I've used a D5300 with the Nikon 18-140 lens as a carry around camera for a while now. First thing was to get an aftermarket 2 battery base to go on it from eBay - large hand problem solved - built in GPS eating batteries problem also solved. The camera has done everything asked of it and done it well. Shot some in Auto but mostly in manual. Video inside with tripod is great - video outside is great but you can't see squat on the rear screen which is the only way the camera will shoot video. So, yes it's good for birds in flight as it will focus well on movement and it's light enough for stills but almost impossible to do video of birds in flight but if you could figure out a way to see the back screen it would do that well also.
I am surprised that you got rid of your V1. They are supposed to be really good for shooting little birds in flight. However, due to higher resolution, the V2 is supposed to be even better. Now that the V3 has bee released, you should be able to get a V2 for a song.
If I were going to get serious about birding, I would get a D4s.