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1 August 2003 POPULATION TRENDS OF MIGRATORY RAPTORS IN WESTERN NORTH AMERICA, 1977–2001
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Abstract

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.

Tendencias Poblacionales de Aves Rapaces Migratorias en el Oeste de Norte América Entre 1977 y 2001

Resumen. Varios observadores entrenados contaron aves rapaces migratorias desde las cimas de montañas en seis sitios en los corredores de vuelo inter-montañas y de las Montañas Rocallosas en el oeste de Norte América: Goshute Mountains, Nevada (otoño 1983–2001); Wellsville Mountains, Utah (otoño 1977–1979 y 1987–2001); Bridger Mountains, Montana (otoño 1992–2001); Manzano Mountains, New Mexico (otoño 1985–2001); Sandia Mountains, New Mexico (primavera 1985–2001); Lipan Point, Arizona (otoño 1991–2001). Analizamos las tendencias específicas de cada sitio en las tasas anuales de paso (número de rapaces por 100 hr de observación), realizando análisis por especie para 13 especies y por edad para las especies Accipiter gentilis y Aquila chrysaetos. Nuestros hallazgos sugieren que hasta finales de los años 1990s (cuando comenzaron a verse tendencias negativas posiblemente relacionadas con sequías) las poblaciones de Cathartes aura, Pandion haliaetus, Buteo platypterus, B. jamaicensis, Falco columbarius y F. peregrinus estaban aumentando por diversos motivos. En contraste, nuestros datos de migración y otras fuentes causan preocupación en cuanto al estatus de A. chrysaetos y B. regalis, e incertidumbre sobre el estatus de A. gentilis y F. sparverius

Stephen W. Hoffman and Jeff P. Smith "POPULATION TRENDS OF MIGRATORY RAPTORS IN WESTERN NORTH AMERICA, 1977–2001," The Condor 105(3), 397-419, (1 August 2003). https://doi.org/10.1650/7146
Received: 9 July 2002; Accepted: 1 April 2003; Published: 1 August 2003
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