This looks amazing! The background is simple enough to not drag the eye away from the subject of the picture but not simple enough to take away from the picture's quality.

I love the effort you put into drawing the feathers on the inside of the wing however you should put just as much effort into drawing the back of the wing instead of leaving it solid.

The shading looks great however I would soften the edges of the highlighted areas like under the belly. It would simply give it a more rounded look, at least to my eyes.

The feathering on the legs looks good. The only thing about the mane and tail that looks off to me is that the horse seems to be moving forward but the mane looks like its moving up and back. The tail says it is moving forward. I would suggest that in future work you put arrows in when you're drawing so you know which way the wind is blowing and you can put the mane and tail both blowing in the same direction.

Overall this is a fantastic piece. Keep up the good work!