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Older Common Terns Had More Mercury in Most Feather Types

Adult common tern standing on the ground
Conservation · Threats

Older common terns (Sterna hirundo) in a long-term study population on Germany’s North Sea coast had higher mercury concentrations in several types of feathers. The study, published on 15 April 2026, also found that mercury in body feathers was related to levels measured in blood, supporting their use for relatively non-invasive monitoring of mercury exposure. Mercury enters birds mainly through contaminated food and can affect health and reproduction; in long-lived seabirds, exposure can also accumulate over time. Mercury circulating in the body is incorporated into feathers as they grow and becomes locked into the keratin once growth is complete, so moult also provides birds with a route for excreting part of their mercury burden.

The researchers analysed dorsal, ventral, primary and tail feathers from 201 common terns of known sex and age that died during highly pathogenic avian influenza outbreaks in 2022 and 2023. Blood samples collected before death were available from 89 of these birds. In addition, 510 dorsal-feather samples collected from 313 individuals between 2017 and 2023 provided repeated measurements across years.

Mercury levels differed among feather types

Across 1,113 feather samples from 313 individuals, total mercury concentrations ranged from 0.17 to 35.15 micrograms per gram of dry feather mass. Ventral feathers had the highest mean concentration, at 3.80 micrograms per gram, while tail feathers had the lowest, at 2.50 micrograms per gram.

Using feather-mercury risk thresholds developed in earlier seabird research, about 18% of samples fell within the no-risk category, 65% within the low-risk category and 16% within the moderate-risk category. Fewer than 1% fell within the high- or severe-risk categories. These categories are comparative toxicity benchmarks rather than health effects measured directly in the common terns, but they show that roughly 17% of samples reached the moderate-or-higher range.

Mercury concentrations were positively correlated across the different feather types within individual birds. Levels in dorsal and ventral feathers, which are grown in the wintering area, were also significantly correlated with mercury measured in blood during the breeding season. The same relationship was not detected for primary or tail feathers.

Older birds had higher levels in three feather types

Males and females did not differ in their feather mercury concentrations. Age, however, was associated with higher concentrations in dorsal, ventral and primary feathers, but not in tail feathers.

Four scatterplots showing total mercury concentration in common tern dorsal, ventral, primary and tail feathers in relation to age
Figure 2 from Bertram et al. (2026): total mercury concentrations in four feather types in relation to age. © The Author(s) 2026, CC BY 4.0.

The repeated dorsal-feather samples allowed the researchers to examine whether the age pattern was likely to reflect changes within birds as they grew older. The within-individual age estimate was not statistically significant on its own, but it was very similar to the age difference seen among individuals. The authors therefore consider mercury accumulation within individuals during ageing a likely explanation for why older birds had higher concentrations in most feather types.

Body feathers may be most useful for monitoring

Feathers provide a way to monitor mercury exposure without routinely taking blood from live birds. The authors recommend body feathers in particular because their mercury concentrations were linked to blood levels, they reflected the age pattern and they can be sampled with relatively little disturbance. They interpret body-feather mercury as reflecting repeatable, relatively short-term dietary exposure, whereas primary and tail feathers integrate exposure over longer periods and are more influenced by internal redistribution of mercury. They also stress that the age of sampled birds should be taken into account when comparing mercury levels.

Sources and image credit

  1. Bertram, J., Garsi, E., Bichet, C., Kürten, N., Schupp, P. J., & Bouwhuis, S. (2026). Differential mercury accumulation and links to blood mercury levels across feather types in a long-lived seabird. Environmental Monitoring and Assessment, 198, 460. https://doi.org/10.1007/s10661-026-15310-4Primary open-access study published 15 April 2026. Common terns from the long-term Banter See colony in Wilhelmshaven, Germany, were analysed for mercury in four feather types, with blood measurements and longitudinal dorsal-feather samples used to examine links among feather type, blood mercury and age.

Image: Hans Hillewaert / Wikimedia Commons — adult common tern photographed at the port of Oostende, Belgium, on 19 June 2010; representative species image, not one of the birds from the Wilhelmshaven study; cropped to 16:9. · CC BY-SA 4.0 · Source

About this content: This story was produced with AI assistance within an editorial workflow developed by Wildlife Vagabond. Editorial responsibility remains with Wildlife Vagabond.How AI is used

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