My research has focused on the behavioral ecology of nesting Bald Eagles at Lake Cascade in the west-central mountains of Idaho. I chose this project to try to understand the consequences to Bald Eagles of nesting in a seemingly high-density population. I was also interested in trying to determine the role of interspecific interactions in the productivity of nesting pairs of eagles. To address these issues, I specifically wanted to ask three questions.
First, how does a change in Bald Eagle nesting density effect home range size/shape and foraging habitat use? To answer this question I observed the movements of Bald Eagles throughout the breeding season of 2000 to determine current home range parameters of eight nesting pairs. I compared these results with those of a study done on the same population in 1990 when there were only five breeding pairs. The results suggest that the eagles did not change the size of their home ranges, but they did change the shape to incorporate new foraging habitat that is away from the main body of the lake. The eagles also decreased the amount of home range overlap with adjacent ranges. These results suggest that the reservoir may be getting close to its Bald Eagle carrying capacity.
My second question addressed the effects of inter- and intraspecific aggressive interactions on the productivity of nesting Bald Eagles. To answer this question, I observed nesting eagles for a total of over 900 hours during the 2000 and 2001 nesting seasons. I observed a total of 119 interactions of which 53 were instigated by nesting Bald Eagles and 66 were instigated by other species as well as non-territorial Bald Eagles. Aggressive interactions mainly occurred when either Ospreys or Red-tailed Hawks attacked territorial Bald Eagles or when territorial Bald Eagles attacked Ospreys or immature Bald Eagles. I was unable to determine any relationship between the number or type of interaction and the eagles’ productivity.
My final question addressed the potential that the population of nesting Bald Eagles at Lake Cascade experienced a population bottleneck after their placement on the endangered species list in the 1970’s. Bald Eagles began nesting at Lake Cascade in 1976, two years before they was federally protected and four years after the federal ban on DDT. Since 1976, the eagle population at Lake Cascade and the vicinity has steadily increased to over 15 pairs. We collected blood samples from nestling Bald Eagles and molted feathers from nesting adults to determine the genetic relatedness of nesting pairs on the reservoir. These samples will be sent to the Donana Biological Station in Seville, Spain where collaborators will analyze them. The results of the genetic analysis will be used to determine the degree of relatedness between nesting eagles. The results may also suggest the amount of inbreeding and outbreeding that may be occurring within this population.
Support for my research has been primarily funded through the U.S. Bureau of Reclamation, the Idaho Department of Fish and Game, and the U.S. Bureau of Land Management. Logistical support has been provided by the Boise State University Raptor Research Center and Department of Biology as well the U.S. Geological Survey Snake River Field Station. Some equipment has been provided by CMI Corporation.