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.

Sexually monomorphic species have been historically overlooked in the sexual/social selection literature, but there is growing evidence that mutual ornamentation can be driven by selective forces such as mutual sexual selection or selection for individual recognition. Examining the properties of a trait may elucidate which forces most likely play a role, especially when comparing the characteristics of quality and identity traits. Atlantic puffins (Fratercula arctica) are an example of a mutually ornamented monomorphic species, where both males and females display a bright orange-red bill and orange gape rosette during the breeding season and are ornamented to similar degrees. In this study, we investigate whether the properties of the colorful bill and rosette, specifically lability across the breeding season and condition-dependence, more closely align with signals of quality or identity. Our findings support prior work that the bill is sexually monochromatic from an avian visual perspective. We also determined that the bill changes in a discriminable way within individuals and is especially dynamic in the fleshy cere and rosette. However, no metric of color on any region of the bill or rosette were significantly related to current body condition. Ultimately, we argue that bill color likely functions as a quality signal, although further study is needed to determine which aspect of quality coloration signals, if not condition. These results provide a basis for experimentally testing the signal value of the colorful bill in Atlantic puffins, and more broadly, a framework for investigating the properties of mutual ornamentation in avian species.

Age‐related changes in the production of sexually selected assessment signals have been identified across a diverse range of taxa, and in some cases, these changes have been shown to affect receiver response to those signals. One important type of change occurs even after a signaler reaches breeding age, a phenomenon known as delayed maturation. Delayed maturation has been observed in the songs of several bird species, with potential fitness consequences for males as a byproduct of female choosiness or male competition. Here, we analyzed songs recorded across the first three years of life in a cohort of hand‐reared song sparrows Melospiza melodia to detect early‐life age‐related changes in song. We focused on three measures of song complexity, including within song type variation, the average number of notes and the number of unique note types for the most common variant of each song type, and five measures of song production patterns, including singing rate, time interval between songs within a bout of the same song type, time interval between bouts (i.e. when the song type changes), within‐song stereotypy and between‐song consistency. All measures of song complexity and most measures of song production patterns (excluding within‐song stereotypy) changed significantly within individuals as birds aged from one to two years as well as from one to three years (excluding within‐bout time interval), whereas no significant changes occurred from two to three years of age. Based on these features, a linear discriminant model could distinguish between the song of young (age 1) and older (age 2 and 3) adult males, providing support that song could serve as an indicator of age in this species. We discuss potential implications of these results for mate choice and male–male competition in song sparrows.

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