A bird's song is a focus of Nature VS Nurture. A lot of the time it is a mix between what it has learnt from parents, picked up from other birds, noises that he or she has found work well in their song; and what is instinct. Budgies and other parrots specialize in copying and therefore their song will mainly consist of things he or she has found where other species usually have a characteristic song for the species as this is what works when attracting a mate! It really is a beautiful and complex work of nature but all it boils down to is the individual and it's species 
