>> “I am worried that the constant refocusing as the little devils fly by will damage the lens mechanism. Will this constant extremely rapid change with autofocus degrade the lens. Are there any precautions?”
The direct answer to your question is – No, what you describe will not damage or degrade the lens in any way. – There are no precautions necessary.
The 70-200/2.8 is a professional lens designed for hard use.
As stated above, humming birds are a big challenge for any camera and more so for the photographer. The D800 has a state of the art, second to none, AF system and is certainly up to the task of shooting these fast little birds.
Dave Summers Lowden, Iowa Nikonians Photo Contest Director
Nikonians membership - "My most important photographic investment, after the camera"