Post-migration weight gain of Swainson's Hawks in Argentina

Hornero, 2006

We examined the fasting migration hypothesis for Swainson's Hawks (Buteo swainsoni) by estimating the length, duration, and speed of the migration between North and South America and measuring changes in their body masses at various times throughout the year. We instrumented 34 adult Swainson's Hawks with satellite radios on their breeding grounds in western North America to determine the duration, length, and speed of the migration. Migrating south at 188 km/day, it took Swainson's Hawks 51 days to complete their 13504 km migration to their austral summer grounds. Averaging only 150 km/day on their return migration, it took them 60 days to complete the shorter 11952 km migration back to North America. Adult male and female Swainson's Hawks had average body masses of 872 g and 1131 g, respectively, when they departed from North America in September and their body masses upon arrival in Argentina averaged 759 g for adult males and 933 g for adult females, indicating they lost only an average of 18% of their body masses during migration. Adult male and female Swainson's Hawks had body masses of 792 g and 1013 g, respectively, in February prior to their departure from Argentina and they returned to the breeding grounds in North America weighing 802 g and 1087 g in April. Our results indicate that the fasting migration model does not predict the actual body masses of Swainson's Hawks during the migration season and it should be modified to account for either lower energy expenditures during migration or the possibility that the birds use a stopover strategy during migration, feeding and regaining, or maintaining, fat stores along the migration route.

Comparative Biochemistry and Physiology Part A: Molecular & Integrative Physiology, 2004

We assessed the nutritional condition and established reference values for serum chemistry parameters in a long distance migrant bird of prey, the Swainson's Hawk (Buteo swainsoni), wintering in central Argentina. We analyzed serum concentration of urea, uric acid, cholesterol, and triglycerides and assessed age and sex related differences in these parameters. A body condition index was obtained from the resultant residuals of the regression of body mass and a morphometric measure. No statistical differences were observed among sex and age groups for urea, uric acid and triglyceride serum concentration. However, cholesterol concentration differed among male and female hawks, which could be related to the gain of body mass in wintering grounds at differential rates. The mean values of the four parameters were in the range of those recorded in the Common Buzzard (Buteo buteo), indicating good nutritional condition of the population we studied. Forearm length was the morphometric variable that better correlated with body mass. The resultant body condition index was only correlated with triglyceride concentration, suggesting that this index could be valuable in future work dealing with the assessment of body fat storage in wintering and breeding hawks, as well as in stopover points on the migratory route. ᮊ 2004 Elsevier Inc. All rights reserved.

Resumen. -Status migratorio del Aguilucho Andino (Buteo albigula): ¿qué sabemos hasta ahora? -El Aguilucho Andino (Buteo albigula) es un pequeño accipítrido distribuido en los Andes en dos áreas diferentes: los bosques templados australes de Argentina y Chile (únicas áreas reproductivas conocidas), y luego de un hiato aparente, los Andes tropicales (desde el noroeste de la Argentina hasta Venezuela). La posibilidad de que la especie fuera migratoria se consideró a partir de las primeras observaciones de esta especie en la región de las Yungas argentinas. Por observaciones hechas durante 1995-2005 en el NO de la Patagonia argentina, confirmamos que los Aguiluchos Andinos son residentes reproductivos de verano. El patrón de residencia veraniego de las poblaciones del sur coincide tanto con grupos de Aguiluchos Andinos observados en Chile central migrando hacia el norte (otoño austral) y hacia el sur (primavera austral). En contraste, el estatus de residencia en las áreas tropicales es desconocido. El propósito de este trabajo es proveer nuevos registros de la migración de este aguilucho, tanto en los Andes del sur de Chile como en las Yungas argentinas, y revisar la evidencia concreta existente que sostiene las distintas hipótesis de migración de esta especie. Con los datos presentados aquí, generamos y discutimos básiccamente dos hipótesis alternativas: (1) los Aguiluchos Andinos son migradores parciales, los bosques australes no son sus únicas áreas reproductivas, y solamente las poblaciones sureñas migran luego de la reproducción, o (2) los Aguiluchos Andinos son migradores totales, la reproducción ocurre exclusivamente en el sur de Argentina y Chile, y hay posiblemente una población no reproductiva que permanece en las áreas tropicales durante la estación reproductiva de la población austral.

The Condor, 2011

Journal of Field Ornithology, 2006

ABSTRACT We examined the diet of White-throated Hawks (Buteo albigula) during the incubation and nestling periods in the southern temperate forest of Argentina. Pellets (N= 74) and prey remains (N= 59) were collected at 10 nests from 1998 to 2003, and preys delivered to two nests were monitored during the 2001–2002 breeding seasons. White-throated Hawks fed on small mammals, birds, lizards, and insects. The three methods of identifying prey (pellets, prey remains, and direct observation) produced different results. All types of prey except large birds were detected in pellets, and arthropods may have been over-represented in pellets due to secondary consumption. No remains of either arthropods or reptiles were identified among prey remains collected at nest sites and, during nest observations, we were unable to identify many of the prey items delivered by adults. Our results indicate that accurate determination of the diet of White-throated Hawks requires more than one method of identifying prey.Reportamos la dieta del Aguilucho Andino durante los períodos de incubación y crianza en los bosques templados australes de Argentina. La dieta se estudió en base a egagrópilas (N= 74) y restos de presas (N= 59) colectadas en 10 nidos durante las estaciones reproductivas de 1998–2003, y observaciones de presas llevadas a dos nidos durante la estación de cría 2001–2002. Los aguiluchos se alimentaron de pequeños mamíferos, aves, lagartijas e insectos. Los tres métodos empleados dieron resultados diferentes. Las egagrópilas representaron todas las categorías de presas pero no detectaron las aves grandes, y los artrópodos pueden haberse sobrevaluado debido al consumo secundario. No se encontraron artrópodos ni reptiles en los nidos, y no pudimos identificar muchas de las presas llevadas al nido. Como conclusión, sugerimos que para una determinación ajustada de la importancia proporcional de los taxa de insectos y vertebrados en la ecología trófica de esta especie a lo largo de su distribución geográfica, debería considerarse el uso combinado de los tres métodos propuestos en este estudio.