Thanks for your question. Unfortunately without examining the baby it is very difficult to know why it is not developing at the same speed as the other. It may not have formed correctly in the egg, or it may not be getting enough nutrition from the parents since it is last in line. Supplemental feeding won’t hurt, however a comprehensive physical exam is the best way to fully answer your question.
We have an avian specialist on staff. Please contact us to schedule an exam, or if you have any further questions.