The Boreal Owl at the Southern Terminus of the Rocky Mountains: Undocumented Longtime Resident or Recent Arrival?

Twenty-three Boreal Owls (Aegolius funereus) documented from April 1987 to August 1993, including three fledged juveniles, unequivocally establish the species as a breeding bird in northern New Mexico where they responded at a rate of 0.17 owls/survey hr during both breeding and non-breeding seasons. Their congener, the Northern Saw-whet Owl (A. acadicus), was not recorded during the breeding season and responded at the rate of 0.03 owls/survey hr during the non-breeding season in the same areas. In mountain ranges apparently not occupied by Boreal Owls, Northern Saw-whet Owls responded at rates of 0.35 and 0.27 owls/survey hr for breeding and non-breeding seasons, respectively. Potential habitat for Boreal Owls away from the Rocky Mountains was typically in isolated, small patches that likely would not sustain viable populations. Paleontological, archaeological, and late 19th-early 20th century autumnal specimens from the southern Rockies support the hypothesis that the Boreal Owl has been present in Colorado and New Mexico for centuries, possibly since the Pleistocene. They remained largely undocumented because their high elevation sub-alpine conifer forest habitat is snowbound during their most vocal period (February-April).