Population and Demographic Trends of Ferruginous Pygmy-owls in Northern Sonora, Mexico 2000-2008

Cactus ferruginous pygmy-owls (Glaucidium brasilianum cactorum) are of significant conservation concern in southern Arizona and are now being considered for listing as endangered by the U.S. Fish and Wildlife Service. Persistence and recovery of pygmy-owls in Arizona may depend on efforts to restore habitat and foster immigration from neighboring Sonora, Mexico where pygmy-owls are more abundant. As such, information on status and trends of populations of pygmy-owls in northern Sonora are important to assess long-term prospects for regional persistence, to identify potential source populations of immigrants, and to answer questions that are fundamental to management and recovery in Arizona. In 2000, I began monitoring population and demographic trends of pygmy-owls in northern Sonora in a large region (25,000 km2) that is immediately adjacent to Arizona and within 110 km of the international boundary (Fig. 1). During each successive year, I continued to estimate population and demographic parameters of pygmy-owls for a total of nine consecutive years. To assess environmental factors that could potentially drive trends, I considered annual estimates of rainfall and prey abundance measured at regional scales and vegetation and land-use factors measured along transects where I monitored owls. This report summarizes the major findings of this program through the 2008 field season. Methods used to sample populations and to analyze data are described elsewhere (Flesch and Steidl 2006, Flesch 2007).