Population status and trends of Ferruginous Pygmy-owls in northern Sonora Mexico: A 22 year perspective


Aaron D. Flesch

Ferruginous Pygmy-Owls (Glaucidium brasilianum) are of major conservation concern in southern Arizona due to marked declines in distribution and abundance over the past century and threats driven by climate and landcover change. Populations of pygmy-owls in adjacent northern Mexico are important for recovery in the U.S. and the focus of extensive long-term monitoring and research efforts by University of Arizona staff that began in 2000. To assess the current status and trends of populations, I estimated abundance and territory occupancy across a vast region of northern Sonora, Mexico immediately south of Arizona in 2021, and evaluated spatiotemporal trends across as many as 22 years with data gathered between 2000 and 2021. Although evidence of systematic population declines between 2000 and 2014 was strong, large increases in abundance and occupancy in 2015-2016 eliminated statistical evidence of declining linear trends across this time period. In 2021, however, I found that high levels of abundance and occupancy in years 2015-2016 were not sustained, and documented marked contractions in distribution and abundance and estimates of these parameters that were low or very low compared to historical values for these populations. Annual estimates of territory occupancy in 2021 from a top-ranked generalized linear mixed effects model, for example, were the lowest observed (0.435) among all 16 annual estimates across the study, and 22% lower than the long-term average (0.560). Trend estimates for occupancy between 2001 and 2021 also provided suggestive evidence of a systematic linear decline across the broader study area but results depended on model assumptions. Abundance also declined markedly from levels last observed in 2015-2016, and was 6% lower than the long-term average. Such results indicate complex population dynamics and the importance of consistently monitoring populations of concern across time and space so that short-term changes in populations can be distinguished from long-term declines. In 2021, I also observed a broad range of new landscape disturbances linked to changes in land use and landcover within or immediately around many territory patches. These changes occurred within the last ~5 years, and impacted 22% of 93 patches I surveyed in 2021, with 29% of new disturbances at sites that had no prior impacts. Such disturbances included vegetation clearing linked to agriculture, grazing, and woodcutting that can eliminate or degrade pygmy-owl habitat and landscape connectivity. Historically low estimates of occupancy from 2021 together with observed impacts to habitat and anticipated increases in aridity across this region suggest high potential for future declines of pygmy-owls in this portion of the Sonoran Desert. Given the value and potential insights offered by long-term datasets, additional monitoring of historical sites that are part of this program and assessment of factors that are associated with population dynamics should be priorities for future efforts.

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Technical Reports