Ecology of bald eagles wintering and nesting near Caballo Reservoir, New Mexico

The bald eagle, Haliaeetus leucocephalus, was recently downlisted from endangered to threatened status. Each population and nesting pair are important in the overall recovery of the species. Bald eagles winter and nest near Caballo Reservoir, New Mexico. This project was initiated to determine if fluctuating reservoir pool levels affect the number of wintering bald eagles. Additionally, the effects the fluctuating reservoir pool may have on the fish prey base and on the physical components of the reservoir, specifically the availability of suitable perches, was investigated. A nesting pair of bald eagles was monitored to determine if the reservoir pool affected production.; Over a three-year period with varying reservoir pool levels (1993-1995), data indicated that reservoir pool fluctuations had contradicting effects on the numbers of wintering eagles during the two extreme reservoir pool levels. Fish availability and foraging success were high when the reservoir pool was low. Bald eagle activity areas were concentrated in locations with suitable perches. Existing suitable perches were increasingly damaged and destroyed during extreme reservoir pool fluctuations. Artificial perch structures were utilized on a very limited basis. Foraging experiments indicated that bald eagles wintering on Caballo Reservoir preferred Gizzard shad, Dorosoma cepedianum, over other fish species. The largest gizzard shad was selected over other size classes in most experimental trials.; The nesting pair of bald eagles were monitored over the same three year period (1993-1995); five eaglets fledged from the nest. Gizzard shad were the most frequently delivered prey item. Food caching was common during all three nesting seasons.; Data indicated that reservoir pool fluctuations may not have influenced the number of wintering bald eagles, however, the impacts on perches were evident. The prey base did not appear to be negatively impacted by reduction in reservoir pool, in fact, fish availability was highest during the lowest reservoir pool and lowest during the highest reservoir pool. Foraging success followed the same trend. There was no indication that reservoir pool levels affected bald eagle nest productivity.