How much does mercury contamination affect body condition in birds?

Mercury is poisonous for all living organisms but quantitatively proving the impact of this chemical element is challenging.  A new meta-analysis that included data from populations monitored by SEAPOP has examined whether increased mercury concentrations lead to reduced body condition in birds.

Synthesis of previous studies

Mercury contamination is a major threat to the global environment and is still increasing in some regions despite international regulations. The methylated form of mercury is hazardous to biota, yet its sublethal effects are difficult to detect in wildlife. Body condition can vary in response to a variety of stressors, but previous studies have shown mixed effects of mercury in wildlife. Using birds as study organisms, researchers have provided the first quantitative synthesis of the effect of mercury on body condition in animals. They also explored the influence of intrinsic, extrinsic and methodological factors potentially explaining cross-study heterogeneity in the results. The material they considered was experimental and correlative studies carried out in adult birds and chicks, and mercury exposure inferred from blood and feathers.

No clear negative association

Most experimental investigations (90%) showed a significant relationship between mercury concentrations and body condition. Experimental exposure to mercury disrupted nutrient (fat) metabolism, metabolic rates, and food intake, resulting in either positive or negative associations with body condition. Correlative studies also showed either positive or negative associations, of which only 14% were statistically significant. Therefore, the overall effect of mercury concentrations on body condition was null in both experimental (estimate ± SE = 0.262 ± 0.309, 20 effect sizes, five species) and correlative studies (−0.011 ± 0.020, 315 effect sizes, 145 species). Of the 15 moderators tested, only wintering status explained cross-study heterogeneity in the correlative data set: free-ranging wintering birds were more likely to show a negative association between mercury and body condition. However, wintering effect sizes were limited to passerines, further studies should thus confirm this trend in other taxa.

Confounding factors

Collectively, the results suggest that (i) effects of mercury on body condition are weak and mostly detectable under controlled conditions, and (ii) body condition indices are unreliable indicators of mercury sublethal effects in the wild. Food availability, feeding rates and other sources of variation that are challenging to quantify likely confound the association between mercury and body condition in natura. Future studies could explore the metabolic effects of mercury further using designs that allow for the estimation and/or manipulation of food intake in both wild and captive birds, especially in under-represented life-history stages such as migration and overwintering.

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• Quantitative meta-analysis reveals no association between mercury contamination and body condition in birds