Probing of heavy metals in the feathers of shorebirds of Central Asian Flyway wintering grounds

Shorebirds migrate long distances between breeding and wintering grounds and use southern Indian wetlands such as Point Calimere Wildlife Sanctuary (PWLS) and Pichavaram Mangrove Forest (PMF) as critical stopover and wintering sites along the Central Asian Flyway (CAF) and East Asian–Australasian Flyway (EAAF). These wetlands have experienced increasing pollution from development, agriculture, pesticides, and other anthropogenic activities, with heavy metals being prominent contaminants that bioaccumulate and biomagnify in food webs. Heavy metals can adversely affect shorebird physiology, behavior, reproduction, and migration. Ethical constraints limit destructive sampling, making feathers a suitable non-invasive bioindicator for metal exposure. Despite prior studies measuring metals in various avian tissues, no studies had quantified metal burdens in feathers of migratory shorebirds at PWLS and PMF. The study’s objective was to evaluate concentrations of seven heavy metals (Cr, Co, Cu, Pb, Hg, Ni, Zn) in feathers of 15 shorebird species at these two wintering grounds to provide baseline information on contamination levels and interspecific variation.

Previous research has documented environmental contamination by heavy metals affecting soils, sediments, aquatic ecosystems, fish, and bird tissues. Feathers, eggs, blood, eggshells, liver and kidney have been used as bioindicators, with feathers offering a non-destructive tool suitable for free-ranging birds. Metals including Cr, Cu, Co, Hg, Pb, Ni, and Zn enter food webs and can biomagnify, impacting wildlife health and breeding ecology. Regional reports indicate environmental pressures from industrialization and agriculture in southern India, with halophytic plants in Pichavaram showing high metal concentrations. However, no prior study assessed heavy metals in feathers of shorebirds at PWLS and PMF, representing a key knowledge gap this work addresses.

Study area: Two southern Indian wetlands were investigated. PWLS (10°18′N, 79°51′E), a Ramsar site at the Great Vedaranyam Swamp, is a monsoon-influenced, mixed fresh and seawater swamp separated from the Bay of Bengal and Palk Strait, with salt production units and seasonal hydrology. PMF (11°29′N, 79°46′E) is a 1,100 ha natural mangrove system (Vellar–Coleroon estuarine complex) with 51 islets and extensive waterways, supporting abundant benthic prey and migratory shorebirds. Sampling: Feathers from 15 shorebird species (Black-winged Stilt, Common Redshank, Common Sandpiper, Curlew Sandpiper, Dunlin, Eurasian Curlew, Kentish Plover, Lesser Sandplover, Little-ringed Plover, Little Stint, Marsh Sandpiper, Painted Stork, Spotted Redshank, Temminck's Stint, Wood Sandpiper) were collected between 2014 and 2019. Twelve dead carcasses from PWLS and 13 from PMF were sampled; additionally, some individuals (e.g., Temminck's Stint 2015, Dunlin 2016/2017, Black-winged Stilt 2016) were captured using mist nets, breast feathers collected, and birds released unharmed. Three individuals per species were used. Breast feathers were chosen to represent plumage and be less influenced by molt than flight feathers. Sample preparation: Feathers were rinsed with double-distilled water, oven-dried at 60 °C for 24 h to constant mass, and weighed (±0.001 g). Digestion used 65% nitric acid and 70% perchloric acid in a 4:1 ratio. Digests were diluted to 10 mL with deionized water and stored in metal-free glassware at 20 °C until analysis. Analytical determination: Metals (Cr, Co, Cu, Pb, Hg, Ni, Zn) were quantified by double-beam atomic absorption spectrophotometry (AAS). Calibration curves were prepared for each metal at 0.5, 1, 2, 5, and 10 ppm. Daily working solutions were prepared by appropriate dilution of stock standards with 65% HNO3, 30% H2O2, and water (v/v/v = 1:1:3). Reagent blanks were used to zero the instrument for every sample. Glassware was acid-washed (30% HNO3), rinsed with deionized water, and air-dried. Results were expressed in ppm. Quality control: A QC sample was injected every ten samples. Reagent blanks were included with each batch. Six replicate measurements were used to derive average values. Accuracy was expressed as relative standard deviation (RSD) of 5–10% (SD/mean). Separate calibration curves were used per metal. Statistics: Data are presented as mean ± SE. Normality was assessed with Shapiro–Wilk. One-way ANOVA tested differences among species. Inter-correlation analyses among metals were conducted. Analyses used SPSS 16.0, with significance assessed using standard procedures.

- Metals measured in feathers of 15 shorebird species at PWLS and PMF showed significant interspecific differences (p < 0.001). - Overall accumulation pattern in shorebird feathers: Zn > Ni > Co > Cr > Cu > Pb > Hg. - High Zn: Dunlin (90.08±4.29 ppm), Little-ringed Plover (75.5±2.25 ppm), Little Stint (419.09±178.1 ppm), Marsh Sandpiper (91.9±1.42 ppm) at PWLS; at PMF, Zn was also high in Common Sandpiper (95.6±4.08 ppm) and Little-ringed Plover (328.4±140.21 ppm). - High Ni: Common Sandpiper (70.9±9.75 ppm) and Little-ringed Plover (114.8±1.18 ppm) at PWLS. - High Cu: Dunlin (PWLS 20.9±2.72 ppm; PMF 87.2±10.62 ppm) and Marsh Sandpiper (PWLS 82.3±2.32 ppm). - High Cr: Marsh Sandpiper (PWLS 22.1±0.31 ppm; PMF 114.7±5.39 ppm); Little Stint also elevated (PWLS 28.01±0.94 ppm). - High Co: Marsh Sandpiper (PWLS 53.08±0.94 ppm) and Little Stint (PMF 128.7±54.72 ppm); Dunlin elevated at PMF (30.8±16.52 ppm). - High Pb: Elevated in multiple species. PWLS examples include Black-winged Stilt (9.6±0.42 ppm), Common Redshank (9.6±0.26 ppm), Curlew Sandpiper (9.6±0.57 ppm), Eurasian Curlew (6.9±1.09 ppm), Lesser Sandplover (9.5±0.52 ppm), Temminck’s Stint (6.1±0.34 ppm). At PMF, high Pb in Black-winged Stilt (10.2±1.20 ppm), Common Redshank (10.6±1.53 ppm), Curlew Sandpiper (12.005±0.92 ppm), Temminck’s Stint (10.6±0.58 ppm), Kentish Plover (8.3±0.66 ppm), Spotted Redshank (10.6±0.58 ppm), Wood Sandpiper (9.5±0.39 ppm). - High Hg: At PWLS, higher in Kentish Plover (8.3±1.03 ppm), Painted Stork (8.1±0.64 ppm), Spotted Redshank (7.8±0.55 ppm), Wood Sandpiper (7.7±0.82 ppm). At PMF, higher in Eurasian Curlew (7.7±0.46 ppm), Lesser Sandplover (5.8±0.79 ppm), Painted Stork (8.6±0.60 ppm). Several species showed Hg >5 ppm. - Correlations: Hg showed negative associations and was not positively correlated with any other metal in both wetlands. At PWLS, Cr correlated positively with Cu (r=0.944). Co correlated positively with Cu (r=0.812) and Cr (r=0.688), and negatively with Hg (r=-0.550). Pb correlated positively with Cu (r=0.804), Cr (r=0.786), and Co (r=0.672). Ni correlated positively with Cu (r=0.461), Co (r=0.478), and Pb (r=0.431). Zn correlated positively with Cu (r=0.659), Cr (r=0.700), and Hg (r=0.476). At PMF, Cr correlated negatively with Hg (r=-0.686). Co correlated negatively with Pb (r=-0.561). Pb correlated negatively with Hg (r=-0.483) and positively with Cr and Co. Ni correlated positively with Cr (r=0.848), Co (r=0.415), and Hg (r=0.505). Zn correlated negatively with Hg (r=-0.322).

Findings demonstrate substantial heavy metal burdens in feathers of shorebirds using two major wintering/stopover wetlands in southern India. The significant interspecific differences reflect species-specific diets, foraging methods, and migratory strategies along CAF and EAAF that influence exposure and bioaccumulation. Elevated Zn, Ni, Co, and Cr in several species align with known high metal levels in benthic invertebrates (molluscs, polychaetes, crustaceans) and sediments, consistent with the birds’ benthic foraging and incidental sediment ingestion. Sources include saltpan operations, industrial and domestic effluents, agriculture (fertilizers and pesticides), waste burning, highways, and aquaculture. Elevated Hg in piscivorous or benthic-feeding species (e.g., Painted Stork, Eurasian Curlew) is consistent with trophic transfer in aquatic food webs. Correlation patterns suggest differing environmental sources and biokinetics among metals, with Hg behaving independently (no positive correlations) relative to other metals. The observed burdens, including multiple species with Pb near or above levels of concern and Hg frequently >5 ppm, indicate potential risks to behavior, physiology, and reproduction, underscoring the need for pollution management at these internationally important wetlands and along migratory flyways.

This first assessment of seven heavy metals in feathers of 15 shorebird species at PWLS and PMF shows significant interspecific differences and an overall accumulation order of Zn > Ni > Co > Cr > Cu > Pb > Hg, with several species exhibiting high burdens of particular metals (e.g., Zn, Ni, Co, Cr, Pb, Hg). Results highlight contamination pressures at key CAF/EAAF wintering and stopover sites and the relevance of diet and habitat quality to exposure. The study provides baseline data for conservation and management of these Ramsar/IBA wetlands and supports the use of feathers as a non-invasive biomonitoring tool. Future work should: expand temporal and spatial coverage across flyways and stopover sites; include additional metals and contaminants (e.g., arsenic, organochlorines/organophosphates); integrate sediment, water, and prey sampling; assess physiological and reproductive impacts; and evaluate mitigation effectiveness.

- Sampling size was limited to three individuals per species, potentially constraining statistical power and generalizability. - Sampling spanned multiple years (2014–2019) and included both carcasses and live-captured individuals, introducing temporal and methodological variability. - Only feathers were analyzed; internal tissues and other matrices (blood, eggs, organs) were not assessed. - Only seven metals were measured; other relevant contaminants (e.g., arsenic) were not evaluated in this study. - Migratory species may accumulate metals at multiple stopover sites along flyways; thus, feather burdens may not exclusively reflect PWLS/PMF exposures.