Birds shape biogeochemistry and ecosystem function globally.

Birds inhabit nearly every environment on Earth, shaping ecosystems through diverse functions that include contributions to biogeochemical cycles. By transporting and recycling nutrients, birds influence the availability and distribution of chemical elements that regulate key ecological processes. Yet, as bird populations decline, their biogeochemical roles remain poorly synthesized, limiting our understanding of the broader consequences of biodiversity loss. Here, we systematically review the role of birds in biogeochemistry, with consequences for ecosystem function, structure, and connectivity globally. We highlight key mechanisms, ecological impacts, and future research and management priorities. We find that birds - by virtue of their broad distribution and abundance, flight-enabled mobility, nutrient-rich diets, and high metabolic demands - play distinct roles in nutrient transport. Although research has focused largely on waterbirds, growing evidence reveals birds' substantial chemical impacts on diverse environments, locally restructuring plant, microbial, and animal communities. Delivery of key limiting elements, such as nitrogen and phosphorus, can boost primary productivity and increase autotroph nutrient content, with bottom-up effects on higher trophic levels. However, bird subsidies can also promote soil acidification, contaminant accumulation, plant damage, and eutrophication. Because birds vary in their diet, size, life history, and behavior, declines in diversity can trigger uneven and cascading ecosystem effects. Human-driven changes in bird abundance, distribution, movement, and nutrition are modifying and diminishing avian-mediated biogeochemical processes across the biosphere, with potential consequences for ecosystem fertility, productivity, functional connectivity, and carbon storage. Nevertheless, opportunities remain to harness bird biogeochemistry in ecological restoration to revitalize Earth's natural nutrient networks.