Egg weight and length of the incubation period are the major determinants of avian embryonic growth and the developmental state of the neonate. Precocial birds lay larger eggs relative to adult body size than do altricial species,[6'7] and they take longer to hatch. Portmann argued that altricial and precocial species progress through the same developmental stages, but that altricial chicks hatch relatively earlier in this progression than precocial. Indeed, the major difference in the weight of altricial and precocial embryos and neonates is water. More water than dry matter would indicate less tissue growth and maturity. Extremely large birds such as the ostrich lay large eggs with variable lengths of incubation that can hatch anywhere from 43 to 49 days of incubation, suggesting great plasticity in the length of incubation at the extremes of egg size.
When differences in egg size and incubation period are taken into account, the slopes of growth curves of embryos do not differ between altricial and precocial species. Embryonic growth would then appear to be an evolutionarily conserved trait, and most variation in egg size may be related to adult size. Larger birds lay larger eggs with longer incubation periods. Smaller birds exhibit slower growth and more rapid maturation. Therefore, regardless of the developmental state of the neonate, differences among egg sizes are accommodated primarily by the uniform acceleration or deceleration of growth rate over most of the developmental period. In other words, egg size and incubation period seem to be closely related and inseparable biologically.
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