Daily energy budgets of avian embryos: the paradox of the plateau phase in egg metabolic rate.

The metabolic rate of precocial bird eggs reaches a plateau when about 80% of the incubation period has passed. This is unexpected, as in many species the embryo continues to grow and maintenance costs must therefore increase. To investigate this paradox, daily energy budgets were constructed for embryos of four galliform species according to two models that used empirical data on egg metabolic rate and embryo growth. In the first model, embryonic synthesis costs were estimated, with an assumed synthesis efficiency, before calculating the maintenance costs. In the second model, embryonic maintenance was calculated first, and no assumptions were made on the synthesis efficiency. The calculations show that assumptions of the synthesis efficiency had a major impact on the energy budget calculations, because embryonic growth rate was high. During the plateau phase, a galliform embryo allocated energy in favor of its maintenance costs in three ways: by decreasing growth rate, by increasing synthesis efficiency, and by depressing the formation of glycogen. Our study suggests that a reduction in growth rate plays a minor role. An increase of synthesis efficiency is more likely to explain the plateau in energy expenditure, since small increases in synthesis efficiency can lead to great savings on synthesis costs.