I had been seeing Turkey Vultures and Red-tailed Hawks (three of them!) soaring around the western part of the trail, but didn't think much of it. But then a large black shape soared overhead and I saw both a white head and white tail. I tried to get a picture but it was gone. Then I walked back upstream and saw it perched in a cottonwood across the river. Apparently the hawks and the vulture had been expressing a certain amount of concern because they were all circling the perched eagle. The eagle was unperturbed and was still perched in the same spot when I left towards home.