Remember how bird feet work? When the ankle joint bends, the leg and
toe tendons tighten and pull the toes closed. When the joint is
extended, the tendons loosen and the toes open. (You can do this
yourself if you find a dead bird.) Interestingly, there are little
projections on the toe tendons that mesh with ribbing on the underside
of the toe bones; when a bird settles on a perch with ankles bent, as
long as the weight of the bird is opposed under the toes by the perch,
the tendons remain locked and the feet will grip the perch even when the
bird is asleep.

So, all an eagle has to do to open its (empty) foot is extend its leg.
It would be incredibly maladaptive if an animal, especially one that
flies, couldn't open its feet unless it was on the ground or on a perch.
If an eagle's talons are embedded in a large fish, that's another story;
the bird may not be able to unhook those long, curved talons quickly
enough to keep from being dragged under the water. This can happen to
Ospreys, too. I wonder how often this actually happens.

Rachel Lawson
Seattle
RachelWL at msn.com