Cardinal Chicks

Looking somewhat like Muppets, two Northern Cardinal chicks realize there is no food forthcoming from the camera. Normally at this stage in their careers, they are all about open mouths — wide, wide mouths, like so:These birds will quickly get bigger, feather out, and fledge, or fly out from the nest. (This site gives details on Cardinal plumage colors and molts; technical, but still a great comparision.)

Fledged songbirds stick close to their parents at first — people often find a young bird on the ground and mistakenly think it has been abandoned, but the parents are just hiding from you (read this on baby birds; in general, you should not interfere). Fledged birds will follow their parents and loudly call to be fed. A whole new chorus of bird sounds is now being heard: hungry, insistent youngsters.
This Starling youngster has yet to develop the glossy black plumage of an adult bird.

So some birds are already fledged. Others, having just arrived, like Baltimore Orioles, are only now weaving their suspended nests.