. La méta‐analyse de survie apparente, qui comprenait seulement des hiboux adultes, a confirmé une tendance croissante de survie avec le temps. Les taux de survie étaient plus élevés pour les hiboux de SKC que pour ceux des autres aires d'étude.Nous avons analysé les données obtenues à partir de 1 865 observations de succès de reproduction de hiboux tachetées femelles. La proportion des hiboux femelles sous‐adultes parmi toutes les femelles territoriales d'àge connu a varié de 0,00 à 0,25 selon les aires et les années d'étude. La proportion des sousadultes parmi les hiboux tachetés femelles a été négativement corrélée avec le succès de reproduction (nombre de jeunes hiboux par femelle territoriale) pour ELD et SIE. La forêt nationale Eldorado et la forêt nationale Lassen ont montré une tendance à alterner selon un cycle biennal pour ce qui est du succès de reproduction, avec un taux plus élevé pendant les années paires. La moyenne du succès de reproduction annuel était de 0,988 ± 0,154 pour ELD, de 0,624 ± 0,140 pour LAS, de 0,478 ± 0,106 pour SIE, et de 0,555 ± 0,110 pour SKD. La forêt nationale Eldorado a montré une tendance décroissante ainsi que la plus grande variation dans le succès de reproduction avec le temps, alors que SIE et SKC, qui ont eu le succès de reproduction le plus bas, ont connu la variation temporelle la plus basse. La méta‐analyse a confirmé que le succès de reproduction variait selon les aires d'étude. Le succès de reproduction a été le plus élevé pour les adultes, puis pour les sous‐adultes de deux ans, et ensuite pour les sous‐adultes d'un an.Nous avons utilisé 842 hiboux marqués, adultes et sous‐adultes, pour estimer le taux de changement de la population. La modélisation a indiqué que Λ t était soit stationnaire (LAS et SIE), soit croissant après une baisse initiale (ELD et SKC). La moyenne estimée Λ t pour les 4 aires d'étude était: 1,007 (95% IC = 0,952–1,066) pour ELD; 0,973 (95% IC = 0,946–1,001) pour LAS; 0,992 (95% IC = 0,966–1,018) pour SIE; et 1,006 (95% IC = 0,947–1,068) pour SKC. Le meilleur modèle de méta‐analyse pour la tendance de la population a indiqué que Λ variait selon le temps mais suivait la même tendance selon les aires d'étude. Nos estimations du changement de population réalisé (Δ t ) étaient fondées sur les estimations de Λ t des aires d'étude individuelles et n'ont pas nécessité d'estimation de la taille annuelle de la population pour chaque aire d'étude. Les tendances représentaient la proportion de la taille de la population pendant la première année qui s'est maintenue chaque année subséquente. De même que λt sur lesquels ils étaient fondés, ces δt ont apporté des preuves de déclin pendant la période d'étude pour LAS et SIE. Le meilleur modèle a indiqué que le recrutement des hiboux adultes et sous‐adultes mâles et femelles variait de 0,10 à 0,31 nouveaux individus territoriaux à un temps t pour un nombre d'individus territoriaux à un temps t‐1 et qu'il en était de même dans chaque aire. Nous avons également procédé à une analyse de viabilité de la population (AVP) fondée sur les résultats de notre méta‐analyse. Cette AVP a été d'une utilité limitée pour les aires d'étude ELD et SKC parce que les intervalles de confiance de 95% sur la probabilité du déclin ou de la croissance couvraient l'intervalle [0, 1] sur une période de 5 à 10 ans. Lorsque nous avons réduit les inférences à 7 ans, la probabilité estimée d'un déclin >10% pour SIE était de 0,41 (95% IC = 0,09–0,78); pour LAS la probabilité était de 0,64 (95% IC = 0,27–0,94). Al'opposé, la probabilité estimée d'une croissance >10% en 7 ans pour SIE était de 0,23 (95% IC = 0,01–0,55) et pour LAS elle était de 0,10 (95% IC = 0,00–0,34). Afin de comparer, nous avons simulé une AVP pour une population hypothétique ayant une moyenne Λ = 1,0 et la même variation temporelle que celle observée dans nos populations de hiboux. Notre AVP a suggéré que les populations de SIE et de LAS avaient de plus grandes probabilités de déclin que de croissance sur une période de 7 ans, mais qu'il serait difficile de déterminer si une population présentait un léger déclin graduel. La collecte des informations pour nos 4 aires d'étude foumit aux personnes chargées de la gestion un modèle riche de données permettant de suivre l'impact sur les hiboux des actions de gestion à l'avenir (par exemple, les nouvelles stratégies de gestion du Plan pour la Forêt de Sierra Nevada). Cette collecte foumit également des preuves importantes afin d'évaluer le statut du hibou pour une classification potentielle sur la liste des espèces menacées.

The range expansion of the Barred Owl (Strix varia) into California is being documented incidentally during the annual monitoring of the Northern Spotted Owl (S. occidentalis caurina) and the California Spotted Owl (S. o. occidentalis) at sites of timber sales and demographic studies in northern California and along the Sierra Nevada. Barred Owls were first recorded in northwestern California in 1981 and have subsequently been observed over much of the northern third of the state, as far south as Nevada County in the Sierra Nevada (Dark et al. 1998) and Marin County along the Coast Range (D. Adams, Point Reyes National Seashore, pers. comm.) (Figure 1).

.................................................................................................................................................... STUDY AREAS ............................................................................................................................................. LASSEN STUDY AREA ................................................................................................................................... ELDORADO STUDY AREA ............................................................................................................................. SIERRA STUDY AREA ..................................................................................................................................... SEQUOIA AND KINGS CANYON NATIONAL PARKS STUDY AREA ........................................................... SAN BERNARDINO STUDY AREA ................................................................................................................. METHODS ....................................................................................................................................................... FIELD METHODS ............................................................................................................................................ Surveys .................................................................................................................................................... Estimation of reproductive effort ............................................................................................................ Capture, banding, sex and age identification, and resighting ofowls ............................................... Pre-analysis data screening ................................................................................................................... Meta-analysis workshop format ........................................................................................................... DATA ANALYSIS ............................................................................................................................................ Changes inanalytical methodology from previous Spotted Owl studies .......................................... Estimating adult survival ...................................................................................................................... Estimating fecundity ............................................................................................................................... Estimating rates of population change .................................................................................................. Comparison f Sierra nd Sequoia and Kings Canyon ational parks tudy areas ......................... RESULTS .......................................................................................................................................................... ADULT SURVIVAL .......................................................................................................................................... FECUNDITY ..................................................................................................................................................... RATES OF POPULATION CHANGE ................................................................................................................ Meta-analysis across tudy areas ........................................................................................................... COMPARISON OF SIERRA AND SEQUOIA AND KINGS CANYON NATIONAL PARKS STUDY AREAS ..... DISCUSSION .................................................................................................................................................. GENERAL INFERENCES .................................................................................................................................. Apparent survival .................................................................................................................................... Fecundity ................................................................................................................................................ Population trend ....................................................................................................................................... STUDY-AREA-SPECIFIC INFERENCES .......................................................................................................... Lassen study area ..................................................................................................................................... Eldorado study area ................................................................................................................................. Sierra and Sequoia nd Kings Canyon ational parks study areas .................................................... San Bernardino study area ..................................................................................................................... CONCLUSION AND RECOMMENDATIONS ................................................................................................... ACKNOWLEDGMENTS ............................................................................................................................. LITERATURE CITED ................................................................................................................................... APPENDICES ................................................................................................................................................. 1

May 1987 through October 1992 and from July through August 1998, we studied diets of California spotted owls (Strix occidentalis occidentalis). Regurgitated pellets were collected at roost and nest sites between 1,000 and 7,600 ft elevation in the Sierra National Forest and were examined for remnant bones, feathers, and insect exoskeletons. Remains of 2,038 individual prey were identified in 1,140 pellets. Woodrats (Neotoma spp.) were the predominant prey in low-elevation oak woodlands and riparian-deciduous forests, accounting for 74.3 percent of the biomass in diets during the breeding period and 81.9 percent during the nonbreeding period. In coniferous forests, northern flying squirrels (Glaucomys sabrinus) were the major prey, comprising 45.6 percent and 77.3 percent of prey biomass during the breeding and nonbreeding periods, respectively. Pocket gophers (Thomomys spp.) were the second largest component of owl diets, by biomass, in both vegetation types. In the breeding period, birds were a larger part of the owl's diet in coniferous forests (12.9 percent) than in the riparian-deciduous and oak habitats (1.8 percent). Other small mammals, insects, and lizards were also found in pellets. Diets differed among years. Concern over the status of the California spotted owl (Strix occidentalis occidentalis) increased when the U. S. Fish and Wildlife Service listed the northern spotted owl (S. o. caurina) and then the Mexican spotted owl (S. o. lucida) as threatened. Although not currently federally listed, the California spotted owl is classified as a sensitive by the Pacific Southwest Region (Region 5) of the USDA Forest Service. Several studies have examined diets of California spotted owls in the Sierra Nevada (Kadoch 1997, Laymon 1988, Marshall 1942, Thrailkill and Bias 1989, Verner and others 1992), but additional information on diets is needed to better understand the relations between owl occurrence and breeding and the availability of prey. An adequate understanding of diets can then guide studies of the ecological requirements of key prey species and help to guide forest management in ways that will favor conditions for the prey species and, in turn, for the owls. We studied California spotted owl diets in the southern Sierra Nevada to describe prey composition and to examine annual and seasonal variations in the diets. We also compared diets between owls in coniferous forests and owls in oak woodlands and riparian-deciduous forests.

years (1990-1998) of demographic data on California spotted owls (Strix occidentalis occidentalis) in two study areas on the western slopes of the Sierra Nevada—one in the Sierra National Forest (SNF), the other in Sequoia/Kings Canyon National Parks (SNP)—are summarized. Numbers of territorial owls fluctuated from 85 to 50 in SNF and 80 to 58 in SNP over the period from 1990 to 1998, and demographic parameters indicate significantly declining populations in both study areas during the same period. Owl densities in conifer forests, reproductive performance, and survival rates did not differ significantly between the two study areas. These results suggest that factors influencing population trends may be more than local in scope, such as weather and/or prey populations. Local forest management may compound the regional variation, however, as may be reflected in the lower adult/ subadult survival rates in the SNF study area compared to the SNP study area. Continual timber harvest has occurred in SNF since the late 1800's, but SNP has had little harvesting activity. Prescription burning and recreation continue to occur on both sites. Declining trends in owl numbers also reflect poor breeding success from 1995 through 1998, apparently attributable to unseasonal storms during the breeding period, especially during the incubation and nestling phases. Results suggest the hypothesis that spotted owls are pulse breeders that exhibit unusually successful reproduction only at intervals of several years, when all conditions are favorable. Continuation of these studies as part of the Kings River Sustainable Forest Ecosystems Project within boundaries of SNF will provide opportunities to explore relations among spotted owl demographics and timber harvest, weather, and prey availability. California spotted owls (Strix occidentalis occidentalis) in the western Sierra Nevada show a close association with dense forest and woodland ecosystems and their nesting/roosting stands typically have much decadence in the form of standing dead trees (snags), diseased and/or deformed live trees, and decaying wood on the forest floor. Pairs in Sierra conifer forests use home ranges that average about 3,400 acres during the breeding period and about 9,700 acres during the nonbreeding period (Verner and others 1992). Consequently, maintenance of sufficient habitat to assure viability of the Sierra population represents a substantial cost in allowable timber volume. The California spotted owl is not currently listed as a threatened or endangered species, although major environmental organizations are poised to petition the U.S. Fish and Wildlife to list the subspecies. Nests are typically in large trees—mean of about 45 inches in diameter at breast height (dbh)—with broken tops and cavities large enough to accommodate a female owl and two or three young (Steger and others 1997b). Nesting in the southern Sierra Nevada is intermittent, involving from none to as many as 95 percent of pairs in any given year (50 percent of pairs nesting is regarded as a

Although the spotted owl (Strix occidentalis) has been intensively studied, factors influencing its reproduction are not well understood. We examined a 9-year demographic study of 51-86 pairs of the California spotted owl (S. o. occidentalis), weather conditions, and forest structure at nest sites in oak (Quercus sp.) woodland and conifer forests to assess if weather or nest-site variables are associated with reproduction. Mean reproduction was highly variable between years, but within a given year was largely synchronous among all owl pairs across forest types (i.e., oak woodlands and conifer) with different prey bases. There was no significant difference in reproduction between owls on National Forest and National Park lands. In oak woodlands and conifer forest, mean fledgling production was negatively correlated with nesting period precipitation, and in conifer forests, positively correlated with April's minimum temperature. For both forest types, live-tree nests were in large (diam at breast height [dbh] > 150 cm), old trees (>225 yr) with large crowns (foliage volume >1,700 m3). Regardless of forest type, all nest sites had similar canopy cover (76%), tree density (312 stems/ha), and foliage volume (45,000 m3/ha). Nests with repeated use produced more young than nests used only 1 year. In oak woodlands, nests with higher reproduction were on shrubby, north-aspect slopes in trees or snags surrounded by a well-developed canopy. In conifer forests, reproductive success was associated with nests overtopped by a canopy with a high foliage volume. Synchronous annual reproduction and the association of nest-period weather and canopy structure with production of young, suggests that reproduction is influenced by both regional weather conditions and nest-site canopy structure, which protects fledglings from detrimental weather.

We described 86 nest sites of California spotted owl (Strix occidentalis occldentalis) and tested for dfferences in vegetation structure at nest locations in coder-dominated stands in 2 study areas, the Sierra National Forest (SNF) and Sequoia and Kings Canyon National Parks (SNP), California. All nests were between 1061 and 2414 m in elevation, 52 were sidecavity nests, 18 topcavity nests, and 9 platform nests. All nests were in live trees (69) or snags (17): 41 in fir (Abies spp.), 17 in black oak (Quercurs kelloggii), 14 in pine (Pinus spp.), and 14 in @ant sequoia (Sequoiadendron g~ganteum). Nest trees had a mean diameter of 118.5 cm (SD = 32. I), except for giant sequoia which were much larger in Qameter (462.9, SD = 144.5 cm). Pines were the only species where we detected a sigdicant difference (P = 0.0001) in m they rely on naturally occurring cavities, platforms from natural accumulation or constructed by birds or mammals, and rock ledges (Verner et al. 1992, Steger et al., in press). Cavity nest sites are more common (66%) in conifer vegetation types (Verner et al. 1992) whereas platform nests are more common (59%) in the oak woodlands (Steger et al., in press). In conifer forests broken tops and cavities develop in old trees, and about onethlrd of all nests are found in snags (Verner et al. 1992). Use of rock ledges for nesting sites is rare in the Sierra Nevada with only 1 recent report (Steger et al., in press). Information on nest sites used by California spotted owls suggests that nests are located in a variety of vegetation types and tree species (Verner et al. 1992, LaHaye et al. 1997, Steger et al., in press). Characteristics such as high basal area of trees, dense canopy cover, and presence of large decadent trees appear to be more critical to successful nesting (l3ias and Gutierrez 1992, Verner et al. 1992) than nest type, species of nest tree, or vegetation type (LaHaye et al. 1997). However, nests studied to date have been on National Forests where nest site availability may have been influenced by historic timber harvest. Descriptions of nest sites not affected by timber harvest, such as on National Parks, may offer add~tional information about the habitat characteristics associated with nesting Our paper describes the characteristics of nest trees, nest structures, associated vegetation, and physical attributes at spotted owl nest sites in the southern Sierra Nevada conifers from the Sierra National Forest and Sequoia and Kings Canyon National Parks, and compares nest site characteristics between National Forest lands and National Park lands. STUDY AREA Our study was conducted in the southern Sierra Nevada in the Sierra National Forest (SNF) and Sequoia and Kmgs Canyon National Parks (SNP), California (Fig 1). The SNF and SNP study areas covered 526 krn2 and 259 km2, respectively; both ranged in elevation fiom 1060 to 3040 m. Spotted owl nests sampled in our study