Egg Composition in Some Australian Birds

Abstract

Egg composition was examined in 17 altricial Australian bud species. The shell fraction comprised 7.4-13.6% (mean 8.7%) of fresh egg mass. Yolk comprised 17.5-29.9% (mean 23.5%), lipid 5.1-8.4% (mean 6.7%) and water-76.4-83.9% (mean 80.3%) of the fresh egg contents. On average, yolk contained 46.2% solids, of which lipids comprised 60.8%, and had an energy density of 13.87 kJg^-1 wet mass. Egg composition was not influenced by diet or clutch size and was similar to that of altricial species breeding elsewhere, emphasising the fundamental nature of its functional relationship with hatchling maturity. However, we identified some possible exceptions to the hatchling maturity-yolk content rule, in which pre-fledging growth rate appeared to influence fractional yolk content. The water fraction of the egg contents and, to a lesser extent, of the albumen component decreased as a function of fractional yolk content. The lipid fraction of the contents, but not of the yolk solids, increased in parallel with fractional yolk content. Consequently, the species whose eggs have larger fractional yolk contents have a relatively greater scope for the biosynthesis and maintenance of embryonic tissue andlor the production of hatchling yolk reserves.