Abundance and habitat associations of elf owls and western screech-owls in the Sonoran Desert

In the arid southwestern United States, populations of native cavity-nesting birds are threatened by urbanization, degradation of riparian vegetation, and competition from exotic species. Unfortunately, little quantitative information exists on the population parameters or habitat associations of these species. From 1994 to 1996, we conducted surveys for two secondary cavity-nesters of the Sonoran Desert, the elf owl (Micrathene whitneyi) and the western screech-owl (Otus kennicottii. We used surveys to estimate each species' relative abundance, examine owl-habitat relationships, and determine if owl abundance was related to reproductive success. Abundance of elf owls did not differ among years (P = 0.59) and was greater than abundance of western screech-owls each year. Abundance of western screech-owls declined each year of the study (P < 0.0001). Abundance of elf owls was positively correlated with abundance of western screech-owls (r = 0.46, P < 0.001) and common poorwills, Phalaenoptilus nuttalliis (r = 0.33, P < 0.01). Abundance of elf owls was positively associated (R-adj(2) = 0.67, P < 0.05) with cover of overstory mesquite (Prosopis), cover of overstory perennial vegetation, and density of mature saguaros (Carnegiea gigantea). Abundance of western screech-owls was positively associated (R-adj(2) = 0.53, P < 0.05) with cover of overstory mesquite and cover understory (<1 m) perennial vegetation. Abundance of elf owls was positively associated with the number of nests located per survey station (r = 0.94, P < 0.01) and may be indicative of habitat quality. Stands of mature saguaros, especially where they occur in association with overstory mesquite, should be protected for the elf owl. Such stands are most likely to occur on rocky bajadas and uplands within the mixed-cacti vegetation type. Mature mesquite appears to be important to both species within the region, possibly due to prey availability and the presence of potential nest cavities.