In Creative Photoshop they leave us this great resource, some birds precious made with the technique of digital painting.
In them you can perfectly observe the feathers, the shades and iridescent of each color of the plumage and "the soul" in the eye, which gives them life and makes them much more real.
The format in which you can download them is PNG, with which you can use them without retouching them.
In the same pack there are many more resources that you can see by visiting the original post.
Download | Birds done in digital painting