In species with obligate bi-parental care, investment by both parents in a current reproductive bout is critical to offspring growth and survival. The degree to which an individual can invest in their offspring relies on their quality as a parent. Parental quality can be communicated between individuals in a mated pair via ornamental features, as they may honestly reflect aspects of direct or indirect offspring contribution. In this study, we investigated whether the red-orange bill colouration in Atlantic puffins Fratercula arctica reflects two proxies of parental quality: hatch date and offspring growth. No aspect of paternal colouration predicted hatch date, but several metrics of maternal colouration predicted offspring peak mass and normalized wing growth. We also explored whether hatch date influenced patterns of chick growth and found that timing (early hatch vs. late hatch) but not synchrony with food availability significantly predicted mass and skeletal growth. Specifically, early hatching chicks achieved higher peak masses but exhibited reduced wing growth, potentially reflecting alternative strategies between investing primarily in weight gain or structural development. Together, these results highlight chick growth as a complex metric of parental quality, associated with both phenology and parental phenotype.

Growth of morphological traits in Atlantic puffin (Fratercula arctica) offspring has typically been characterized by linear models, despite clearly displaying nonlinear patterns. We assessed the fit of six typical avian growth models to measurements of puffling mass, wing length, and tenth primary feather length. Across all three biometrics, the first- and second-best performing models were nonlinear, and the worst performing model was the linear model. Specifically, the preferred model for mass, wing, and tenth primary growth was the quadratic model, logistic model, and extreme value function model, respectively. The preferred models were used to generate separate growth curves for individual chicks, from which parameter estimates for growth rate, normalized growth rate, inflection value, and asymptotic value can be extracted. These parameter estimates are easily interpretable and comparable between several of the nonlinear models. We recommend the reported models in future studies incorporating metrics of puffling growth, and encourage the use of this methodology to explore nonlinear growth patterns across avian species.