Lucy does that. My theory is that they see the moving colors, and want to catch it. Lucy especially loves to chase things.
Then again, Lucy once saw a picture of her father with food on his face after feeding babies, and put her beak right where his beak was. She could have wanted to be a baby bird again as well.
it's hard to tell, sometimes.
I played a video to Alex of him before he was weaned, making the feed me cry. He just looked at me all like: "what is the point of this?"