Thanks everyone for the comments.
Woodpeckers can generate up to 1000 Gs of force when hammering away, an amazing number. In my line of work it is hard to imagine that they don't suffer retinal detachments, but nature has adapted to allow it.
re: 500 PF. This lens has changed my photography more than any other single lens. I carry it places that I wouldn't take my reg. 500 or 600/4, the image quality is excellent , fast focusing. I still use the 600 from my boat for loon photography and some select other things, but my Gitzo sits in the closet now because I am able to do most of my BIF hand held with the 500 PF.
Cheers
Randy