Metabolic rates of embryos of precocial birds increase rapidly until about 80% through incubation, then increase slowly remain constant or even decline. In altricial species, embryo metabolic rates increase continuously and at an accelerating rate through out incubation. Total energy cost of development is higher in precocial than in altricial species. Growth patterns of altricial and precocial embryos differ in the same way as does metabolic rate. Embryo growth rates decline late in incubation in precocial species, but increase continuously in altricial species. Embryo metabolic rate in cal/hr (P) is related to embryo mass in grams (M) and growth rate in grams/day (GR) by the equation P = 12.17GR + 1.66M + 1.81. The energy cost of growth in avian embryos is 292 cal/g. The energy cost of maintenance is 1.66 cal/g hr and appeals to be independent of embryo mass. Differences in growth patterns account for the observed differences in metabolic rates and total energy costs of development. High energy costs of maintenance account for high total developmental costs in piecocial species and in species that have unusually long incubation periods.