Chickadees Chicks Hatched

Three days ago, I finally found the Carolina Chickadee parents off of the nest inside the bird box. Expecting to find the eggs in the nest, I was surprised to find that the chicks had already hatched. They were laying limp in the bottom of the deep nest; their bodies dramatically moving with each beat of their hearts. (Click on the photos to get an enlarged view.)
Forty-eight hours later when I opened the side panel of the bird box, I was greeted with four small heads pointing straight up, mouths open, eyes closed, but clearly expecting mom or dad to provide a meal. Today, I finally managed to get some photos. The chicks are clearly larger than just 24 hours later. Today, only one chick greeted me with a headed pointed upward and waiting for a meal.
Each time I approach and open the box, one of the Chickadee parents perches about 5 feet away in the oak tree. They squawk a lot to make sure that I know that they are there.