That's pretty good, moomoose. I can see some of the ways you went about coloring and filling out the original line art sketch you started with. however, I feel the same as you. There is an inconsistency in it. The background makes the cockatrice pop out, but not in the best of ways. It looks good, and aside from completely creating your own background from scratch it is very good. But what gets me about the actual cockatrice is the lack of shadowing. There are dark parts that look very good with it, but the entire image feel flat. There is very little dimension in it.
Looking at it, it appears your main light source is from the top right, sort of where the sun seems to be shining from in the background image. You can use that much more to your advantage. The bottom of the belly on the creature should be darker. Light would have a harder time reaching that since its back is to the light. Granted you could have thought of that and darkened the body to reflect the light source but it's all a flat shadow then. The bottom of the belly would be darker, and there might be some more reflecting light hitting it from bouncing off the ground. The bend in the neck could be darker since it will be creasing there. Creases are good shadow gatherers.
As for the wings, there needs to be some more defining features. It looks as if the wing closest to the viewer blends in and is directly connected to the wing farthest from the viewer. That needs some seperation.
The black parts of the creature look nice, they provide a good contrast, but I would maybe think about trying to avoid outline-like looks. Such as the the outlines around the wings feel unneeded. There is a highligh on the upper right of the wings tha would make for a perfect ending to them instead of defining them with that black outline.
Hope this helps you some.
