Cactus Ferruginous Pygmy-Owl Monitoring and Habitat Assessment on Pima County Conservation Lands
Date Published or Written
To address obligations linked to the recently approved Pima County Multi-species Conservation Plan, I identified and estimated the quality of habitat for the Cactus Ferruginous Pygmy-Owl (Glaucidium brasilianum cactorum; hereafter “pygmy-owls”) and surveyed owls on Pima County Conservation Lands in south-central Arizona in 2017. To identify habitat and prioritize areas for surveys across a vast region of County lands, I used a model of habitat quality developed in neighboring Sonora, Mexico together with aerial reconnaissance for saguaros and remotely-sensed data on woody vegetation cover. I evaluated the existing survey protocol for pygmy-owls recommended by the U.S. Fish and Wildlife Service, developed a more efficient survey protocol, and used this approach to survey owls along 11 transects three times; once shortly before breeding in March, once during nesting in April, and following breeding in October when young owls are dispersing and selecting home ranges. I documented a fairly large population of pygmy-owls in the northern Altar Valley and detected pygmy-owls along 10 of the 11 transects surveyed during at least one season with occupancy and abundance peaking during October when 46% of survey stations were occupied. I located four nests and three likely nests—all in saguaro cacti—that contained an average of 4.5 eggs and were located between 846 and 1,177 m elevation (mean = 1,038 m). In general, pygmy-owls were found to be more common and broadly distributed than previously known in the northern Altar Valley and in southern Arizona in general. In total, I documented 20 distinct territories occupied by territorial male pygmy-owls including 17 territories that were not known before this effort. These new territories roughly doubled the known population of pygmy-owls in the northern Altar and adjacent Avra valleys in Arizona, and increased the total number of historical (i.e., known within the last 20 years) sites in the broader region by 44%. Importantly, I validated the utility of a useful quantitative tool for identifying areas on the landscape to prioritize for surveys, which is broadly applicable for other management and recovery applications for this species. Despite the rarity of pygmy-owls on the landscape, virtually all transects I identified for surveys were occupied by one or more pygmy-owls during one or more survey events. Moreover, observed pygmy-owl distribution was associated with the estimated quality of space in directions predicted by theory as indicated by a positive relationship between persistence in occupancy across the three surveys and the estimated local quality of habitat. In combination with management strategies that preserve and perpetuate the continued existence of habitat, these results confirm the value of Pima County conservation lands for the pygmy-owl. Baseline data collected during this effort and summarized here provide a strong foundation for long-term trend monitoring of pygmy-owls on Pima County conservation lands.
Report for Pima County Office of Sustainability and Conservation