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Kimball, Scott A. – Behavioral interactions of breeding Bald Eagles (Haliaeetus luecocephalus) at Lake Cascade, Idaho. 2009.

bald eagle in skyDuring the 2000 and 2001 breeding seasons I studied the behavioral inter- and intra-specific interactions of Bald Eagles (Haliaeetus leucocephalus) breeding at Lake Cascade, Idaho. In the first chapter, I describe the relationships between an increase in population density and home range and foraging habitat use by Bald eagles. In the second chapter, I describe the relationship between aggressive interactions and reproductive success, as measured by number of nestlings fledged and nestling weight.

Because home range and foraging habitat use show temporal variability, I compared breeding home ranges and foraging habitat use of Bald Eagles at Lake Cascade, Idaho in 2000 with previously obtained results from 1989 and 1990. In 2000, eight pairs nested on Lake Cascade whereas five pairs nested in 1989/90. This increase was correlated with changes in home range and foraging habitat use. The home ranges of the three new pairs included habitat that was previously used by the five pairs of eagles in 1989/90. In addition, several pairs that occupied older nest sites appeared to utilize foraging habitats farther from the main body of the reservoir, including upper reaches of tributary arms and nearby, smaller bodies of water. Pairs from recently established territories also used smaller bodies of water away from Lake Cascade. Thus, home ranges in 2000 differed in shape and foraging habitat use from those in 1989/90. The increase in the number of breeding pairs also was correlated with a decrease in home range overlap. This appears to be the first study to show how a natural population size increase may affect home range size, overlap and shape and foraging habitat use within a bird population.

I conducted a study in 2000 and 2001 to describe the relationship between Bald Eagle inter- and intraspecific aggressive interactions and measures of reproductive success, including number of nestlings fledged and nestling mass, during the breeding season at Lake Cascade, ID. I observed activity of nine breeding pairs in both 2000 and 2001 to quantify the level of territorial aggression exhibited by nesting pairs of eagles. Interactions were classified according to the level of aggression exhibited: vocalizing, chasing, stooping, or kleptoparasitizing. I recorded a total of 124 interactions between territorial eagles and both conspecifics and heterospecifics. Eagles acted as both the aggressors and recipients of aggression in these interactions. Seven pairs of eagles produced 13 fledglings (average mass = 4.00 kg) in 2000 and four pairs produced eight fledglings (average mass = 4.15 kg) in 2001. The results of this study suggest that the level of aggression exhibited by nesting Bald Eagles may be related to the number of fledglings but not to nestling mass.

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