The potential post-hatching growth of domestic birds is sufficiently described by the Gompertz function.

1. It has been hypothesised that, for post-hatching domestic birds reared under good conditions, the relative growth rate R = (dW/dt)/W = dlnW/dt is a linear function of lnW; W is body weight. It followed that dW/dt = B.W. ln(A/W), where A is mature weight, and, by integration, that weight over time is described by a Gompertz function: W = A.exp.(-exp(-(G - B.t)), where W is weight at time t d, B is the rate parameter d, and G = -ln(-ln(W/A)).2. Where growth data published in the literature did not show this relationship, it was likely to have been caused by sub-optimal conditions for maximum growth, such as inadequate nutrition or other factors.3. Published data, which have been diligently examined from both sexes and nine species, over 400-fold range of degree of maturity, collected over nearly a century of data from four continents, strongly corroborated the hypothesis.4. The data were not in agreement with another functions using three parameters and made functions with four or more parameters superfluous. The relationship between R and lnW should be carefully examined before subjecting any data set to further analysis.