Home-Range Size and Examples of Post-Nesting Movements for Adult Golden Eagles (Aquila chrysaetos) in Boreal Sweden
We studied home-range size using 15 GPS-tracked adult Golden Eagles (Aquila chrysaetos) in nine different territories, two in 2011 and those two as well as seven others in 2012, in northern Sweden. Home ranges were represented by 50 and 95% minimum convex polygons (MCPs) and 50, 80 and 95% kernel density estimates (KDE). In 2012, 95% MCPs ranged from 100–525 km2 for males (n = 8), and 60–605 km2 for females (n = 7). Mean home-range sizes for the eagles in our study were among the largest reported. Moreover, we found an inverse relationship between home-range size and the percent of clear-cuts within the range. Together these suggest that eagles in Sweden may compensate for low availability of hunting areas, e.g., lower proportion of clear-cuts in their range, by expanding their range. Some eagles displayed different forms of post-nesting movements (i.e., movements not related to breeding) during the normal breeding season in addition to the ranging within their home ranges: (i) long-distance directional movements (n = 3), (ii) intermediate-distance movements (n = 4), and (iii) movements within an unusually large home range (n = 1). These movements varied considerably, with some eagles travelling nearly 700 km north into northern Finland and Norway. No adults with transmitters reproduced successfully in 2012; in four territories, nests failed and in five territories occupied by pairs we did not know if eggs were laid. Post-nesting movements, which occurred after nesting or breeding failure, occurred in a year with apparently low food supply and may have been triggered by local food shortage. Estudiamos el área de acción utilizando 15 individuos adultos de Aquila chrysaetos rastreados por GPS en nueve territorios diferentes, dos en 2011 y estos dos más siete otros en 2012, en el norte de Suecia. Las áreas de acción fueron representadas por 50 y 95% de polígonos convexos mínimos (PCMs) y 50, 80 y 95% de estimadores de densidad kernel (EDK). En 2012, 95% de los PCMs oscilaron entre 100–525 km2 para los machos (n = 8), y 60–605 km2 para las hembras (n = 7). Las áreas de acción medias para las águilas de nuestro estudio estuvieron entre las más grandes reportadas. Además, encontramos una relación inversa entre el tamaño del área de acción y el porcentaje de claros dentro del área. Todo esto sugiere que las águilas en Suecia pueden compensar la baja disponibilidad de áreas de caza (por ejemplo, una proporción menor de claros en su área), al expandir su área de acción. Algunas águilas presentaron diferentes formas de movimientos post-nidificación (i.e., movimientos no relacionados con la cría) durante la época reproductiva normal además de los movimientos dentro de sus áreas de acción: (i) movimientos direccionales de larga distancia (n = 3), (ii) movimientos de distancias intermedias (n = 4) y (iii) movimientos dentro de un área de acción inusualmente grande (n = 1). Estos movimientos variaron considerablemente, con algunas águilas viajando cerca de 700 km en dirección norte hacia el norte de Finlandia y Noruega. Ningún adulto con transmisor se reprodujo con éxito en 2012; en cuatro territorios, los nidos fallaron y en cinco territorios ocupados por parejas no llegamos a saber si hubo puesta de huevos. Los movimientos post-nidificación, que ocurrieron luego de un anidamiento o cría fallidos, ocurrieron en un año de aparente baja disponibilidad de alimento y pueden haber sido ocasionados por la escasez local de alimento.Abstract
Resumen
Home ranges of adult Golden Eagles during the breeding seasons 2011 (n = 3) and 2012 (n = 15) in northern Sweden according to 50% and 95% minimum convex polygons (MCPs). Home-range sizes in km2 are displayed below illustrations in order of corresponding size. The closed circles denote successful nests with fledglings, shaded circles denote unsuccessful nests and open circles denote occupied home ranges where we were uncertain as to whether eggs were laid.
Home ranges of adult Golden Eagles during the breeding seasons 2011 (n = 3) and 2012 (n = 15) in northern Sweden according to 50%, 80%, and 95% bivariate KDE. Home-range sizes in km2 are displayed below illustrations in order of corresponding size. The closed circles denote successful nests with fledglings, shaded circles denote unsuccessful nests and open circles denote occupied home ranges where we were uncertain as to whether eggs were laid. The nest site is represented by the small dot.
Relationship between home-range size (95% MCPs) and proportion of clear-cuts within the home ranges for Golden Eagles (n = 15) during the 2012 breeding season in northern Sweden with the correlation coefficient shown for the pooled number of males and females. Open circles denote females whereas closed circles denote males.
Ranging behavior of adult Golden Eagles (n = 7) in 2012 from when breeding home ranges in northern Sweden were first deserted (Table 1) through 15 August. (a) long-range directional movements for eagles with ID 3, 4, and 11; (b) eagles with ID 5, 6, 12, and 13 that simply expanded their home ranges and undertook intermediate-distance movements (Table 2). Nest sites are represented by the solid black circles, while No, Sw and Fi represent Norway, Sweden and Finland, respectively.
(a) Minimum convex polygons (MCPs) from a territorial female in northern Sweden showing an exceptionally large ranging area in 2011, and (b) nonbreeding movements by the same adult female in 2012. Scaling and compass directions in (a) are the same for (b), and the solid black circles represent the nest where the female bred in 2010.
Contributor Notes
Associate Editor: Karen Steenhof