Here is a link that has a couple pix's of stunted babies:
Babies in Trouble pictures by Mousebirds-and-more - Photobucket Your baby does not show these signs...yet...but the growth rate appears more from nutritional defeciencies of not being fed enough. From the pix the crop should be 4 times that size shown and packed with food almost around the clock for the first 10-12 days of age. I would suggest that you mix up formula and supplement feed the baby several times a day while in the nest or pull the clutch and start handfeeding them.
1...Formula temps should be 104-106 degrees.
2...Thickness of the formula about the consistency of babyfood applesause.
3...Weigh and calculate 10% of body weight to handfeed per feeding.
4...Never save and reuse mixed formula.
5...ALWAYS allow the crop to empty between EACH feeding.
Since you will be mixing just a small amount of formula per feeding times to avoid waste:
1...Weigh the babies to calculate 10% of body weight. Ex, if the baby is 40 grams, it will be fed 4cc (or ml) of formula.
2...Once you know the calculated weight of the babies measure that amount of water into a cup.
3...Warm the water and then mix in the formula.
4...Use a thermometer to measure the temp.
5...To keep the mixed formula warm (so that it doesn't cool down while feeding) place in a pan of warm water.
6...Hold the head firmly, neck stretched upwards, and formula is fed to the babies
right side of the throat. Release formula slowly as needed watching that the baby is swallowing it.
This forum must've changed how to post pix's
so I will direct you to a thread on another cockatiel forum that I spend most of my time that has some help info and pixs:
Hand-Feeding Chart - Talk Cockatiels Forums