Sex- and age-specific mercury contamination in Central Europe's last gull-billed tern population

Mercury contamination is a growing threat to ecosystems and wildlife due to its adverse effects on animal physiology and its potential to accumulate in long-lived top predators. Endangered species may be especially vulnerable, requiring contamination assessment at both the population- and individual-level. Between 2022 and 2024, we assessed total mercury (THg) contamination in the last population of gull-billed terns (Gelochelidon nilotica) in Central Europe, known to breed in a heavily mercury-polluted area, by collecting blood samples of adults and chicks. Among adults, THg concentrations were variable, but mostly below toxicity thresholds associated with major adverse health effects. They were higher in males than females, and higher when birds were sampled later in the season, but not related to clutch size. Using longitudinally collected data and a within-subject centering statistical approach, females, but not males, were found to accumulate THg as they aged. Chicks exhibited relatively low THg levels, which varied with age (following a bathtub-shape), but not sex. As such, our study suggests sex-specific mercury accumulation in gull-billed terns at levels that are currently associated with low to moderate health effects. We recommend continued monitoring of THg levels and assessing the levels of further contaminants, as well as how they may interact with mercury to affect the birds.