Parental blood mercury levels are correlated, and predictive of those in eggs in a long-lived seabird

Mercury (Hg) is a pollutant that does not only biomagnify along the food chain, but can also accumulate in longlived top-predators, such as many seabirds, and be transferred to the next generation during reproduction. To better understand the transfer of Hg from parents to offspring, as well as its potential negative consequences, we used seven years of data on total mercury (THg) levels in the blood of common terns (Sterna hirundo) breeding at the German North Sea coast, and their eggs. We assessed whether (i) THg levels of pair members correlate, (ii) (changes in) parental THg levels correlate with those in their eggs, (iii) egg THg levels are repeatable within a given clutch and within parents across years, and (iv) parental and egg THg levels correlate with embryonic development, hatching success and hatch mass. Blood THg levels of pair members were positively correlated, but only maternal THg levels correlated with those in shells of hatched eggs, with within-individual increases over time leading to increases in shell THg levels as well. THg levels in shells of hatched eggs showed within-clutch and across-year repeatability. THg levels in shells of unhatched eggs were higher than those of hatched eggs, and were lower when embryos had developed for longer before they died, suggesting Hg absorption from the eggshell. THg levels in the contents of unhatched eggs were positively correlated with those of both parents. Interestingly, hatching success and hatch mass correlated positively, rather than negatively, with maternal THg levels, suggesting that a Hg-rich maternal diet (e.g., large fish or prey of high trophic levels) may enhance reproductive investment, thereby offsetting potential negative effects of Hg, at least under current pollution levels. Testing for downstream effects of transferred Hg on chicks will, however, be important.