Richards’s equation and nonlinear mixed models applied to avian growth: why use them?

Postnatal growth is an important life-history trait that varies widely across avian species, and several equations with a sigmoidal shape have been used to model it. Classical three-parameter models have an inflection point fixed at a percentage of the upper asymptote which could be an unrealistic assumption generating biased fits. The Richards model emerged as an interesting alternative because it includes an extra parameter that determines the location of the inflection point which can move freely along the growth curve. Recently, nonlinear mixed models (NLMM) have been used in modeling avian growth because these models can deal with a lack of independence among data as typically occurs with multiple measurements on the same individual or on groups of related individuals. Here, we evaluated the usefulness of von Bertalanffy, Gompertz, logistic, U4 and Richards’s equations modeling chick growth in the Imperial Shag (Phalacrocorax atriceps). We modelled growth in commonly used morpholog...