A graphical method for dating chicks using bivariate body measurements

Jones et al. (2005, J. Agric. Biol. Environ. Stat., 10, 306–320) developed a bivariate, nonlinear, mixed, growth curve model in order to estimate the ages of tern chicks from paired measurements of wing length and body weight. They found that age estimates obtained from a pair of measurements were much more precise than using either one alone. We present a graphical method for obtaining approximate answers to the same problem, but invoking fewer modeling assumptions. We obtain fiducial limits for dates by assessing whether pairs of measurements fall within confidence ellipses in the predicted morphological spaces of chicks of given ages. However, our conclusions differ markedly from those obtained by Jones et al.: we found that the precision of date estimates obtained using wing lengths alone were similar to those using both body measures. Supplemental materials including R-code and full illustration of the graphical method are available online.