The impact of heavy metal pollution on birds' behavior and cognition: A comprehensive review

Heavy metal pollution represents a persistent and pervasive threat to wildlife. Birds, as ubiquitous, top predators in many aquatic and terrestrial habitats, are particularly vulnerable and commonly used as bioindicators. While the lethal and reproductive effects of heavy metals on avian species are well-documented, sublethal consequences, particularly those affecting behavior and cognition, have received comparatively less attention. This review synthesizes current knowledge about the effect of sublethal exposure to heavy metals, notably mercury (Hg), lead (Pb), cadmium (Cd), and arsenic (As), on birds' behavior and cognition. Across taxa and contexts, heavy metal exposure has been shown to disrupt a wide range of behaviors, including altered general activity, impaired locomotor coordination, reduced parental care and vocal communication, and excessive vigilance. These behavioral changes could be linked to underlying cognitive impairments, such as deficits in learning, memory, perception, and recognition, often resulting from neurotoxic effects of heavy metals on brain regions, particularly the cerebellum and hippocampus. The severity and direction of these effects, however, vary widely across studies, species, and contexts. They are shaped by multiple interacting factors, including metal speciation, dose, developmental timing, exposure duration and route, and may also reflect the substantial methodological heterogeneity among studies. Together, these findings highlight the need for more integrative and ecologically realistic research designs to better characterize the sublethal impacts of heavy metal pollution on avian behavior and cognition. Advancing this understanding is crucial for developing early-warning indicators of environmental degradation and for informing conservation strategies in polluted environments.