Mercury levels in chicks of a long-lived seabird – parental effects and links with growth and survival

Environmental mercury levels continue to rise, leading to increased exposure, especially in long-lived species at high trophic positions, such as many seabirds. During breeding, female seabirds can transfer mercury to their eggs, exposing their offspring to its harmful effects. After hatching, dietary exposure may further compound the risk, although excretion pathways exist as well. Despite extensive research, the relative contributions of maternal transfer, dietary exposure, and excretion to mercury dynamics in chicks, as well as its impacts on growth and survival, remain poorly understood. As such, across seven years, we artificially incubated 578 common tern (Sterna hirundo) eggs, and monitored the resulting chicks from hatching to fledging. We assessed total mercury levels in the eggshells, in chick blood and feathers, as well as in the blood of the parents. Chicks found dead were used to study mercury concentrations in brain, gut, kidney, liver and muscle. Mercury levels in organs were highest among chicks that died young, then lower among chicks that died by approximately day 24, suggesting growth dilution and excretion through feather growth, before being higher again among fully-grown chicks that died around fledging. Chick blood mercury levels predicted brain and feather mercury concentrations, but only showed marginal associations with those in other tissues. Maternal mercury levels were a key predictor of chick levels early in development, whereas paternal mercury levels were less consistently linked with those in chicks. Higher mercury levels in eggshells and chick blood were associated with reduced growth, and, in turn, survival. These findings demonstrate that early-life mercury exposure, shaped partly by maternal transfer, varies with chick development and survival. Future research should clarify the causalities linking mercury to chick growth, survival, recruitment and adult fitness to better understand its population-level consequences.