Elf Owl (Micrathene whitneyi) is the world’s smallest owl and the sole member of its genus. Its small size and distinctive calls can distinguish it from other owls in its range. Elf Owl nests in old woodpecker holes from the river valleys and desert washes up to nearly 2000 m in mountain canyons. It is strictly nocturnal and mostly insectivorous, occasionally eating small reptiles and mammals. Breeding occurs in a limited range in subtropical thorn woodland, upland desert, montane evergreen woodland and riparian forest in several populations along the U.S.-Mexico border and south o Baja California and southern Mexico. Currently, there are six separate breeding populations of Elf Owl: three migratory populations in the southwestern United State and three nonmigratory populations in Mexico. Although popularly associated with saguaro cacti (Cereus giganteus) and desert vegetation, denser and more stable populations of Elf Owl inhabit subtropical thorn woodland, montane evergreen woodland, and riparian forest. It may be the most abundant raptor in upland deserts of Arizona and Sonora, Mexico, but due loss and degradation of habitat in the later part of the 20th century it has become quite rare in parts of its historic U.S. range. Elf Owl was listed as endangered by the California Department of Fish and Game (CDFG) in 1980, when it was estimated that only 10 pairs existed within the state (Cardiff 1979). Efforts to re-establish the bird in riparian woodland along the Lower Colorado River are underway, including establishment of nest boxes.