The influence of avian functional traits on bird–litter interactions: a systematic quantitative review

Summary

Anthropogenic litter poses a significant global threat to birds through ingestion, entanglement, and use of litter as nesting material, yet the literature lacks a quantitative understanding of bird–litter interactions and their drivers, constraining evidence-based management. This review examined how sample method, taxonomy, avian functional traits, and latitude varied with three commonly reported litter measures: frequency of occurrence, litter mass, and litter count. Although some patterns emerged, cross-study consensus was limited by imbalanced research effort (with seabirds dominating the dataset) and inconsistent data reporting and sampling methods. Despite this, analysis provided some insight into how avian functional traits influence bird–litter interactions. For example, wetland birds ingested litter less frequently than coastal birds. Birds with primarily aquatic lifestyles exhibited lower rates of both ingestion and entanglement than generalists. The frequency of use as nesting material was positively correlated with bird body mass. Although frequency of occurrence was the most used measure of bird–litter interactions, there was high variability and few apparent patterns across taxonomic groups or other variables. Overall, our results emphasise that advancing our understanding of bird–litter interactions and reducing the negative impact of litter on birds requires the use of multiple litter measures, more research on non-marine bird species, and improved method and data standardisation.