In my opinion the softness comes from the type of lens used and shutter speed. I always personally go for a shutter speed of 1/1250 sec or greater for fast flapping birds to freeze the wing motion. I also like an fstop around F8 or so...gets a better focus on large wing spans shots!

Birds that move real slow, like flying into the wind you can reduce you shutter speed since they are basically gliding with little movement.

my 2 cents..........

That last shot came out very nice!