POSTFLEDGING DEPENDENCE PERIOD OF MIGRATORY GOLDEN EAGLES (AQUILA CHRYSAETOS) IN DENALI NATIONAL PARK AND PRESERVE, ALASKA

Auk, The, Jul 2006 by McIntyre, Carol L, Collopy, Michael W

ABSTRACT.-

The postfledging dependence period is not well documented for many species of raptors, including Golden Eagles (Aquila chrysaetos). From 1997 to 1999, we used satellite telemetry to estimate the length of the postfledging dependence period, and the finite survival rate of fledglings during that period, for migratory Golden Eagles in Denali National Park and Preserve, Alaska. Length of the postfledging dependence period averaged 50.1 days and ranged from 39 to 63 days. The postfledging dependence period was longer for Golden Eagles that hatched earlier, but hatching date did not influence the date they departed their natal area. Average date of departure from the natal area was 25 September. The departure period spanned a 17-day period from 15 September to 5 October, and coincided with a series of environmental changes including decreases in day length, temperature, and prey diversity. Probability of survival during the postfledging dependence period was 0.98 (95% CI = 0.94 to 1.00).

Received 18 April 2005, accepted 8 November 2005.

Key words: Alaska, Aquila chrysaetos, Denali, Golden Eagle, postfledging dependence period, survival.

Período de Dependencia Posterior al Emplumamiento de Aquila chrysaetos en el Parque Nacional y Réserva Denali, Alaska

RESUMEN.-El período de dependencia posterior al emplumamiento no ha sido bien documentado en muchas especies de rapaces, incluyendo a Aquila chrysaetos. Desde 1997 hasta 1999 usamos telemetría satelital para estimar la duración del período de dependencia posterior al emplumamiento y la tasa finita de supervivencia de los volantones durante este período para el águila migratoria A. chrysaetos en el Parque Nacional y Reserva Denali, Alaska. La duración del período de dependencia posterior al emplumamiento promedió 50.1 días y varió entre 39 y 63 días. El período fue más largo para las águilas que eclosionaron más temprano, pero la fecha de eclosión no influenció la fecha de partida del área natal. La fecha promedio de partida desde el área natal fue el 25 de septiembre. El período de partida se extendió por un período de 17 días (desde el 15 de septiembre hasta el 5 de octubre) y coincidió con una serie de cambios ambientales, incluyendo la disminución en la extensión del día, en la temperatura y en la diversidad de presas. La probabilidad de supervivencia durante el período de dependencia posterior al emplumamiento fue 0.98 (95% IC = 0.94 a 1.00).

THE POSTFLEDGING DEPENDENCE period is an important stage of life for fledgling raptors. During this period, fledglings acquire adequate food reserves before migration (Wood et al. 1998), improve their flight skills (Bustamante and Hiraldo 1989), and develop hunting skills (Newton 1979, Weathers and Sullivan 1989, Bustamante 1993), while often free of the additional pressures of securing food (Amar et al. 2000). Despite its importance, the postfledging dependence period is one of the least-studied life-history stages of many raptor species, apparently because of difficulties associated with monitoring fledglings after they leave the nest (McFadzen and Marzluff 1996, Wood et al. 1998). Advances in radiotelemetry, however, are providing scientists with new tools to overcome some of these difficulties.

Factors regulating the length of the postfledging dependence period in raptors vary among species (Bustamante 1994). Parents may control the length of this period by reducing food-provisioning (Watson 1997) or exhibiting aggressive behavior toward their offspring (Alonso et al. 1987). By contrast, fledglings may lose interest in their parents once they are able to fend for themselves (Brown 1966) and may leave their natal areas as food supply decreases (McCollough 1986) or when migration starts (Bustamante and Hiraldo 1989).

Estimates of the length of the postfledging dependence period for Golden Eagles (Aquila chrysaetos) range from 30 to 180 days and are based on small sample sizes from studies where Golden Eagles are sedentary or short-distance migrants (Walker 1987, Bahat 1992, Watson 1997, Grant and McGrady 1999, O'Toole et al. 1999, Kochert et al. 2002). By contrast, Golden Eagles from northern latitudes of North America generally complete annual long-distance migrations (Kochert et al. 2002). Compared with Golden Eagles originating in temperate latitudes, fledgling Golden Eagles raised in northern latitudes in North America do not have the option of lingering in their natal areas throughout the winter (Kochert et al. 2002); they must migrate or chance overwintering in severe Arctic or sub-Arctic regions where winters are often extremely cold and food resources are limited.

Estimating survival during different life-history stages is essential for understanding population dynamics of raptors (McFadzen and Marzluff 1996, Zelenak et al. 1997, Bennetts et al. 1999), yet few raptor studies include estimates of survival during the postfledging dependence period. Many raptor studies include estimates of nesting success, but equating nesting success with recruitment can lead to erroneous conclusions about trends of individual populations over time (McFadzen and Marzluff 1996).