ON THE COMPOSITION OF BIRD EGGS

From data in the literature an attempt is made to provide a general overview of the composition of bird eggs and how individual components are related to the relative yolk content, which varies from about 15% in some Sulidae to 69% in the Kiwi (Apteryx). Regression equations are given for the relative water content of the fresh egg, ablumen, and yolk, the solid and lipid fractions of egg content, and the calculated caloric density of egg content as a function of relative yolk content. As relative yolk content of eggs increases among 127 species representing 44 families, relative water content decreases. These changes are related to a progressive increase in hatching maturity from altricial to precocial birds according to the classification of Nice (1962). Certain exceptions to this rule are discussed, namely, eggs of the pelagic feeding Procellariiformes and offshore-feeding terns. Both groups have reduced their clutch to a single egg, have unusually long incubation and fledging periods, and larger relative yolk content than predicted. A general model of egg components is presented which relates the ash, carbohydrate, protein, lipid, water, and caloric content to the relative amount of yolk in the freshly laid egg. Lipids make up 58% of all yolk solids, which have an energy content of 33 k.I .cntdot. g-1; total solids in egg contents have an average energy content of 29 k.I .cntdot. g-1. An overview shows how the initial solids and caloric contents are redistributed in the yolk-free neonate and the yolk reserve as well as estimates of the solids and caloric loss during development; energy cost of development is similar in all birds and amounts to ca. 36% of the initial energy content of fresh eggs.