Several things will elicit a feeding response from a baby parrot. "Feeding response" is basically the bird bobbing its head up and down so that as the parrot parents regurgitate food into the baby's mouth it is bounced down the throat and into the crop. When the baby is displaying the feeding response, the opening to his trachea is closed and the mouth, throat, and esophagus are all opened so that food can flow all the way into the crop without the bird needing to swallow. Parrot parents feed by regurgitating partially digested food into the baby's mouth. When regurgitating, the baby bobs his head up and down to pump the food into their own mouth. As the parents lock beaks with the baby, the baby matches this bobbing rhythm causing the food to continue pumping from the parents mouth into the babies and on down into the babies' crop.
Baby Bird Food: Parrot Breeding Supplies - AllParrotProducts
Activity level, especially if there are other parrots around, will vary during the day and affect a baby's interest in food. For instance, an older baby will begin to have less interest in eating and run from the syringe in the morning because as soon as the cage door is opened he is ready to explore the world. However, an hour later he may be crying and demanding that you stop the world to feed him. More information can be found in the weaning section. (this link goes to the weaning section in SECTION 3 part 4)
Parrot Baby Bird Food and Hand Weaning Formulas - All Parrot Products
Some babies are needier than others and need more nurture. An effort should be made to feed on demand, just like a mother would if breast feeding. A hungry baby that keeps coaxing or crying, and is not quite empty, should be fed a little (nurtured) even if it is not on the scheduled time. The same variety of things that cause a human baby to require attention will affect your baby parrot. Too cold, too hot, need to be touched, played with or just security concerns will cause the baby to cry. Because of the nurturing component of feeding, the baby will be reassured that everything is OK and there is nothing to worry about because 'mom' is taking care of him. Even if the baby has food in his crop it is all right to give him a small unscheduled amount, assuming he is otherwise healthy. If you believe the baby is getting enough food and should not be hungry and crying, explore other causes.
Baby Parrots That Cry Or Scream For Food