Still, most of my bird photography is not done on vacation but is done almost every day. My office is within walking distance of an area that has quite a few birds and I take walks there every day during lunch. Therefore, whichever lens I pick up will likely be used at least twice a week.
Ok, I can't imagine you taking either of these lenses to work twice a week, especially the 600/4. I think the 400/5.6 might be lighter, add the 1.4 tc or 2.0tc and boom. Plus you have the 70-200.
+1. That was exactly my thought. Getting one out and setup would take your lunch hour. I sold my 600mm f/4 because it became a big expedition to haul it around and use it.