Population Trends of Ferruginous Pygmy-Owls in Northern Sonora, Mexico and Implications for Organ Pipe Cactus National Monument


Flesch, A. D.

Monitoring wildlife populations that are subdivided by international boundaries can enhance management and conservation efforts in transboundary regions especially when actions in one nation affect populations in the other. In the borderlands region on and around Organ Pipe Cactus National Monument (OPCNM) populations of Ferruginous Pygmy-Owls (Glaucidium brasilianum cactorum) occur in both Sonora, Mexico and in neighboring southern Arizona yet the status and threats to populations vary. In southern Arizona, pygmy-owls were once described as locally common but are now rare, declining, and being considered for listing as endangered by the U.S. Fish and Wildlife Service (Bent 1938, Johnson et al. 2003, Flesch 2010). OPCNM is near the western edge of the geographic range of pygmy-owls and although a population has been present for many decades (Hensley 1954, Phillips et al. 1964), abundance has varied and owls are absent from several localities that were occupied in the late 1990s (Tim Tibbitts, personal communication). In northern Sonora, pygmy-owls are more common yet habitat is also highly fragmented, often restricted to riparian woodlands and nearby stands of large saguaro cacti (Carnegiea gigantea), and abundance declines from east to west along environmental and rainfall gradients as one approaches the arid western deserts south of OPCNM (Flesch 2003, Flesch and Steidl 2010). Given the status of pygmy-owls in the region, persistence and recovery of populations of pygmy-owls on and around OPCNM, and in Arizona in general, may depend on conserving populations in adjacent northern Sonora and on dispersal into Arizona.

Despite the importance of populations of pygmy-owls in Sonora for persistence and recovery in Arizona, habitat loss, habitat fragmentation, and drought threaten prospects for recovery in Arizona. Recently, construction of security fences and associated vegetation clearing along the international border may be reducing the frequency of transboundary dispersal events by pygmy- owls into Arizona. This is because pygmy-owls tend to fly near ground level when crossing open areas and avoid large vegetation gaps during dispersal (Flesch et al. 2010). Moreover, long-term monitoring of populations of pygmy-owls in northern Sonora indicate they have declined by an estimated 36% between 2000 and 2008 (Flesch 2008a), which could further limit transboundary movements by pygmy-owls into Arizona.

With support from the U.S. National Park Service, I estimated abundance, territory occupancy, and reproductive performance of pygmy-owls across a 25,000 km2 area in the borderlands of northern Sonora during spring and summer 2009. I also surveyed potential habitat within approximately 30 km of OPCNM and assessed how population dynamics of pygmy-owls over the last decade varied with annual estimates of rainfall and prey abundance. These data are important for evaluating the status, recovery prospects, and management strategies for pygmy- owls in Sonora, in OPCNM, and elsewhere in Arizona. This effort extended a monitoring program that began in 2000 into its tenth consecutive year and reports trend estimates for abundance, patch occupancy, and reproductive performance for populations of pygmy-owls in northern Sonora since 2000 and 2001.

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Technical Reports