American Robin (fledgling)

They're going to be pretty close in size. All of the eggs would have been laid within days of each other, and then they would have hatched within days of each other. So the one sitting up is probably the oldest, and may be a little bigger, but it won't be much.

Fledgling just means a young bird that is developed enough to leave the nest, but not an adult yet. All the young in the nest are going to be days apart, fledgling is just describing them at a particular stage in life.