Introduction

The Elf Owl is the smallest owl in the world and perhaps the most abundant raptor in upland deserts of Arizona and Sonora, Mexico. Although popularly associated with saguaro cacti (Cereus giganteus) and desert vegetation, denser and more stable populations of this species inhabit subtropical thorn woodland, montane evergreen woodland, and riparian forest. Elf Owls also live in partly urbanized habitats. The species nests in old woodpecker holes in trees, columnar cacti, agave and yucca flowering stalks, fence posts, utility poles, and bird boxes, from river valleys and desert washes to nearly 2,000 meters in mountain canyons. It is strictly nocturnal and mostly insectivorous, but occasionally eats small reptiles and mammals.

Most Elf Owls breed in three populations in the U.S.–Mexican borderlands and winter in southern Mexico. Three other disjunct populations in southern Baja California and Puebla, Mexico, are nonmigratory. Nesting pairs are especially vocal during April–June. Males give high-pitched, puppy (dog)-like yips, unique among North American owls. In the cavity-rich riparian forests of mountain canyons, pairs are often interspersed among screech-owls (Otus spp.) and pygmy-owls (Glaucidium spp.) in avian cavity-nesting guilds of up to 16 species. Elf Owls may nest coincidently in the same trees and cacti as other avian cavity users. They vigorously defend nests and cooperatively mob intruders and predators such as snakes, larger owls, and ringtails (Bassariscus astutus).