First-Year Dispersal of Golden Eagles from Natal Areas in the Southwestern United States and Implications for Second-year Settling
Knowledge of dispersal from natal areas by pre-breeding-age Golden Eagles (Aquila chrysaetos) in North America is sparse, though crucial for conservation planning. During 2010–2015, we used satellite telemetry to document movement behavior of pre-breeding-age Golden Eagles from the Colorado Plateau and Southern Rocky Mountains of the southwestern U.S.A. Here we report on (1) 1st-yr dispersal timing, distances, strategies, and survival; (2) influences of age, sex, nest-mates, and area of origin; and (3) progressive distancing from natal nests and coinciding space use, including overlap between individuals' home ranges at the end of their 1st and 2nd yr. Our dataset included 293,960 GPS fixes from 66 Golden Eagles monitored through at least the onset of dispersal. All dispersed at age <1 yr. Most (66.7%) initiated dispersal during 16 September–21 November (median = 22 October, 191 d after median hatch date). We could assign each of 60 eagles to one of four 1st-yr dispersal categories based on maximum distance traveled from natal nests: short-distance (SD), generally <120 km (66.7% of eagles); moderate-distance (MD), 120–500 km (16.7%); long-distance (LD) >500 km (13.3%); and Other (3.3%). LDs dispersed at younger ages than SDs and were more likely to be from the arid half of our study area. First-year survival was significantly lower for LDs. Overlap of 95% kernel density home ranges at ages 12 and 24 mo was 42.2% (±6.4 SE) for 26 SDs and 26.5% (±9.5) for eight MDs. Our data indicate that, during at least their 1st and 2nd yr of life, most Golden Eagles from the Colorado Plateau and Southern Rocky Mountains remain within 120 km of their nests, where they experience high survival and are relatively settled by the end of their 1st yr. As such, these landscapes are key habitat for at least 1st- and 2nd-yr eagles as well as breeding pairs. El conocimiento de la dispersión de individuos en edad pre-reproductiva desde sus áreas natales en América del Norte es escaso, pero es crucial para planificar la conservación. Durante 2010-2015 utilizamos telemetría satelital para documentar el comportamiento de movimientos de individuos en edad pre-reproductiva de Aquila chrysaetos en la Planicie de Colorado y en el sur de las Montañas Rocosas en el suroeste de los Estados Unidos. En este trabajo informamos sobre (1) tiempos, distancias, estrategias y supervivencia de la dispersión de individuos durante su primer año de vida; (2) la influencia de la edad, el sexo, las parejas y el área de origen y (3) el distanciamiento progresivo desde los nidos de nacimiento y el uso coincidente del espacio, incluyendo la superposición entre áreas de campeo hacia el final del primer y segundo año. Nuestros datos incluyeron 293,960 localizaciones GPS de 66 individuos de A. chrysaetos seguidos al menos durante el inicio de la dispersión. Todos los individuos se dispersaron a la edad de <1 año. La mayoría (66.7%) inició la dispersión entre el 16 de septiembre y el 21 de noviembre (mediana = 22 de octubre, 191 días después de la fecha mediana de eclosión). Pudimos asignar cada una de las 60 águilas a una de cuatro categorías de dispersión de primer año, basados en la distancia máxima viajada desde los nidos de nacimiento: de corta distancia (CD), generalmente <120 km (66.7% de las águilas); de distancia moderada (DM), 120–500 km (16,7%); de larga distancia (LD) >500 km (13.3%); y otra (3.3%). Los individuos LD se dispersaron a una edad menor que los individuos CD y tuvieron mayor probabilidad de provenir de la porción árida de nuestro área de estudio. La supervivencia del primer año fue significativamente menor para los individuos LD. La superposición de las áreas de campeo de acuerdo con el kernel del 95% a las edades de 12 y 24 meses fue del 42.2% (±6.4 EE) para 26 individuos CD y de 26.5% (±9.5) para ocho individuos DM. Nuestros datos indican que, durante por lo menos el primer y segundo año de vida, la mayoría de los individuos de A. chrysaetos de la Planicie de Colorado y del sur de las Montañas Rocosas permanecen a menos de 120 km de sus nidos, donde exhiben una supervivencia elevada y donde están relativamente establecidos hacia finales de su primer año. De esta manera, estos paisajes son un hábitat clave por lo menos para las águilas del primer y segundo año así como para las parejas reproductoras. [Traducción del equipo editorial]Abstract
Resumen
Fifty-three Golden Eagle breeding areas in the Colorado Plateau and Southern Rocky Mountains of the southwestern U.S.A, where 66 Golden Eagle nestlings were tagged with PTTs during 2010–2014 and monitored during dispersal from natal areas. Dashed line approximates the boundary between breeding areas in the arid, lower-elevation (1700–2700 masl) part of the study area and those in the relatively less arid, higher-elevation (2100–3600 masl) part of the study area. Disperser strategy type of eagles from different natal areas, as described in text, include: (1) SD – short-distance disperser, (2) MD – moderate-distance disperser, (3) LD – long-distance disperser, and (4) Other – individually unique dispersal strategy. At some natal areas, more than one eagle was tagged; individuals from some such sites exhibited different strategies, as indicated. Natal area locations on lands of Native American tribes are obfuscated (±50 km) for purposes of confidentiality.
Date of onset of dispersal from natal areas for 66 juvenile Golden Eagles tagged as nestlings with PTTs. Dates were determined by using Method 7 in Weston et al. (2013). Forty-four (66.7%) of the eagles dispersed during the 10-wk period bounded by dashed lines. All eagles dispersed by age 1 yr.
Movement patterns of Golden Eagles before and just after onset of dispersal from natal nests: (a) typical pre-dispersal pattern including multiple, 1- to 4-d excursions to 20–150 km from natal nest; (b) less common pattern, with no excursions before dispersing or just one to two excursions 2–12 d before dispersing. Vertical dashed lines represent date of the onset of dispersal, determined by using Method 7 in Weston et al. (2013).
Maximum distance (km) from natal nest during 1st-yr dispersal for 63 Golden Eagles tagged with PTTs as nestlings in the Colorado Plateau and Southern Rocky Mountains of the southwestern U.S.A. during 2010–2014. Each distance category value (x-axis label) is ±50 km.
Maximum distances (km) moved and direction to point of maximum distance moved by 63 Golden Eagles from the Colorado Plateau and Southern Rocky Mountains during their 1st yr of life. Each eagle is represented by a line extending from the central point; maximum distances of eagles that died before 1 yr of age are indicated by numeric values at ends of their respective lines. Radii of inner to outer circles are 120 km, 500 km, and 1000 km.
Examples of dispersal onset and subsequent movement through ca. the 1st yr of life for Golden Eagles exhibiting three dispersal strategies, based on maximum daily distances from the natal nest: (a) two short-distance dispersers, (b) two moderate-distance dispersers, and (c) three long-distance dispersers. For each strategy type, time-distance graphs are accompanied by movement tracks of the respective individual eagles to provide spatial context; black dots represent natal nests. Onset of dispersal (vertical lines) is based on changes in distance from natal nest per Method 7 in Weston et al. (2013). Maximum daily distance is derived from GPS locations collected hourly during 0900–1600 H.
Relationship between percentage of 66 Golden Eagles surviving through their 1st yr of life and “greater than” maximum distance traveled from the natal nest. This distance includes all eagles that moved farther than any particular x-axis value, e.g., 12 eagles moved >400 km, and six (50%; y-axis) of these survived through the end of their 1st yr of life. Data are truncated at 800 km to simplify the illustration; five individuals had maximum distances of 941 to 1379 km and none of these survived to age 1 yr.
Progressive distancing and spatial use by 1st-yr Golden Eagles exhibiting short-distance (SD; n = 38) or moderate-distance (MD; n = 10) strategies for dispersing from natal nests through the end of their 1st yr (April): (a) mean distance between natal nests and centroids of monthly home ranges (95% kernel density estimates); (b) mean month-to-month, cumulative home-range size; (c) percentage overlap between successive monthly home ranges. Only individuals that survived ≥1 yr are included. Error bars are ±SE.
Spatial overlap between 1st-yr and 2nd-yr, early-spring home ranges of eight Golden Eagles classified as short-distance dispersers and one (GE 121733) classified as a moderate-distance disperser, and relationship to their respective natal nest locations. For simplicity, home ranges are estimated by 95% minimum convex polygons. Arrows link natal nests of three eagles to their respective home ranges. Early spring is 16 March–15 April, corresponding roughly to age 12 mo in the 1st yr and 24 mo in the 2nd yr. Some natal nest locations are intentionally inexact to protect sensitive areas.
Contributor Notes
Associate Editor: Ian G. Warkentin