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

This is a Preprint and has not been peer reviewed. This is version 2 of this Preprint. You must log in to post a comment. There are no comments or no comments have been made public for this article. This is a Preprint and has not been peer reviewed. This is version 2 of this Preprint. Add a Comment You must log in to post a comment. Comments There are no comments or no comments have been made public for this article. Many animals construct nests. Nests are often considered extended phenotypes that shape survival and reproduction beyond the builder’s body. Birds are key examples of nest builders, and many add fresh 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, or direct enhancement of offspring condition. Here, we conducted a pre-registered systematic review and meta-analysis of 28 experimental studies (published and unpublished), spanning seven bird species and 274 effect sizes, to test whether green nest material influences fitness, and to evaluate the competing functional explanations. Our synthesis shows that while green nest material can enhance avian fitness, this effect depends on the fitness proxy used, and its functional role remains unresolved. The strongest predictor was experimental design, strongly influencing the outcomes, challenging the role of aromatic compounds in the fitness benefits of green nest material. Our meta-analysis supports the importance of green nest material on fitness, but calls for more research to elucidate its mechanisms. https://doi.org/10.32942/X2X65Z Behavior and Ethology, Ecology and Evolutionary Biology, Evolution, Other Ecology and Evolutionary Biology Green nest material, Ornithology, Birds, Drug hypothesis, Avian pathogen, Avian parasite, Nest protection hypothesis, Courtship hypothesis, Post-mating sexual selection, Sexual display, Secondary sexual trait Published: 2025-10-30 11:03 Last Updated: 2025-10-30 11:03 CC BY Attribution 4.0 International Conflict of interest statement: None Data and Code Availability Statement: All data and code associated with this project can be found on GitHub at [https://github.com/shreyadimri/Green_Nest_Material]. Language: English