If you feel your cockatiel is too skinny but otherwise is alert, happy and healthy, you might want to look into a couple of possible causes.
Many new cockatiel owners do not realized that they need to refresh and clean their bird's seeds everyday. Cockatiels will not eat the shell of the seeds. Make sure your cockatiel always have new and complete seeds everyday.
"Cockatiels love millet and that is OK, but millet should only be use as a treat and not as a main source of a cockatiel's diet. Millet to cockatiels is like popcorn to humans."
If your cockatiel is very active, it might not be receiving enough calories in its diet. The best cockatiel diet is variety, do check our nutritional section for ideas about your tiel eating right.
If you feel that your cockatiel do have a proper diet and is eating but still is too skinny, you might want to see an avian vet, as it might be an underlying reason for certain illnesses like diabetes or parasites.
As always the information offered here is to provide guidance and is not intended to be a substitute for the good advice provided by your own avian vet. When in doubt always consult your own veterinarian.