Human-eagle interactions on the lower Columbia River Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/jw827g98x

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  • During the past decade (1978-87), breeding success and productivity of bald eagles on the lower Columbia River (LCR) has been far below state and regional averages and well below levels required for delisting under the Endangered Species Act by the Pacific States Bald Eagle Recovery Plan. Human disturbance was suspected as one possible cause for this depressed productivity. I investigated the response of breeding bald eagles to human activities in foraging areas on the LCR during Spring and Summer, 1985 and 1986. Based on preliminary observations I developed a conceptual model for understanding human-eagle interactions in foraging areas. This model contrasts two forms of human disturbance. In the first type of interaction, a moving or actively approaching human forces a direct confrontation with an eagle. This type of interaction is extremely rare on the LCR and accounts for a minor proportion of an eagle's time-energy budget. Only 20% of all moving human activities observed during this study resulted in close contact (i.e. <500 m) with eagles; less than 6% of all human-eagle encounters within 500 m resulted in a visible disturbance to an eagle. In the second type of interaction, an eagle is presented with several alternative foraging destinations, several of which may have human activities occurring nearby. In this situation, the eagle has the freedom to choose an activity pattern given the existing pattern of human activities. This type of interaction represents the major form of human-eagle interaction on the LCR. To investigate this, I studied six pairs of eagles in each of two years; each pair was sampled three times during the breeding season, roughly corresponding to incubation, nestling, and fledgling stages of the nesting cycle. Each sample consisted of a 3-day control period, during which I monitored "normal" eagle activity patterns; and a 3-day influence period, during which I "disturbed" (i.e. stationary boat with observer) a high-use foraging area. I compared eagle activity patterns within 1200 m of the experimental disturbance between sampling periods. On the average, eagles avoided an area within 300-400 m of the human activity. In most cases, eagles spent less time and had fewer foraging attempts in the entire sample area during the influence period. Eagle responses were consistent among pairs and among nesting stages; although, eagle foraging activity increased dramatically and was more concentrated in the high-use areas during the later nesting stages. Based on these results, I developed a model of human-eagle interactions in foraging areas. I used this model and the results of this study to develop several alternative management recommendations involving temporal and spatial restrictions of human activity. I recommend buffer zones 400 in wide around high-use foraging areas as the single most appropriate and practical strategy.
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