Population Trends and Implications for Monitoring Cactus Ferruginous Pygmy Owls in Northern Mexico

Although once described as common in lowland central and southern Arizona (Bendire 1888, Fisher 1893, Breninger 1898, Gilman 1909, Bent 1938), cactus ferruginous pygmy owls (Glaucidium brasilianum cactorum; hereafter, pygmy owls) have been extirpated throughout much of their former range in Arizona. As a result, this northernmost subspecies of ferruginous pygmy owls (Van Rossem 1937, Johnsgard 1988) are now listed as endangered by the U.S. Fish and Wildlife Service (USFWS 1997, Hardy et al. 1999).

Immediately south of Arizona in northern Sonora, Mexico, pygmy owls occur primarily in desert-scrub and grassland vegetation communities where woodlands are near stands of large saguaro cacti (Carnegia gigantea) (Flesch 2003a). Because pygmy owls are thought to be abundant in northern Sonora, these populations may prove critical for recovery in Arizona as well as for long-term persistence of pygmy owls in the Sonoran Desert. Natural or facilitated dispersal of pygmy owls from Sonora may augment populations in Arizona, especially when combined with habitat management (USFWS 2003). Numerous threats to pygmy-owl habitat exist in northern Sonora, however, including woodcutting, vegetation clearing for agriculture or buffelgrass (Pennisetum ciliare), and overgrazing, and there are few regulatory mechanisms in place to protect habitat.

Despite the importance of pygmy-owl populations in northern Sonora, there are currently no data on population trends or on design parameters for population monitoring. Therefore, between 2000 and 2004 we monitored relative abundance of pygmy owls in northern Sonora, estimated temporal variation in relative abundance, and assessed the efficacy of different sampling designs for monitoring population trends. Our goals were to assess population trends, determine environmental factors that explained variation in trends, and evaluate the statistical power of our monitoring program for future monitoring efforts.