Mercury contamination is an invisible threat to declining migratory shorebirds along the East Asian-Australasian Flyway

Abstract

Exposure to pollutants is a potentially crucial but overlooked driver of population declines in shorebirds along the East Asian-Australasian Flyway. We combined knowledge of moult strategy and life history with a standardised sampling protocol to assess mercury (Hg) contamination in 984 individuals across 33 migratory shorebird species on an intercontinental scale. Over one-third of the samples exceeded toxicity benchmarks. Feather Hg was best explained by moulting region, while habitat preference (coastal obligate vs. non-coastal obligate), the proportion of invertebrates in the diet and foraging stratum (foraging mostly on the surface vs. at depth) also contributed, but were less pronounced. Feather Hg was substantially higher in South China (Mai Po and Leizhou), Australia and the Yellow Sea than in temperate and Arctic breeding ranges. Non-coastal obligate species (Tringa genus) frequently encountered in freshwater habitats were at the highest risk. It is important to continue and expand biomonitoring research to assess how other pollutants might impact shorebirds.

Fig. 1. Total Hg (THg) concentration in feathers of different shorebird species along the East Asian-Australasian Flyway.

Vertical red dashed lines show Hg toxicity thresholds^, from left to right, representing low risk (1.62 mg/kg), moderate risk (4.53 mg/kg), high risk (9.14 mg/kg), and severe risk (10.99 mg/kg). For each species, the number in parentheses indicates the sample size. Boxplots show median and 25% and 75% quartiles. Species are ranked by median feather THg concentrations. Scientific names can be found in Supplementary Data 1 and 2 (all species) and in Table 1. The x-axis is truncated at 18 mg/kg for clarity; one individual data point of 38.63 mg/kg from a Marsh Sandpiper is not shown.

Fig. 2. Hg concentration in feathers based on model estimations vs. observed calculations.

The number of observations used here was 742. Left: the estimated marginal means (log₁₀ transformed THg concentration) for the region on the full model (logHg ~ Region + Habitat Preference + Foraging Stratum + Diet + (1|Species) + (1|Year)). Right: the observed means of total Hg concentration in feathers. The standard errors of both means are indicated by red horizontal bars. The blue vertical lines indicate the estimated 95% CI while the black vertical dash lines indicate the observed 95% CI.

Fig. 3. Sampling sites along the East Asian-Australasian Flyway.

1: Yalujiang; 2: Tiaozini; 3: Chongming Dongtan; 4: Yangkou; 5: Mai Po; 6: Leizhou; 7: Taylors Lagoon; 8: Coconut Well Beach; 9: Lake Eda; 10: Roebuck Bay; 11: 80 Mile Beach; 12: Beachport; 13: Canunda; 14: Nene Valley; 15: Brown Bay; 16: Port Phillip Bay; 17: Western Port; 18: Corner Inlet. Sites are numbered in order of latitude. Map layers were provided by Esri, FAO, NOAA, and USGS (ArcGIS Pro 3.0.2).