im an expert in photoshop, moreover you dont need special move to arrange everything in one scene ..
in my opinion , your project needs to be place on layer, arrange them, mixing in the same level (brightness, contrast, and any other adjustment), than make them in one resolution,
1. every assets like what you got in one scene (arranged)
2. everything made in pixelrate (like your bird)
3. everything in a high resolution (like the background)
am i right?
this is my first chance in freelancer, but i wont let you down for my skill ..