I'm thinking it's because the birds are going toward the sun, thus starting the silhouette process (figures becomes darker and darker). That's what I can think of at the moment.
Also, another thing I noticed in this scene is that the birds are very small, yet the holes in their 'cage' are very big, it seems that they could easily escape the cage.
I also noticed that when she opens the cage the first time and pets one of the birds they all stay in, but when she does it a second time they fly away.