Reference: Psittacine Aviculture -Perspectives, Techniques and Research by Richard Schubot, Kevin and Susan Clubb.DVM

" STUNTING: It should be kept in mind that stunted birds also gain weight daily, but in inadequate and sporadic amounts. The trained eye is the best judge of condition."
"Stunted birds seem thin and disproportionate with the head appearing too large for the body. Toes and wings are a better indicator of adequate weight then breast muscle in very young birds. Toes, wings or backs should not be excessively thin."
"Eye opening may be delayed in a stunted or sick nestling. The skin of a normal chick will be thick, supple, opaque and often yellowish due to subcutaneous fat deposits.
Abnormal feathering patterns may develop on the head of a stunted chick, and the emergence and maturity of the body feather may be delayed"

Cherane writes;   Often, I show in the videos the backs, hocks and feet/claws of the Hyacinth and Blue Throated Macaw chicks with reference to the skin color, supple and texture, hydration, muscles, including feather development of the head and body. Most viewers can see eye developememt as well personality development of the chicks THE TRAINED   EYE   throughout the 95 or so video's.   In the  flick,  I also address the feathering of the head, body  to demonstrate the lack of black and stress demarcation in his feathers, all of which can be associated with the stunted Hyacinth Macaw or any bird, inadequate handfeeding formula, environmemnt or and  event/disturbance including Metabolic disturbance on  day of formation.