Why do birds use green nest material? A systematic review and meta-analysis of experiments

Many animals build nests. As external structures that can influence survival and reproduction, nests are often considered extended phenotypes. Birds are key examples of nest builders and some species add green plant material to their nests. Yet, the adaptive value of this behaviour remains debated. Non-mutually exclusive hypotheses propose roles in courtship signalling, parasite defence and enhancement of offspring condition through pharmacological effects independent of parasite reduction. Here, we conducted a pre-registered systematic review and meta-analysis of 28 experimental studies (26 published, 2 unpublished), spanning seven bird species and 274 effect sizes, to test whether green nest material enhances fitness and to evaluate competing functional explanations. Our meta-analysis shows that green nest material can increase fitness; however, this effect varied depending on the fitness proxy investigated, being strongest for morphological proxies. We found no compelling evidence to preferentially support the courtship, nest protection or drug hypothesis. Nonetheless, experimental design (i.e. treatment-control comparison type) was the moderator explaining most effect size variation, challenging the traditionally held role of aromatic compounds in the fitness benefits of green nest material. Our synthesis provides evidence for the adaptive significance of green nest material and highlights the need for further research into the underlying mechanisms.