Distribution, Abundance, and Habitat of Cactus Ferruginous Pygmy-owls in Sonora, Mexico

Recovery of the federally endangered cactus ferruginous pygmy-owl (Glaucidium brasilianum cactorum) in Arizona may be facilitated by emigrants from neighboring Sonora, Mexico. In northern Sonora, however, pygmy-owls were thought to be rare or absent within 200 Jan of Arizona. In 2000 and 2001, I quantified abundance, distribution, and habitat characteristics of pygmy-owls across Sonora. I detected 524 pygmy-owls at 2,812 stations along 392 transects (1,113 km). Relative abundance (no. males/stations± SE) was high in both northern (0.38 ± 0.04) and southern (0.67 ± 0.07) Sonora and low in central (0.13 ± 0.03) Sonora. Density (no. males/100 ha ± SE) was highest in Sinaloan Deciduous Forest (2.00 ± 0.82) and moderate in Semidesert Grassland (0.99 ± 0.45) and Sinaloan Thornscrub (0.85 ± 0.37). Density of owls in the 4 vegetation subdivisions of the Sonoran Desert combined was 0.67 ± 0.32 and ranged from 1.47 ± 0.61 in Arizona Uplands to 0.08 ± 0.04 in Central Gulf Coast desertscrub. Density on upper bajadas was 2.5 to 17 times greater than on lower bajadas in Semidesert Grassland, Sinaloan Thornscrub, and on the Plains of Sonora, and was 4.5 times lower in Arizona Uplands. Relative abundance was higher on the Coastal Plain (0.18 ± 0.02) than in interior valleys and foothills of the Sierra Madre Occidental (<0.01 ± <0.01). Over 72% of pygmy-owls I located were aggregated along only 18.9% of transects and abundance was moderately clumped across the state. In the north, abundance corresponded closely with geographic trends in the environmental factors important in habitat selection; in the south, however, there was little correspondence at higher elevation where Colima pygmy-owls (Glaucidium palmarum) often replaced ferruginous in tall tropical forests.

Although associations between pygmy-owls and specific environmental factors varied somewhat geographically, owls typically occupied areas with moderate vegetation volume, taller upland canopy height, riparian areas dominated by mesquite (Prosopis sp.) and chino (Havardia mexicana), and upland areas dominated by mesquite, ironwood (Olneya tesota), tropical deciduous forest species, and large columnar cacti. Large columnar cacti had a strong influence on occupancy and abundance of pygmy-owls especially where large cacti were less common. Factors related to riparian vegetation and the size and density of drainages had a strong influence on occupancy and abundance of pygmy-owls in arid northern Sonora but not in the relatively mesic south. Occupancy and abundance were lower in areas altered by agriculture, and were higher in areas with low to moderate levels of woodcutting in Sinaloan Thornscrub, which reduced vegetation volume improving conditions for owls. Although occupancy was often lower where grazing intensity was high, the influence of grazing intensity varied with elevation and topography. Pygmy-owls were detected regularly in both riparian and upland vegetation areas, however almost all owls were detected in uplands when volume of vegetation between 3 and 6 m above ground exceeded 20%. Existence of a well-distributed population of pygmy-owls in northern Sonora enhances recovery potential in neighboring portions of Arizona. Management strategies designed to augment or develop habitat characteristics that promote occupancy by pygmy-owls is an important step toward recovery of pygmy-owls in Arizona.