Effects of white-tailed eagle (Haliaeetus albicilla) nestling diet on mercury exposure dynamics in Kopački rit Nature Park, Croatia.

The present study assessed for the first time the magnitude and dietary ecological source of total mercury (THg) exposure in a southern population of white-tailed eagles (Haliaeetus albicilla), an apex predator species shown valuable for environmental biomonitoring. This population depends on the Kopački rit Nature Park - the most important breeding site. We assessed THg exposure, using nestling body feathers collected between 2014-2019 (n = 72), and potential dietary ecological sources, proxied by prey remains and stable isotope analysis. Results show THg concentrations vary significantly over the years, though not showing any time trend. Prey remains analysis shows nests with aquatic prey remains to exhibit higher THg concentrations (median: 7.57 μg g dw; min - max: 6.00-13.16 μg g dw) compared to those with terrestrial remains (median: 3.94 μg g dw; min - max: 0.28-12.04 μg g dw) or evidencing a mixed diet (median: 7.43 μg g dw; min - max: 3.38-12.04 μg g dw). Nests with a predominant aquatic diet show elevated lower δC and higher δN values, indicating agreement between both dietary approaches. The model selection reveals a combination of year and δN best explain the variability in feather THg concentrations. Complementing these predictors with a dietary descriptor based on prey remains results in a poorer model fit and lowered explanatory power, similar to sexing the nestlings. The observed body feather THg concentrations (median: 6.99 μg g dw; min - max: 0.27 - 17.16 μg g dw) exceeded putative biogeochemical background levels (5.00 μg g dw) in 71% of the nestlings, though, did not seem to exceed a threshold at which detrimental physiological effects are expected (40 μg g dw). Continued monitoring is warranted as the studied population is likely exposed to a larger cocktail of contaminants while resident-protected bird areas.