One of my Java sparrow chicks has a white area of feathering above its beak. Is it suffering from a vitamin deficiency?

While white patches of feathering in some young birds such as crows are often a sign of a nutritional problem, this is not the case with your young Java sparrow. The chick is a pied, and as such, this area of white plumage will remain constant throughout its life. When a nutritional deficiency is involved however, the affected area of feathering may be replaced with normal plumage at the next moult, or it could expand further.