Baby not growing feathers on head

by Maritza
(Miami, FL)

Hello, I just purchased a couple of cockatiel birds in December 2010. The female laid a clutch of 8 eggs and only 1 survived of the eggs that were fertile. The baby is now 6 weeks old and has grown feathers everywhere except his head. Also, I've noticed that the feathers that he has grown on his body has something like a something dark brown/black on the tips of the feathers. Not sure what's going on. The baby has been fed by the parents and til this day is still being fed by them. First time we have birds that have laid eggs. Not sure what to do or what could be wrong. Please help!