Cactus Ferruginous Pygmy-Owl monitoring and habitat on Pima County Conservation Lands


Aaron D. Flesch

To address obligations linked to the Pima County Multi-species Conservation Plan, my colleagues with the Pima County Office of Sustainability and Conservation and I developed a monitoring program for the Cactus Ferruginous Pygmy-Owl (Glaucidium brasilianum cactorum; hereafter “pygmy-owls”) on Pima County Conservation Lands in south-central Arizona in 2017 and performed baseline surveys. These efforts included extensive habitat assessments to guide placement of 11 transects that we surveyed in each of three different seasons. In 2020, we surveyed pygmy-owls along eight of these 11 transects and along five new transects 1-2 times per year during the breeding season in April and in early fall. In contrast to efforts in 2017, we transitioned to both transect- and territory-based monitoring and in 2020 surveyed all 21 territories we first documented in 2017. To assess changes in populations between years, we compared data sets from all transects and territories that were surveyed in both years and fit linear and generalized linear mixed-effects models with relative abundance (log no. of territorial males/station) and occupancy (occupied or undetected) as response variables, year and season as fixed effects, and transect or territory identity as random effects. In 2020, we detected pygmy-owls along 83% of transects during both the spring and fall survey seasons. We also detected an estimated 23 individuals all of which were males in spring, 33 individuals including 13 likely females in fall, and confirmed nine nests in 2020. Across all seasons and years, we have now documented a total of 33 distinct pygmy-owl territories on Pima County Conservation Lands in the Altar and Avra valleys, which included 12 new territories in 2020. Along transects, estimates of relative abundance of all individuals combined, males, number of present points (includes same individuals detected at multiple points), and number of overall detections (also includes same individuals detected at multiple points) did not vary between years (p ≤ 0.32). However, relative abundance of females increased in 2020 somewhat (p = 0.097) after adjusting for the effects of season (p = 0.036), due likely to later survey timing in 2020 when females are typically more responsive. Among territories, occupancy declined in 2020 compared to baseline estimates from 2017 (p < 0.001), and estimates of occupancy that were adjusted for the effects of season equaled 0.847 in 2017 and 0.739 in 2020. However, occupancy estimates from 2017 were likely biased high somewhat due to the sampling and territory discovery process given it was the initial year of sampling, when only occupied territories are generally discovered. Data we gathered document a significant population of pygmy-owls on Pima County Conservation Lands, and provide a strong foundation for future monitoring and efforts to understand processes that drive spatiotemporal variation in populations. Combined with efforts to conserve habitat and continue monitoring, our results confirm the value of Pima County Conservation Lands for the pygmy-owl.

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Technical Reports