Mercury and other trace elements in gull chicks from a highly urbanised environment: effects on chick growth, physiology, and health condition

Coastal urbanisation has increased the exposure of urban-dwelling organisms to contaminants, such as trace elements (TEs) in opportunistic seabirds. This exposure may cause detrimental health effects, especially during sensitive early life-stages, before detoxification mechanisms are fully developed. We monitored yellow-legged gull (Larus michahellis) chicks in the city of Porto, Portugal, and collected down feathers and blood for mercury (Hg) and other TE analyses. We aimed to i) evaluate the effects of maternal exposure to Hg (inferred from down feathers) on early chick development, and ii) examine the impact of blood TE contamination on chick physiology and health condition. Aligned with previous research, blood Hg decreased as chicks aged (up to the 5^th week), due to the growth dilution effect and depuration into the growing feathers. Additionally, down feather Hg concentrations showed a limited transfer of Hg from females to the embryo and was unrelated to hatching mass or early growth rate. While low maternal Hg exposure resulted in no detectable effects on early-life development or chick survival, dietary exposure (inferred from blood) to other TEs incurred physiological costs. Blood arsenic (As) and lead (Pb) concentrations were associated with a faster erythrocyte sedimentation rate, suggesting impaired immune response and inflammation. This highlights a dichotomy between transgenerational Hg transfer and post-hatching dietary exposure to Hg and other TEs. Future work should aim to define toxicity thresholds for TEs in seabird chicks, investigating the implications in the health since early life-stages, to better integrate the observed physiological responses.