Revisiting avian wingtip shape indices: a comparison of methods with empirical data

The study of avian wing shape helps in understanding how birds can adapt to their habitats and respond to changing environmental conditions. An important challenge in this endeavour is obtaining estimates of wing shape that are truly independent of body size, particularly when there is an allometric relationship between them. Size‐constrained component analysis (SCCA) has been proposed as the most appropriate method for estimating wingtip shape from flight feather measurements. Its implementation, however, has been hindered by the lack of open, accessible and reproducible software, leading many researchers either to adopt alternative multivariate methods or to continue relying on traditional, simpler to obtain, indices (e.g. hand‐wing index). Using data from 24 Western Palaearctic passerine species, we demonstrated that SCCA‐derived wingtip shape indices outperform five alternative methods in predicting interspecies variation in migration distance, the most influential life‐history trait shaping wing morphology. Although the scores of the shape indices derived from alternative methods were positively correlated with those obtained using SCCA—indicating that they captured a substantial proportion of the variation—our results provide empirical support for the value of SCCA, particularly in comparison with methods based on ratios of morphological measurements. In light of these findings, we strongly advise adopting SCCA as the gold standard for size‐independent wingtip shape estimation. To facilitate the implementation of this best practice, we provide an open‐source R code for SCCA calculation.