Mark, this is a great example of the fact that whites don't need to be "blown" to be lacking in detail. When there is this much bright light on a white bird, the light just scatters between the feathers and really robs the scene of detail. I can see by the catch light in the eye that it was late in the day, but there is still some pretty strong light here. As a reference, I shoot with my 400mm at f/5.6 and ISO 400 a lot and I would probably be starting with a shutter speed of 1/4000 sec here. I know that's a full stop under what you used (and I'm sure your 1D is slightly different) but you really need to contain all that light in this situation. Take a look at my snowy post here -
http://www.birdphotographers.net/for...read.php/82433
Notice that I was shooting at ISO 200 and used the same shutter speed that you did, which means that at ISO 400 I would be at 1/4000. While the whites here are mostly in the 230's, I still have a slight loss of detail in the under-wing area.