Seasonal Variation in Space Use by Nonbreeding Bald Eagles Within the Upper Chesapeake Bay

Access to food resources is essential to self-maintenance and reproduction and, for species of conservation concern, foraging areas are considered critical habitat. Human disturbance is an important factor restricting access to prey resources for Bald Eagles (Haliaeetus leucocephalus) and guidelines in the Chesapeake Bay have been developed to mitigate its impact. However, our ability to implement such guidelines has been limited by a lack of information on important foraging areas. We used Brownian bridge movement modeling to develop a population-wide utilization probability surface for Bald Eagles along shorelines within the upper Chesapeake Bay. We used locations (n = 320 304) for individuals (n = 63) tracked with GPS satellite transmitters between 2007 and 2011 in the analysis. We examined seasonal variation by developing utilization surfaces for summer and winter. Although shoreline use was widespread, segments receiving high levels of activity were relatively rare. Shoreline classified as having the highest category of use and accounting for 10% of the total utilization made up 0.41% and 0.55% of the total shoreline for winter and summer, respectively. From a management perspective, there is a clear pattern of diminishing returns in conservation value for including sequentially lower-use shorelines in land-use management plans. Shoreline use shifted dramatically in both location and extent between seasons. During the summer months, use was highly concentrated on shorelines along the main stem of the Chesapeake Bay or along major (>1 km wide) tributaries. During the winter months, use shifted away from the main stem of the bay and was more focused on minor (<100 m wide) tributaries and inland ponds. Seasonal shifts in shoreline use suggest the need for season-based management objectives.