Dietary Calcium Availability and Reproduction in Birds

Reproduction is energetically costly. Egg formation in birds requires 37–55% of basal metabolism for small passerines and 160–216% for ducks and the Southern Brown Kiwi (Apteryx australis) (Walsberg 1983). The production of eggs requires an adequate supply of water, macronutrients (carbohydrate, protein, and fat), and micronutrients (essential fatty acids, amino acids, vitamins, and ions). Of this latter group of nutrients, calcium is probably the most limiting micronutrient required by the laying bird (Burley and Vadehra 1989); 98% of the dry mass of the eggshell consists of calcite, a crystalline form of calcium carbonate (CaCO3) (Romanoff and Romanoff 1949). The eggshell encases all of the nutrients required for embryonic development and survival during the incubation period, preventing the developing chick from being crushed by the incubating adult. The shell also protects the embryo from dehydration, prevents the entry of pathogens from the external environment, and allows gaseous exchange. It also acts as a source of calcium for skeletal development of the embryo. The withdrawal from the skeleton results in “natural thinning” of the eggshell from ∼6% (e.g., Balkan et al. 2006) to 21% (e.g., Booth and Seymour 1987) as the embryo absorbs calcium from the shell during incubation. After egg production, the calcium requirements of the adult in many species must remain elevated during chick-rearing, when birds feed calcium-rich foods to their young (see Table 3 in Graveland 1996a) for the continued mineralization of their skeletons (Starck 1998).