Stephen W. Hoffman

POPULATION TRENDS OF MIGRATORY RAPTORS IN WESTERN NORTH AMERICA, 1977–2001

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. Las tendencias evidentes en los censos de aves reproductivas y los conteos navideños generalmente corroboraron nuestros resultados. Los datos de migración mostraron la existencia de variación regional en el estatus de algunas especies, revelaron información nueva con respecto a las migraciones de A. chrysaetos y A. gentilis e indicaron los efectos de una sequía actual generalizada. Nuestros análisis demuestran el valor de conteos estandarizados de aves rapaces migratorias a largo plazo para monitorear el estatus regional y las tendencias poblacionales de estas aves en el oeste de Norte América.

Differential autumn migration of sharp-shinned and Cooper's Hawks in Western North America

Sharp-shinned Hawks (Accipiter stria tus) and Coopers Hawks (A. cooperii) showed differ ential migration timing during autumn migration in New Mexico and Nevada. Age-sex classes passed through both sites in this order: juvenite females, juvenile males, adult females, and adult males. We com pared the magnitude of differential migration timing in these two species. The number of days separating mean passage dates of age classes (juvenile and adult) was greater for Sharp-shinned Hawks than for Coopers Hawks. The number of days separating the mean passage dates of sex classes was mostly similar be tween the two species. In the Manzano Mountains of New Mexico, however, adult male and female Sharp shinned Hawks were more separated than adult male and female Coopers Hawks. In Sharp-shinned and Coopers Hawks, it seems likely that differences in rate of travel between males and females may best explain sex-specific differential migration timing and that for aging efficiency (i.e., hunting skill level) may best ex plain age-specific differential migration timing.

North American population estimates of waterbirds, vultures and hawks from migration counts in Veracruz, México

Summary Continental-scale bird population estimates are used as a decision-support tool in conservation plans. The calculation of these estimates includes the use of density values and survey data from one or multiple sources extrapolated to the geographic scale of interest. In this paper, we use migration count data from a migration monitoring project in Veracruz, Mexico, to revise existing North American population estimates of seven species of waterbirds, New World vultures, and diurnal raptors. In two species of waterbirds, we suggest that existing estimates are low and propose a mechanism to correct these figures. In the remaining five cases, we also determine present estimates are low, but use our data to provide new continental-scale values. We discuss the importance of refining population estimates using data from a diverse suite of field methods as a means to overcome the limitations of the single-survey-based estimates. Resumen Las estimaciones de poblaciones de aves a escala continental son utilizadas como una herramienta

Seasonal, Diel, and Spatial Dispersion Patterns of Golden Eagle Autumn Migration in Southwestern Montana

In this report we describe three behavioral phenomena of Golden Eagle (Aquila chrysaetos) fall migration: agespecific seasonal differential migration, age-specific diel patterns of migration, and the spatial dispersion pattern of migrating Golden Eagles. Immature individuals of many North American raptor species migrate earlier in the season than adults (see Appendix 2 in Kerlinger 1989). As resource levels decline in the fall, less efficient foragers (Hoffman 1985) or subordinate birds (Gauthreaux 1978) are likely to be the first to seek resources elsewhere. Hoffman and Potts (1985) reported that immature Golden Eagles migrate earlier in the season than adults although Kerlinger (1989) reported the reverse. We examined our data to address whether there was seasonal differential migration between age classes of Golden Eagles migrating along the Bridger Range. Because different travel requirements or behavioral differences between age classes may result in age-specific migration patterns within periods of the day, we also investigated whether immature and adult eagles had different diel patterns of migration. A migrating raptor searching for lift may locate an updraft by flying towards other soaring birds. Such visual interaction among migrants would result in aggregation. This is similar to Kerlingers (1985) aggregation model for flock formation. While his model applies to species that typically migrate in flocks, other species that do not form discrete flocks may still exhibit nonrandom dispersion due to visual interaction. Alternatively, Golden Eagles may avoid other eagles during migration. Pairs of Golden Eagles space out the nests that they use in a given year more regularly than random (Watson and Rothery 1986) and their territorial behavior may carry over to the migration season. In that case we would observe even dispersion. For aggregation or repulsion due to visual interaction to occur, concentrations of migrants would have to be sufficiently dense that individuals could see each other. Furthermore, aggregations of aerodynamically dissimilar birds would be ephemeral because of different climb and glide characteristics so it is appropriate to focus on concentrations of aerodynamically similar birds, e.g., birds of the same species, when testing for nonrandom dispersion. Golden Eagles passed our lookouts in sufficient densities that if they respond to the presence of other soaring Golden Eagles we should have observed nonrandom dispersion.

Combining migration and wintering counts to enhance understanding of population change in a generalist raptor species, the North American Red-tailed Hawk

ABSTRACT An increasing body of scientific evidence supports the idea that many avian species are changing their migratory behavior as a result of climate change, land-use change, or both. We assessed Red-tailed Hawk (Buteo jamaicensis) population trends in 2 parts of the annual cycle (fall migration and winter) to better understand regional population trends and their relationship to changes in migration. We conducted 10 yr, 20 yr, and 30 yr trend analyses using pan–North American standardized fall migration counts and Christmas Bird Counts. We quantitatively compared trends in seasonal counts by latitude within the eastern and western migratory flyways. Our combined analysis of migration and wintering count data revealed flyway-specific patterns in count trends suggesting that Red-tailed Hawks are undergoing substantial changes in both migratory behavior and population size. Decreasing Red-tailed Hawk wintering and migration counts in southern regions and increasing winter counts in northern regions were consistent with other observations indicating changes in migratory strategy; an increasing number of Red-tailed Hawks do not migrate, or migrate shorter distances than they did in the past. Further, Red-tailed Hawk populations have been stable or increasing across much of North America. However, we found strong negative count trends at the northernmost migration sites on the eastern flyway, suggesting possible breeding-population declines in the central and eastern Canadian provinces. Our findings demonstrate the benefit of using appropriate data from multiple seasons of the annual cycle to provide insight into shifting avian migration strategies and population change.

Neotropical records of Nearctic Raptors: observational data from Veracruz, Mexico

ABSTRACT Extralimital records of birds, including diurnal raptors, are frequently set in localities along migration flyways. Normally, these records pertain to a few individuals that are away from their usual flight routes and found in areas that range from tens to a few hundred km from their usual distribution. Here, we report 77 migration records of Nearctic hawks and eagles from a site in the Neotropical region. These records belong to five well-known Nearctic species: Golden Eagle, Northern Goshawk, Bald Eagle, Ferruginous Hawk, and Rough-legged Hawk. The Veracruz River of Raptors Projects (VRR) observational data represent the most reliable and abundant data source of raptor migration along the region and country, including the southernmost cluster of records of these Nearctic species in the continent.