Energy in avian eggs and hatchlings: utilization and transfer

Energy content of eggs, hatchlings, and egg components (albumen, yolk, true hatchling = hatchling without spare yolk, and spare yolk) and the energy spent for metabolism were analyzed in 50 species of birds divided into four maturity types. Mass-specific energy density on the basis of fresh egg content mass appears to be mass-independent within each maturity group but different among the groups. Mass-specific energy densities calculated on the basis of dry component mass are the same for albumen, yolk, and spare yolk in all maturity groups but are different for true hatchlings, precocial hatchlings being richer in energy than all others. The gross production efficiency (hatchling energy/egg energy) of 63.7% +/- 7.8 SD does not differ significantly among maturity types. Total production efficiency (true hatchling energy/egg energy minus spare yolk energy) averaged 57.0% +/- 7.0% (SD) in all types. The inefficiency attributed to fuel loss in metabolism is 34.7% +/- 11.0% (SD) of the total energy used; hence losses in extraembryonic tissues and meconium average 8.3% of the total energy used. The cost of transforming the chemical potential energy in the egg into living tissues (including maintenance costs) is about 0.5 J X J-1. The energy densities of the dry matter in the egg and the energy transformation efficiencies and costs seem to be independent of maturity type. The differences among maturity types reside in the different water concentrations in eggs and hatchlings, in the density of chemical potential energy in the dry matter of true hatchlings, and in the different amounts of energy transferred untransformed from the egg to the spare yolk.