The first body part to grow the fastest on a macaw is the foot. Around day 25 the foot is near adult size and ready to support the baby chick who will be gaping its mouth open waiting to be fed:
The second body part to grow at the fastest rate is the entire body. The parents spend all day foraging to bring back food for these hungry chicks. Parents will lay anywhere from 1-3 eggs focusing on feeding the healthiest individuals first.
The beak size grows as well with a steady rate reaching near adult size by age of 80 days.
Lastly the wings start to grow. Around day 40 the size growth accelerates as the chicks prepare to fledge and leave the nest. Around day 80 they start to say goodbye.