Black Swan Eggshells Reveal Hidden Metal Pollution in Urban Wetlands

The discarded shells of black swan eggs are quietly keeping a chemical diary of Australia’s wetlands, and a new study has learned to read it. Researchers analysing post-hatched eggshells from two very different Victorian breeding sites have found that swans nesting on an artificial lake in the heart of Melbourne carry significantly higher concentrations of manganese, zinc and lead in their eggshells than swans breeding on a comparatively natural coastal island in the Gippsland Lakes. The findings, published in the Archives of Environmental Contamination and Toxicology, mark the first time trace metals have been measured in the eggshells of black swans (Cygnus atratus) and one of the first such assessments for any Australian waterbird. Because the shells can be gathered after chicks hatch without touching adult birds or eggs, the study points to a remarkably simple, ethical and repeatable way to track pollution in wetland ecosystems that are increasingly squeezed by urban growth.

The research team, led by Damien Nzabanita of RMIT University with colleagues from the University of Melbourne, Curtin University and BirdLife Australia, collected 37 eggshell samples during the 2021 breeding season. Seventeen came from nests around Albert Park Lake, an artificial wetland ringed by dense metropolitan infrastructure and fed by stormwater draining one of Australia’s largest cities. The remaining twenty were gathered from Pelican Island, a low-lying island within the Gippsland Lakes system in eastern Victoria, surrounded predominantly by natural and rural land uses. Fragments from each nest were combined into a single composite sample, cleaned of debris and residual membranes, rinsed in deionised water, dried and ground to a fine powder with a mortar and pestle before chemical analysis.

The analytical workhorse of the study was inductively coupled plasma tandem mass spectrometry, an Agilent 8900 triple quadrupole instrument capable of quantifying ten elements at extremely low concentrations: arsenic, cadmium, chromium, copper, iron, lead, manganese, nickel, selenium and zinc. Roughly 75 milligrams of powdered shell from each nest was digested in concentrated nitric acid and hydrogen peroxide until the solution ran clear, then diluted to a known volume with ultrapure water. Rigorous quality control accompanied every batch, with ultrapure water blanks and certified reference materials derived from mussel tissue and human hair run alongside the samples. Recovery values for all reported elements fell within acceptable ranges, giving the team confidence that the spatial patterns they observed were real rather than analytical artefacts.

The results revealed a clear urban fingerprint. Manganese concentrations averaged 3.20 milligrams per kilogram in Albert Park Lake eggshells, nearly three times the 1.19 milligrams per kilogram recorded at Pelican Island, a difference that was highly significant statistically. Zinc followed the same pattern, averaging 4.28 milligrams per kilogram in urban shells against 3.01 at the coastal site, and lead told the most striking story of all: 0.40 milligrams per kilogram in city shells compared with just 0.15 in coastal ones. Arsenic also trended higher in urban samples, though the difference fell just short of statistical significance. By contrast, chromium, iron, nickel and copper showed no meaningful differences between the two wetlands, suggesting that the urban signal is specific to particular contaminant pathways rather than a blanket elevation of all metals.

Not every element could be compared. Cadmium was excluded entirely because all 37 samples fell below the instrument’s limit of quantification, while selenium was dropped after 14 of 37 samples, including more than three-quarters of the urban shells, proved too low to quantify reliably. Nickel, with roughly a quarter of samples below the detection threshold, was handled with a specialised Peto–Peto statistical test designed for left-censored data rather than simple substitution, an approach the authors note avoids the distortions that fabricated values can introduce. The remaining comparisons relied on two-sided Wilcoxon rank-sum tests, chosen because several elements showed non-normal distributions even after logarithmic transformation. Iron and zinc both displayed considerable variability within sites, including one urban shell with markedly elevated iron at 38.93 milligrams per kilogram and a single coastal sample reaching 38.99 milligrams per kilogram of zinc.

Why should a city lake leave such a distinctive chemical signature in swan eggshells? The answer lies in the anatomy of urban catchments. Zinc is a classic urban contaminant, shed continuously from tyre wear, galvanised infrastructure and vehicle emissions before being swept into stormwater and deposited in receiving wetlands. Manganese enrichment can reflect altered sediment chemistry and runoff from urban infrastructure, while lead persists in soils and sediments decades after Australia phased out leaded petrol, a legacy of historical emissions, atmospheric deposition and contaminated urban ground. Wetlands act as depositional basins where these metals accumulate and remain biologically available through sediment disturbance and food web transfer. Black swans, which forage extensively in shallow water on aquatic vegetation and benthic material, are ideally positioned to pick up sediment-associated contaminants and pass them to their eggs.

The eggshell findings dovetail with earlier work on the same resident Albert Park Lake population. Previous studies had already detected elevated per- and polyfluoroalkyl substances, or PFAS, in swan serum and excrement, and feather analyses had identified exposure to multiple trace metals including raised zinc. The new data add a reproductive dimension to that picture: because eggshells form during egg development, their chemistry reflects what breeding females were transferring to their offspring at the moment of reproduction. Maternal transfer is ecologically significant because it represents a direct exposure pathway for embryos during their most sensitive developmental stages. Lead is of particular concern as a non-essential metal with no known biological function, capable of impairing neurological, physiological and reproductive processes in birds even at relatively low concentrations, although the levels recorded here remain far below those seen in heavily contaminated systems elsewhere.

The authors are careful about what the data can and cannot show. The concentrations measured do not, on their own, demonstrate harm to swans, and species- and matrix-specific toxicity thresholds for eggshell metals have not been established. The elevated manganese, zinc and lead in urban shells are best read as evidence of greater environmental exposure and maternal transfer rather than proof of adverse reproductive effects. The study also carries limitations: only two wetlands were sampled, the interval between hatching and shell collection was not recorded, and environmental matrices such as sediment, water and vegetation were not analysed concurrently, preventing direct identification of contaminant sources. Eggshell chemistry can additionally be influenced by embryonic development stage, as calcium and associated trace elements are mobilised during shell resorption. Future work, the team suggests, should pair eggshell data with environmental sampling, egg membranes, feather analysis and measures of shell thickness and structural integrity.

Even with those caveats, the broader message is compelling. Eggshells offer a practical, non-invasive biomonitoring matrix that can be collected season after season across many locations without disturbing breeding birds, complementing feathers, which record longer-term metal deposition during growth. For a species as widespread, long-lived and site-faithful as the black swan, the approach could turn one of Australia’s most familiar birds into a national sentinel for wetland contamination. As urbanisation continues to press against wetlands across the continent, the humble eggshell, usually trampled into the mud after hatching, may become one of the most valuable pollution records conservationists can collect, providing baseline ecotoxicological data for a native waterbird that straddles both freshwater and coastal worlds.

Subject of Research: Trace metal contamination in black swan eggshells as a non-invasive biomonitor of urban and coastal wetland pollution in southeastern Australia.

Article Title: Trace Metal Concentrations in Black Swan (Cygnus atratus) Eggshells from Urban and Coastal Wetlands in Southeastern Australia

Article References: Trace Metal Concentrations in Black Swan (Cygnus atratus) Eggshells from Urban and Coastal Wetlands in Southeastern Australia. (n.d.). https://doi.org/10.1007/s00244-026-01220-6

Image Credits: AI Generated

DOI: 10.1007/s00244-026-01220-6

Keywords: black swan, eggshells, trace metals, urban wetlands, biomonitoring, lead contamination, zinc, manganese, Gippsland Lakes, Albert Park Lake, ecotoxicology, waterbirds