The new X-T3 is improved (vs X-T2) in several ways that can be interesting for bird photographers.
It now has a 26MP BSI sensor (APS-C). Standard ISO: 160 to 12800. To me it sounds like better performance at high ISO and also better dynamic range.
"Autofocus speeds 1.5x faster than before, enhanced subject tracking ability, enhanced face/eye detection and sensitivity down to -3EV."
Battery (booster) grip is no longer needed to get maximum fps.
A blackout free "sports finder mode" with a "range finder" like view producing 16.6 MP files:
"The X-T3 can shoot at up to 30fps using its electronic shutter in 1.25x crop mode or 11fps in normal shooting. At the 11fps setting it has an exceptionally good buffer capacity of 145 lossless compressed raw files."
A bit better battery performance, CIPA rated to 390 shots.
Video specs seem to be close to Lumix GH5 and low light performance probably on level with GH5S or even some FF cameras. The X-T3 can capture 4K video at up to 60p saving it internally as 4:2:0 10-bit footage (4:2:2 can be saved externally).
Overall a very interesting package especially if you are interested in video as well.
More specs and comparison with X-T2 here:
Below is some links to a guy using the previous X-T2, the 100-400mm lens and 1.4x TC for bird/wildlife photography with very nice results:
Review of XF 100-400mm: