One of the things I have learned is that when you are shooting dark subjects like this bird against a lighter background as you have here is that the use of +1 to +1 1/3 stops of exposure compensation (EC) will help bring out the detail in the darker bird's feathers. In this case, because there is so much contrast difference between the bird and the background, then I might even try as high as +2 on the exposure compensation scale just to see what it gives me - doesn't cost you anything except some time.. Having said this, then my question is whether or not you tried this (EC) on these shots? I am assuming that you can set EC on your camera, because as BillM has said that if you don't get the detail in the shadows, then there really isn't any way to recover it in post.
Hope this helps.
WesternGuy