Jody,

While Bald Eagles can usurp Osprey nests (as others have already noted), I have witnessed eagles raiding an un-guarded osprey nest for nest material.
In West Seattle there's an eagle nest in the greenbelt with an Osprey nest on a pole across the street (maybe 1/4 mile).
It always amazes me that these Ospreys manage to deliver enough food to their young to fledge them successfully (what with the eagles kleptoparasitizing them and all).
A few weeks ago, as I drove by the Osprey nest, I noticed an adult eagle standing in the nest and decided to check it out. I figured it was too close for another Bald Eagle pair to muscle in, and why would the local pair move from a tree where they've raised young successfully to a uncharacteristic nest site (on top of a pole)?
After watching the eagle poke around in the nest for a few minutes I noticed another adult Bald on a nearby light stand, with a large stick in its talon.
The bird in the nest took off, taking with it a big stick from the nest, closely followed by the other adult. Both flew directly to the traditional eagle nest tree and deposited the sticks there.

Given the risks inherent in collecting dead branches from trees, this makes perfect sense.
I've watched eagles fly through tree canopies and rip branches off in flight. If the branch is heavy, and the canopy is dense, the risk of hitting their wings is real.
One time I watched an eagle miscalculate. The target branch did not break and the eagle ended up hanging upside down from one leg. After a few minutes of looking around (getting its bearings?) the eagle let go, crashed through several layers of branches while righting itself, before barely clearing a nearby road, flapping furiously all the time. It's almost the difference between going into the woods to log your own tree for 2 x 4s or going to the home improvement center ;)

Martin Muller, Seattle
martinmuller at msn.com