There is another aspect of BiF that is worth mentioning, tracking. A number of times I have found it difficult to track a bird that is almost filling the frame and even more so if the bird has an erratic flight pattern, normally I end up with clipped wings, no head or even no bird if it changes direction really fast. With an FX camera it's a lot easier to track a subject and be able to keep up with any changes in direction while still having the whole bird in the frame, it's then just a case of cropping to suit whatever composition you want something easily done with the D800.
It is worth mentioning that a good gimbal head is virtually indispensable with long lenses if you want sharp shots for BiF, there are times when you can get by without but not many.
Hi Mark,
whereas I would agree with your statement about a good gimbal in general I don't agree that you should have to have a gimbal if you want most of your shots sharp. Gimbals are a PIA to transport around. The D800 or D7100 with the new 80-400 AFS VR is superb for getting sharp handheld birds in flight shots time after time. Sure it takes a lot of practice but once achieved it's a joy to use.
All of the images taken in my D7100 set are handheld and many are of fast birds in flight. The link below is to the D7100 set.
http://www.flickr.com/photos/brandon_birder/sets/72157633409947519/