I like this. You did a wonderful job with picking the background, and finding poses that allowed each wyvern its own personality.
There's the brightness/contrast issue; it is a very dark picture, and you loose a lot of the lushness of the forest with it being so dark. You can't make out the moss on the trees, or the ferns beside the stream.
Highlights; Your dark blue sleeping wyvern, and the brown wyvern on the log have a couple of places where they grab the light beautifully - ears, on both, wing tips and tail on the brown one. I'm not sure if this is intentional, or if it's because of the colors that you chose. Because of this, they give the impression that wyverns are glossy, but the three wyverns that should show that glossiness the most - the three flying in the middle of the clearing - lack these bright highlights.
Transparency; Your green wyverns wings aren't entirely opaque, and I love that! But it doesn't carry through to Mr. Pink and Ms. Purple in the corners.
On vs. In; There's a "placed" look to some of the wyverns that gives them away. It's most noticable on the dark blue one sleeping under the tree. He looks on the mulch, not in it. If you lighten the picture, you'll begin to notice it more with the other ones, too.
I have to say, I think the shadow mottling on the brown wyvern is superb.
Thank you for sharing this!
