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Comparative analysis of avian embryonic growth.

The relationships between weight (W) and age (t) of embryos of 27 species of birds are described by parabolic equations of the form W = a*(t-i)b*, where a*,b*, and i are constants and i accounts for a time lag before the onset of parabolic growth. A simple transformation gives values of a and b for the equation dW/dt = aWb. Some mathematical properties of the parabolic equation, relating growth rate (dW/dt) to weight and age, extrapolating different curves to their intersection (point of divergence), and relating the length of the incubation period to constants of the parabolic equation and to hatching weight, are described. Fitted constants for most species formed a cluster in the vicinity of a = 0.4-0.5 and b = 0.65-0.75, within which one could not differentiate precocial and altricial species, nor among precocial families. Pelagic seabirds consistently had reduced values of b and were the only group with i greater than 0 (5-19 days). The relation between fitted constants and incubation periods is discussed. The rate at which embryo growth rate decreases with respect to size (b) is directly related to the rate of accumulation of dry matter in the tissues, suggesting that both express an underlying program of development. This study emphasizes the relationship between growth rate and development of the embryo, illustrates the degree of evolutionary flexibility and diversification within and among taxonomic groups, and reveals the sensitivity of the very young embryo to environmental conditions.