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The duration and rate of yolk deposition in the formation of avian eggs are species specific and are apparently fixed characteristics. We determined these properties for eggs from 40 species of marine birds. The mass of yolk laid down daily during rapid yolk deposition in the ovary was calculated and used to construct and compare yolk enlargement curves for each species. The spectrum of curves are all best fitted by simple second-order polynomial regression equations, with the very rapidly enlarging yolks of Alcidae at one extreme and the slow-forming eggs of Procellariiformes at the other. Calculated daily energy allocation to yolk deposition during the last, and most demanding, phase of yolk formation ranged from 3% to 44% of standard metabolic rate, considerably lower than other estimates.