1. We compare biases in estimates of the rate constant of the logistic and Gompertz functions applied to avian growth using three methods of parameter estimation. The methods differ in how the asymptote is determined, and consist of either floating asymptotes estimated from the full data (floating A) or data truncated at 70% of the adult mass (70%A), or fixing the asymptote at the adult mass (fixed A).2. First, using data for two passerine species exhibiting different growth patterns, we truncated mass measurements at different ages to explore how each method of growth rate estimation responded to simulated differences in nestling period. We bootstrapped growth rates and error measures from these data to produce unbiased error terms, which we used to compare the different models.3. We tested the effects of age truncation on both real and simulated data, and found that the fixed A method produced less bias and better fits than using estimates of floating asymptotes from the full growth curve (floating A) or truncated at 70% of the adult mass (70%A). Logistic and Gompertz models with a floating asymptote generally provided poorer fits than those with a fixed asymptote, although fits were improved by including a variable accounting for the ratio of asymptotic to adult mass.4. To evaluate the performance of the methods across an array of species, we applied the three methods to growth data for 45 species of New World songbirds spanning diverse rates of growth, developmental periods and geographic origins. To determine whether choice of method influenced interpretation of regional and life‐history differences in avian growth, we used model selection to estimate the influence of three independent variables (adult mass, nestling period and tropical vs. temperate region) with known effects on nestling growth rate calculated by each method. The coefficients of determination in these analyses suggest that the most appropriate method to estimate growth rates, regardless of the growth function (i.e. logistic, Gompertz), is the fixed A method, taking care to use accurate and appropriate estimates of adult mass.
Methods in Ecology and Evolution
Ricklefs, Robert; Austin, Suzanne; Robinson, Tara; and Robinson, W., "Potential Biases in Estimating the Rate Parameter of Sigmoid Growth Functions" (2010). Biology Department Faculty Works. 63.