Wing length canalisation and behaviour across birds: a phylogenetic meta-analysis of variance

We examined the expectation that stronger stabilising selection leads to a decrease in trait variation across species by investigating individual variation in wing length. We hypothesised that species that heavily rely on aerial feeding, as well as long-distance migratory species, show higher canalisation (lower coefficients of variation, i.e., CV values) in wing length than non-aerial feeders and non-migratory species. We collected summary statistics on wing length for males and females from the literature and analysed them using recently developed meta-analytic metrics for integrating phenotypic variance estimates. Our phylogenetic multilevel meta-analysis showed relatively low heterogeneity among CV values, indicating generalisability of the overall CV value (2.6%). Although not all pairwise comparisons were statistically significant, all our analyses consistently showed higher canalisation in aerial compared to non-aerial feeders, and in migratory compared to non-migratory species. We conclude that wing length in bird species relying on their wings more heavily is likely under stronger (stabilising) selection, which in turn would have led to the observed higher canalisation on this trait for those species. Our study showcases how to combine already available descriptive statistics for phenotypic traits with underused meta-analysis of variance approaches to test often-neglected evolutionary predictions.