The relationship between nutrition and physiology is intriguing and, when considered closely, may have wider implications than at first thought. To illustrate this statement, I wish to consider, in a representative bird, what may be regarded as a relatively simple dietary constituent.As in mammals, the domestic hen requires calcium for normal metabolic function and for bone formation during growth. Maintenance requirements are relatively similar to those for mammals and, for both hens and cocks, are (Berg et al. 1964). However, the hen differs from the mammal because reproduction in birds is accomplished through the production of an egg with a calcified shell.There is now extensive literature on Ca metabolism, though most concerns the domestic fowl. Much is known of the requirements for Ca, the role of the digestive tract, absorption from the tract, the importance of medullary bone, shell formation and the relationship between dietary Ca and shell calcification (Taylor