Population trends of migratory raptors in western North America, 1977-2001

Trained observers counted migrating raptors and vultures from ridgetops at six sites in the Intermountain and Rocky Mountain Flyways of western North America: Goshute Mountains, Nevada (autumn 1983-2001); Wellsville Mountains, Utah (autumn 1977-1979 and 1987-2001); Bridger Mountains, Montana (autumn 1992-2001); Manzano Mountains, New Mexico (autumn 1985-2001); Sandia Mountains, New Mexico (spring 1985-2001); and Lipan Point, Arizona (autumn 1991-2001). We analyzed site-specific trends in annual passage rates (raptors per 100 hr of observation), conducting species-level analyses for 13 species and age-specific analyses for Northern Goshawks (Accipiter gentilis) and Golden Eagles (Aquila chrysaetos). Our findings suggest that, until the late 1990s when possibly drought-related downturns began, Intermountain-Rocky Mountain populations of Turkey Vultures (Cathartes aura), Ospreys (Pandion haliaetus), Broad-winged Hawks (Buteo platypterus), Red-tailed Hawks (B. jamaicensis), Merlins (Falco columbarius), and Peregrine Falcons (F. peregrinus) were increasing for diverse reasons. In contrast, our migration data and other sources indicate concern over the status of western Golden Eagles and Ferruginous Hawks (B. regalis), and uncertainty about the status of western Northern Goshawks and American Kestrels (F. sparverius). Breeding Bird Survey and Christmas Bird Count trends generally corroborated our results. The migration data reveal regional variation in the status of some species, give new insight concerning Golden Eagle and Northern Goshawk migrations, and suggest effects of an ongoing, widespread drought. Our analyses demonstrate the value of standardized, long-term migration counts for monitoring the regional status and trends of migratory raptors in western North America.