All vertebrates require certain amounts of energy, protein and ‘essential’ nutrients. Dietary sources of energy include carbohydrates, protein and fat. Dietary protein is necessary for satisfying an animal’s protein requirements. Certain dietary nutrients are classified as ‘essential’ because they are required for synthesis or functioning of crucial chemical compounds, yet they cannot be synthesized by the organism in sufficient amounts to satisfy requirements. Thus, essential nutrients must come from diet. For most birds and mammals, about 38 nutrients are required in the diet including 10 essential aminoacids, 1–3 essential fatty acids, 13 vitamins, and at least 14 essential minerals (Robbins, 1993;Murphy, 1996).