Aplomado Falcons and Grazing: Invoking History to Plan Restoration

Abstract

The federally endangered northern aplomado falcon (Falco femoralis septentrionalis) disappeared as a breeder from its historic nesting range in the southwestern United States in the early to middle 1900s. Since 1995, a small breeding population has been restored to former range in South Texas grasslands, and interest has escalated in restoring the bird to northern Chihuahuan Desert grasslands in southeastern Arizona and southwestern New Mexico. In these latter areas, intensive livestock grazing and associated shrub (brush) encroachment have been theorized to have contributed importantly to the bird's demise, and thus grazing management has been identified as an important restoration issue. A review of the bird's abundance in the context of the grazing history of these areas suggests it was common when grazing, both by livestock and black-tailed prairie dogs (Cynomys ludovicianus), was intense and widespread (1880s-1920s) in the bird's habitat. It declined in abundance and disappeared coincident with declines in livestock abundance and the extirpation of prairie dogs (1930s-1940s). Most locations where observers historically encountered the bird had little brush at the time of its demise. In the Chihuahuan Desert grasslands of Arizona, New Mexico, and West Texas where the bird occurred historically, the avian prey base is presently an order of magnitude or more lower than that in higher-rainfall habitats of the bird in eastern Mexico and South Texas. The avian prey base is similarly depauperate where 2 small populations of aplomado falcon exist in Mexico's northern Chihuahuan Desert, and the bird's reproductive success there is lower than that in eastern Mexico. The historic extirpation of prairie dogs caused a reduction in the prey biomass available to diurnally feeding raptors such as the aplomado falcon. Efforts to restore the falcon to Chihuahuan Desert grasslands in the United States ideally should include monitoring the responses of released birds to levels of grazing, to shrub abundance, to prey abundance, and to black-tailed prairie dog colonies should any exist in release areas.