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Common Tern Eggs Had More PCBs, HCB and Mercury Than Oystercatcher Eggs in 40-Year Wadden Sea Study

Common tern guarding a nest with speckled eggs
Conservation · Threats

Four decades of bird-egg monitoring in the Wadden Sea found higher concentrations of 18 consistently monitored individual PCB compounds, hexachlorobenzene (HCB) and total mercury in common tern (Sterna hirundo) eggs than in Eurasian oystercatcher (Haematopus ostralegus) eggs. The study analysed pollution records spanning 1981–2023 and was published online in late August 2026.

The monitoring formed part of the Trilateral Monitoring and Assessment Programme (TMAP) for the Wadden Sea. Across the study period, the researchers examined 77 pollutants or pollutant compounds in eggs from the two bird species. These included several groups of persistent organic pollutants (POPs) — chemicals that break down slowly and can remain in ecosystems for long periods — as well as total mercury. PCBs are a family of long-lived chlorinated industrial compounds. The study included DDT-related compounds, chlordanes, hexachlorocyclohexanes, up to 62 individual PCB compounds, HCB and mercury; 18 PCB compounds were analysed consistently across the monitoring period.

Common tern eggs carried the higher contaminant burden

Common tern eggs contained higher concentrations of the consistently monitored PCB group, HCB and total mercury than oystercatcher eggs. The researchers also found that concentrations of most of the organochlorine pollutants were correlated with one another, whereas total mercury followed a different pattern. The authors therefore highlight concern about combined effects from simultaneous exposure to several contaminants.

Pollution was not evenly distributed across the Wadden Sea. Hotspots occurred around the estuaries of the Elbe, Ems and Weser rivers. Despite the long-term decline in many contaminants, the study reports that levels of most pollutants in almost all eggs still exceeded the safety thresholds used in the assessment.

Most pollutants declined, but some have risen again

The overall long-term trend was nevertheless downward. Concentrations of every pollutant group except summed chlordane decreased over the monitoring period. For mercury, the proportion of eggs above the study’s low-risk threshold for detrimental effects fell from 74% in the 1980s to 22% in the 2020s.

The decline was not uniform through time. Several pollutants showed slight increases again in recent years, which the authors identify as a reason to continue long-term monitoring and to add emerging contaminants to future surveillance. Bird eggs are useful for this purpose because repeated sampling can reveal both geographic differences and long-term changes in contamination experienced by breeding females and their environment.

Sources and image credit

  1. Forty years of pollution monitoring in the Wadden Sea using bird eggs: insights from the Trilateral monitoring and assessment programEnvironmental Pollution study released online in late August 2026. Bibliographic services differ by one day on the first online date (28–29 August). The study analysed 77 pollutants in common tern and Eurasian oystercatcher eggs across the Wadden Sea from 1981 to 2023.
  2. Contaminants in bird eggs – Wadden Sea Quality Status ReportOfficial Wadden Sea background on TMAP bird-egg monitoring and the use of common tern and Eurasian oystercatcher as indicator species.

Image: A common tern guarding its nest on Masonboro Island, North Carolina. Photo: NOAA National Estuarine Research Reserve Collection. Source: Wikimedia Commons. Public domain. Representative image; not a bird from the Wadden Sea study. · Public Domain (NOAA) · 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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SPECIES IN THIS STORY

Species in this story

Common Tern Sterna hirundo Explore species Eurasian Oystercatcher Haematopus ostralegus Explore species

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