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Garwood, Robin R. (Spahr) – Factors affecting the distribution of Bald Eagles and effects of human activity on Bald Eagles wintering along the Boise River. 1990.

three bald eagles in a treeBald eagle numbers, foraging, distribution, and response to human activity in an urban area were studied for 2 winters along the Boise River in Idaho. Eagles arrived on the study area in November and left by March during both winters. Peak counts of eagles (21 in 1987-88 and 25 in 1988-89) occurred during the first week of February both winters. Eagle numbers were negatively correlated with air temperature and with wind velocity. Eagles were observed foraging mainly on mule deer carcasses, small mammals, and fish.

Eagle locations were compared to random locations along the river, and regression models were developed to predict the presence or absence and abundance of eagles in an area. Eagles were found at pools more often than expected, and at areas with low human development containing a high number of perch trees. Variables that predicted eagle use of an area were the number of perch trees and the number of commercial buildings present, the river habitat type, and the river width.

Effects of human activity were studied by observing human/eagle encounters and conducting experimental disturbances. Based on disturbance indexes, developed from observed flushing frequencies and distances, eagles were least tolerant of walkers, then bicyclers and fishermen. Based on experimental disturbances, eagles were least tolerant of disturbers that approached slowly. Eagles were more tolerant of activity along the most developed stretch of the river. Eagles in this study were, in general, more tolerant of activity than eagles studied in other areas. A probability of flushing curve was developed which predicts the proportion of eagles that will flush at a given distance.

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