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

In the arid southwestern United States, populations of native cavity-nestin g birds are threatened by urbanization, degradation of riparian vegetation, and competition from exotic species. Unfortunately, little quantitative in formation exists on the population parameters or habitat associations of th ese species. From 1994 to 1996, we conducted surveys for two secondary cavi ty-nesters of the Sonoran Desert, the elf owl (Micrathene whitneyi) and the western screech-owl (Otus kennicottii. We used surveys to estimate each sp ecies' relative abundance, examine owl-habitat relationships, and determine if owl abundance was related to reproductive success. Abundance of elf owl s did not differ among years (P = 0.59) and was greater than abundance of w estern screech-owls each year. Abundance of western screech-owls declined e ach year of the study (P < 0.0001). Abundance of elf owls was positively co rrelated with abundance of western screech-owls (r = 0.46, P < 0.001) and c ommon 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 cove r 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 co ver of overstory mesquite and cover understory (<1 m) perennial vegetation. Abundance of elf owls was positively associated with the number of nests l ocated per survey station (r = 0.94, P < 0.01) and may be indicative of hab itat quality. Stands of mature saguaros, especially where they occur in ass ociation with overstory mesquite, should be protected for the elf owl. Such stands are most likely to occur on rocky bajadas and uplands within the mi xed-cacti vegetation type. Mature mesquite appears to be important to both species within the region, possibly due to prey availability and the presen ce of potential nest cavities.