Is it a defense mechanism? Mating mechanism? Etc.
-
Probably a sex-recognition token. Dull colors indicate to the opposite sex that a bird is poor condition, possibly due to parasites, and that therefore the genetic basis of the immune system may not be desirable. Might also be a species recognition token since, if I remember, cockateils flock together.
-
It isn't a defence mechanism. Predators of cockatiels include raptors such as falcons, and reptiles like goannas and snakes which will take eggs and young. The cocaktiel nests in a tree hollow so its colouring matters little on the nest. The colours are for species recognition and sexual display. The males have bright colours while the females and immatures have duller facial colours. Their colours stand out and the birds can be recognised quickly at some distance.
-
Most birds use plumage color patterns as a form of sex and species recognition. Different species have different plumages, and often (but not always) males have different plumages than females. Quite often juveniles also have different plumages than the adults. In addition to plumage color pattern, birds use different songs and/or dances to identify individuals of the same species. Species identification is important because if a bird mates with a different species than its own, the hybrids are almost certainly less fit than individuals of either of the parental species. In the worst case scenario, such matings may not even produce embryos.