That's a cool concept!

what an idea and everything is cute :-O

hehe, nice one.. i'm totally diggin' these enemy birds; clouds and the other object are well done too, though (as you mentioned by yourself already) this needs to be filled;

a good idea would be to draw something in the background - like some mountains or the silhouette of a city..  or even (as your horizon already looks more like a room border) to make the sky look like a wallpaper by extending the shadows and adding a door and a light switch or something =)

↔ You could make a green enemy as well, perhaps one that is flying higher than all others.

Then add mountains to the background? =.

I do like this style though; Everything looks so very cute. :3 I especially like the grass, and the light line that parts horizontally, with the dots under it. I would've never thought of that! o_o