The study was carried out during the 2023 breeding season by Mohammadreza Ahmadi, Rasool Zamani-Ahmadmahmoodi, Seyed Mehdi Amininasab and Atefeh Chamani, researchers affiliated with Shahrekord University, Sari Agricultural Sciences and Natural Resources University and Islamic Azad University. The team deliberately selected three species with distinct ecological roles: the common tern (Sterna hirundo), a fish-eating predator near the top of the aquatic food chain; the black-necked grebe (Podiceps nigricollis), an intermediate feeder on invertebrates and small fish; and the Eurasian coot (Fulica atra), a largely herbivorous omnivore feeding low on the chain. By comparing metal burdens across this trophic gradient, the researchers could test whether contamination accumulates as it moves upward through the ecosystem, a pattern known as biomagnification.

Eggshells are an unusually convenient biomonitor. As the shell mineralizes inside the female, calcium is drawn from her body and deposited as calcium carbonate, and any lead circulating in her blood can hitch a ride on that process. The mechanism behind this is ionic mimicry: the lead ion Pb2+ is close enough in size and charge to the calcium ion Ca2+ that the biological machinery building the shell cannot reliably tell them apart, so lead substitutes for calcium in the crystal lattice. Cadmium, by contrast, enters the egg through different physiological routes and is generally present at lower levels. Because shells are abandoned naturally after hatching, sampling them leaves breeding colonies undisturbed, a major advantage over studies that require tissue collection from live or dead birds.

Metal concentrations in the collected shells were quantified with an Agilent 240FS AA atomic absorption spectrometer, a workhorse analytical instrument that atomizes a digested sample and measures how much light of a characteristic wavelength each metal absorbs. The results were stark. Lead concentrations exceeded cadmium in every species examined, and mean values for both metals surpassed widely used hazard thresholds of 4 milligrams per kilogram for lead and 2 milligrams per kilogram for cadmium. In other words, every one of the three species is carrying shell metal burdens associated in the ecotoxicological literature with potential reproductive and developmental harm.

The trophic pattern was unmistakable. The common tern, the high-trophic-level piscivore, showed the highest mean concentrations by a large margin: 424.29 milligrams of lead per kilogram of shell material and 23.39 milligrams of cadmium per kilogram. The black-necked grebe recorded intermediate values of 174.60 milligrams per kilogram for lead and 1.75 milligrams per kilogram for cadmium, while the low-trophic Eurasian coot carried 44.58 milligrams of lead and 2.38 milligrams of cadmium per kilogram. The tern’s lead burden was nearly ten times that of the coot, a difference consistent with the idea that predatory fish-eaters integrate contamination from every link below them in the food web. Each step up the chain concentrates the pollutant further, so a tern eating contaminated fish effectively eats the accumulated exposure of the smaller organisms those fish consumed.

Statistical analysis backed up the visual pattern. A one-way analysis of variance confirmed that the differences among species were significant at the conventional P < 0.05 threshold, supporting the trophic-level hypothesis of bioaccumulation in this system. Interestingly, Pearson correlation analysis found no significant relationship between lead and cadmium concentrations across samples. That statistical independence matters, because it suggests the two metals are not arriving from a single shared source. The authors point to likely distinct origins: industrial effluents for lead, and agricultural runoff for cadmium, which is a common contaminant of phosphate fertilizers and can wash from farmland into wetlands during irrigation and rainfall.

Gandoman Wetland sits in Iran’s Chaharmahal and Bakhtiari province and is recognized under the Ramsar Convention as a wetland of international importance, valued for its bird diversity and its role as a stopover and breeding site for waterbirds. Previous work has already documented heavy metal contamination in its sediments, and recent studies have flagged climate-driven vegetation change and other pressures on the wetland’s bird populations. The new eggshell data add a biological dimension to that picture, showing that contamination is not merely present in the substrate but is actively moving through the food web into the reproductive tissues of the birds themselves, with the potential to affect hatching success and chick development.

The concentrations reported are striking by any comparative standard. Lead levels in the tern shells, at over 400 milligrams per kilogram, are orders of magnitude above the 4 milligram per kilogram hazard threshold cited in the study, and even the coot, the cleanest of the three, exceeded both the lead and cadmium benchmarks. Eggshell metal burdens reflect the maternal body burden during egg formation, so these figures indicate that female birds at Gandoman are carrying substantial internal loads of lead and cadmium during the breeding season. Chronic lead exposure in birds is associated with neurological impairment, anemia and reduced reproductive output, while cadmium accumulates in the kidney and can disrupt calcium metabolism and eggshell quality, creating a feedback loop in which contamination may itself weaken the shell’s structure.

The non-invasive eggshell approach has grown into a respected tool in ecotoxicology, with studies of herons, egrets, gulls, cranes, terns and even flamingos using shell residues to trace environmental metal pollution across Europe, Asia and the Americas. What the Gandoman study adds is a clean demonstration of trophic structure within a single wetland, using three co-occurring species that share the same water but occupy different feeding niches. Because the three species forage at different levels, their shells function like a set of nested barometers, each reading the contamination of its own slice of the food web. The strong species-level differences, and the divergence between lead and cadmium, give managers a way to distinguish pollution pathways rather than just measuring a generic toxic load.

The authors describe Gandoman as a high-risk environment and argue that stringent pollution control is urgently needed to protect its wetland-dependent birds. Practically, that means addressing the two suspected sources separately: controlling industrial discharges that carry lead into the watershed, and managing agricultural runoff that delivers cadmium from surrounding farmland. The study also underscores the value of continued eggshell monitoring as a low-cost, repeatable surveillance method that can track whether remediation is working without ever handling a live bird. For a Ramsar site whose international designation rests on its birdlife, the message written into these fragile calcium shells is hard to ignore: the contamination that begins in factory outfalls and farm fields is ending up, concentrated and measurable, in the very animals the wetland was designated to protect.

Subject of Research: Lead and cadmium bioaccumulation in waterbird eggshells at Gandoman Wetland, Iran

Article Title: Trophic drivers of lead and cadmium bioaccumulation in waterbird eggshells: a non-invasive assessment at Gandoman Wetland, a Ramsar site in Iran

Article References: Ahmadi, M., Zamani-Ahmadmahmoodi, R., Amininasab, S. M., & Chamani, A. (2026). Trophic drivers of lead and cadmium bioaccumulation in waterbird eggshells: a non-invasive assessment at Gandoman Wetland, a Ramsar site in Iran. Environmental Monitoring and Assessment, 198(11), Article 1159. https://doi.org/10.1007/s10661-026-15984-w

Image Credits: AI Generated

DOI: 10.1007/s10661-026-15984-w

Keywords: heavy metals, lead, cadmium, eggshells, waterbirds, biomagnification, Gandoman Wetland, Ramsar site, ecotoxicology, biomonitoring, Iran, common tern

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