Check out the Buy & Sell forum here or on Fred Miranda. There are all sorts of great long glass options (Nikon or Sigma) for you that are much less expensive then buying new. Buying from these two sites seems to be far more reliable them eBay or Craig's List.
You do not want to use the 80-400 for birds in motion. The AFS is awfully slow & you'd be wasting the state of the art AFS technology you have on your D800.