I'm using a D90 with 300 f4 with TC1.4 for birding.
If I were to upgrade, it will be either the D7100 with the 80-400 zoom, or the 7D2 with then 400L5.6.
Having said that, it all depends on your style of birding. If you do a lot of bird-in-flight shots, the 7D2+400L5.6
combo is the one to get. For long exposure shots in dark places like the Malaysian rainforests here, you'd need
a Nikon, a fast lens (at least a 300 f/2.8) and a sturdy tripod. A friend of mine got good results with the
lightweight combo of V3 with the 80-400.