Metabolism and energetics of avian embryos.

Energy within an avian egg is used primarily to support the metabolic costs of growth and maintenance, deposited in embryonic tissues, or retained in residual yolk. In altricial species, embryonic growth rate and metabolic rate increase continuously during incubation, whereas in precocial species, embryonic growth rate declines shortly before hatching so that metabolic rate usually reaches a plateau before hatching. These differences mean that when altricial and precocial birds are compared using egg mass as a scaling variable, altricial species have lower costs of development, smaller hatchlings, and shorter incubation periods. Their eggs also have lower energy densities, however, and when egg energy content is used as the scaling variable altricial and precocial species cannot be distinguished except by egg size and energy density. This suggests that before hatching all avian embryos expand about the same percentage of the energy stored in their eggs. Altricial and precocial species do not appear to differ in energy cost per gram of yolk-free, dry hatchling produced (15.4 kJ/gm). The major difference between altriciality and precocity lies in the timing of hatching during the developmental process (earlier for altricial species than for precocial species) and in the amount of energy the parent originally deposits in the egg. We suggest that the first step in the evolution of altricial bird species from precocial ancestors was hatching early. This shortened the incubation period and reduced the amount of energy needed prior to hatching. Consequently, the female could reduce her energy investment per egg by decreasing size and increasing water content of the egg.