I wanted to give a quick run at photographing some birds in trees in the backyard yesterday with both the 1Ds3 and 800 as well as the D700 and 200-400. There were many Mourning Doves hanging around so I gave it a shot.
I am very impressed with the performance of the 800 on the 1Ds3 even wide open. The colors are great and the images are very sharp.
I was impressed with the D700 + 200-400 combo as well.
Obviously the reach won't be the same and the extra resolution of the 1Ds3 gives it a distinct advantage at low ISOs. However I'm hoping to do BIFs (birds in flight) with the D700, especially larger birds and I think for that purpose it'll do a fine job.