Movement ecology of Golden Eagles (aquila chrysaetos) in eastern North America

Open Access
- Author:
- Miller, Tricia Ann
- Graduate Program:
- Ecology
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- June 21, 2012
- Committee Members:
- Robert Brooks, Dissertation Advisor/Co-Advisor
Robert Brooks, Committee Chair/Co-Chair
Margaret Brittingham Brant, Committee Member
Michael R Gannon, Committee Member
George Young, Committee Member
Todd Katzner, Special Member - Keywords:
- golden eagle
movement
ecology
resource selection
home range
seasonal
migration
modeling - Abstract:
- The eastern North American population of golden eagles (Aquila chrysaetos canadensis) has been relatively understudied and little was known about their movements throughout the annual cycle. Additionally, golden eagles are at high risk for negative interactions with wind power development in other parts of their range. Because this population is thought to be small, <2000 individuals, and because the majority of the population is thought to concentrate twice annually during migration in central PA where significant wind power development is occurring, there is a very high potential for conflict. Until I undertook this work, knowledge of the spatial ecology and movements of this population was based on bird band recoveries and four birds tagged with ARGOS satellite telemetry devices. My purpose in conducting this research was to improve the ecological knowledge about the movements and space use of this population and to map risk from a known threat, industrial wind power development. To address these issues and others, 42 golden eagles were trapped and fitted with telemetry devices between 2006-2012. I examined inter- and intra-seasonal variation in movement during summer and winter by calculating home range size (a proxy for amount of movement) using adaptive local convex hulls (aLoCoH) and by developing a new movement index, the central place foraging index (CPFI), which is the ratio of the aLoCoH to the minimum convex polygon. For each season, I examined the influence of age class, latitude, and habitat type on space use and movement. I found that during summer, space use depended upon eagle age and that winter space use was negatively correlated to forest coverage. Mean summer range size was 812.7 ± 344.7 km2 (±SE, n = 10) for adults, 3,684.9 ± 2,397.9 km2 (n = 3) for sub-adults, and 2,917.3 ± 1,833.9 km2 (n = 4) for juveniles. Time of year also influenced movements. During winter, range size was smaller and much less variable than during summer (p = 0.018). Mean winter range sizes were 164.2 ± 132.2 km2 (n = 10) for adults (A), 695.8 ± 4,577.7 km2 (n = 9) for sub-adults (SA), and 372.3 ± 199.4 km2 (n = 10) for juveniles (J). Finally, during summer, adult movements were more centralized, presumably around a nest site, than those of sub-adults or juveniles (CPFI = 0.28 ± 0.03 (A) vs. 0.21 ± 0.03 (SA) vs. 0.05 ± 0.15 (J); p = 0.011), which is consistent with the expected breeding behavior of adults. However, during winter, CFPI was similar for all age classes (p > 0.05). The complex interactions between season and age I documented show the importance of considering such parameters in evaluating movement ecology. Mechanisms controlling migratory flight behavior also have direct consequences for fitness. I explored the influence of several biotic (flight speed, individual age and sex) and abiotic (weather, day of year, topography) factors on directness of migratory flight, measured by the straightness index, at multiple spatial scales. At a daily scale, spring flight paths were controlled by weather and topography. Conditions better for thermal development - increases in solar radiation and tailwinds - resulted in straighter flight paths because drift in thermals occurred parallel to the axis of migration. Conversely, increasing speed of headwinds resulted in more sinuous, less direct paths as birds drifted opposing the primary axis of migration. Long-linear ridges of the Ridge and Valley Province in central Pennsylvania provide leading lines where near straight-line orographic lift is available. When conditions for thermal development were poor, flight within this region was straighter than flight paths outside of the region because flight is constrained by the topography. At broader scales, regional and complete migratory paths, biotic factors were the drivers of flight path directness. At the regional scale, the interaction between age and sex was an important determinant of directness of flight; non-adult females flew the least directly and non-adult males flew the most directly. These differences may be a result of variation in physiology, development, or possibly anatomy. At the full migratory scale, distance, age, and speed were important determinants of directness of flight. Wind drift and age-related differences in the learned ability to compensate for drift, as well as exploration by non-breeders, and aggressive territorial defense by breeding birds are likely mechanisms that push non-breeding birds off course. To assess risk during migration from industrial wind power developments, I modeled resource selection of low flying eagles and of wind turbines in three regions, the Allegheny Mountains/Plateau (AM) of west central PA, Plateau Provinces (PL) of northern PA and the Ridge and Valley (RV) region of central PA. I combined these models to create regional spatially explicit risk models. I found that risk varied regionally. Areas that are suitable for wind development that do not overlap with eagle habitat are: 14% in AM, 9% in PL, and 2% in RV. Of the total area suitable for development in AM, PL and RV, 22%, 16%, and 80%, respectively, fell in the high-highest risk categories. In addition to determining regional levels of risk, these models can be used to make site-level recommendations to wind developers regarding turbine placement that have the potential to reduce risk to low flying eagles and allow for wind energy generation.