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• Pb isotope ratio analysis was done to determine Pb sources polluting eiders in 2018. • Pb isotopic ratios suggest the highest exposure is due to ingestion of Pb pellets. • Pb isotopic ratios also indicate multiple sources of Pb pollution. • Additional Pb sampling in the Baltic Sea is needed to identify specific Pb sources. The Christiansø colony of common eiders (Somateria mollissima) in the central Baltic Sea were exposed to high levels of Pb during the 2018 breeding season that were not present in 2017. Due to these high Pb blood levels, the present study investigated possible Pb sources and Pb dynamics within this vulnerable colony. We analyzed body mass and lead isotopic ratios (Pb-IRs) in blood taken from the same incubating eiders at the early (day 4) and late (day 24) stages of incubation during the 2018 breeding season (n = 23). Pb-IRs 208/207, 208/206, 206/207, and 207/206 were analyzed using high resolution inductively coupled mass spectrometry. We found largely similar Pb-IRs from the different stages of incubation indicating a predominantly constant endogenous source of Pb exposure. We suggest the increasing Pb levels come from pre-nesting and nesting foraging and from medullary bone release. The similar Pb-IRs also indicate continued metabolization of the medullary bone to meet the nutritional and energy demands of incubation. Comparisons to Pb-IR reports from the Baltic Sea showed multiple sources of pollution distinguished by a difference between Pb-IRs in individuals with Pb blood concentrations >500 μg/kg ww and <500 μg/kg ww. The most highly contaminated individuals in the present study had Pb-IRs similar to those of Pb ammunition indicating shot pellet uptake. This study further emphasizes the need for continued biomonitoring of the Christiansø colony, including fecal sampling and environmental field sampling to identify the origin and extent of dietary Pb exposure on Christiansø. As a representative unit of the Baltic Flyway population; the Christiansø colony provides an important opportunity for continued investigation into Pb contamination, population dynamics, and declines. [ABSTRACT FROM AUTHOR]

The ingestion of lead (Pb) shot pellets is a well-known cause of avian mortality, but exposure of birds to Pb may have other sublethal effects that can affect population sustainability. One of these effects is the alteration of the immunological status. Pb can affect most components of the avian immune system and imbalance the relationship among them. Pb exposure typically alters the ratio between T h1 - and T h2 -type responses mounted by different classes of T-lymphocytes, causing the depression of the T h1 responses that are associated with cell-mediated immunity. Immunodepressing effects of Pb on birds are observed at blood levels above 50 μg dL−1, but developing birds show immunodepressing effects at much lower concentrations (> 10 μg dL−1). Impacts of Pb on the avian immune system also relate to reduced resistance to infection. We review immunotoxic effects of Pb on birds affected by shot ingestion as well as by other sources of exposure to this element. Unlabelled Image • Lead poisoning is among the main causes of wild bird mortality. • Lead disrupts the balance among immune system components in birds. • Immunodepressing effects of lead on birds appear at blood levels >50 μg/dL. • Developing birds are especially sensitive to immunotoxic effects of lead. [ABSTRACT FROM AUTHOR]