Methylmercury Effects on Birds: A Review, Meta‐Analysis, and Development of Toxicity Reference Values for Injury Assessment Based on Tissue Residues and Diet

Abstract

Birds are used as bioindicators of environmental mercury (Hg) contamination, and toxicity reference values are needed for injury assessments. We conducted a comprehensive review, summarized data from 168 studies, performed a series of Bayesian hierarchical meta‐analyses, and developed new toxicity reference values for the effects of methylmercury (MeHg) on birds using a benchmark dose analysis framework. Lethal and sublethal effects of MeHg on birds were categorized into nine biologically relevant endpoint categories and three age classes. Effective Hg concentrations where there was a 10% reduction (EC10) in the production of juvenile offspring (0.55 µg/g wet wt adult blood‐equivalent Hg concentrations, 80% credible interval: [0.33, 0.85]), histology endpoints (0.49 [0.15, 0.96] and 0.61 [0.09, 2.48]), and biochemical markers (0.77 [<0.25, 2.12] and 0.57 [0.35, 0.92]) were substantially lower than those for survival (2.97 [2.10, 4.73] and 5.24 [3.30, 9.55]) and behavior (6.23 [1.84, >13.42] and 3.11 [2.10, 4.64]) of juveniles and adults, respectively. Within the egg age class, survival was the most sensitive endpoint (EC10 = 2.02 µg/g wet wt adult blood‐equivalent Hg concentrations [1.39, 2.94] or 1.17 µg/g fresh wet wt egg‐equivalent Hg concentrations [0.80, 1.70]). Body morphology was not particularly sensitive to Hg. We developed toxicity reference values using a combined survival and reproduction endpoints category for juveniles, because juveniles were more sensitive to Hg toxicity than eggs or adults. Adult blood‐equivalent Hg concentrations (µg/g wet wt) and egg‐equivalent Hg concentrations (µg/g fresh wet wt) caused low injury to birds (EC1) at 0.09 [0.04, 0.17] and 0.04 [0.01, 0.08], moderate injury (EC5) at 0.6 [0.37, 0.84] and 0.3 [0.17, 0.44], high injury (EC10) at 1.3 [0.94, 1.89] and 0.7 [0.49, 1.02], and severe injury (EC20) at 3.2 [2.24, 4.78] and 1.8 [1.28, 2.79], respectively. Maternal dietary Hg (µg/g dry wt) caused low injury to juveniles at 0.16 [0.05, 0.38], moderate injury at 0.6 [0.29, 1.03], high injury at 1.1 [0.63, 1.87], and severe injury at 2.4 [1.42, 4.13]. We found few substantial differences in Hg toxicity among avian taxonomic orders, including for controlled laboratory studies that injected Hg into eggs. Our results can be used to quantify injury to birds caused by Hg pollution. Environ Toxicol Chem 2024;00:1–52. Published 2024. This article is a U.S. Government work and is in the public domain in the USA. Environmental Toxicology and Chemistry published by Wiley Periodicals LLC on behalf of SETAC.

Egg Mercury Concentration and Egg Size Varies with Position in the Laying Sequence in two Songbird Species

“…In tree swallows the largest difference in least squares mean egg THg concentrations was between egg 1 (0.272 µg/g fresh wet wt) and egg 5 (0.228 µg/g fresh wet wt), and in house wrens the largest difference was between egg 5 (0.184 µg/g fresh wet wt) and egg 2 (0.168 µg/g fresh wet wt). Although these differences in THg concentrations among eggs may be unlikely to result in differences in egg hatching success, these THg concentrations may still result in sublethal effects on the chicks that hatch from these eggs (Ackerman et al, 2024). In both tree swallows and house wrens, the largest decrease in average egg THg concentration was observed between the first and second egg laid (−14% in tree swallows, −6% in house wrens), although this decrease was only significant in tree swallows.…”

“…Mercury pollution is a widespread and continuing problem affecting wildlife health (Driscoll et al, 2013;Krabbenhoft & Sunderland, 2013;Wiener et al, 2003). Avian reproduction is particularly susceptible to mercury toxicity, and elevated mercury concentrations have been associated with reproductive impairment (Ackerman et al, 2016a(Ackerman et al, , 2024Scheuhammer et al, 2007). Mercury exposure can impair avian reproduction by altering parental care behavior, such as reducing incubation effort and increasing egg neglect (Evers et al, 2008;Hartman et al, 2019;Heath, 2002;Herring et al, 2010), and through direct embryotoxicity resulting in embryo deformities and mortality (Braune et al, 2012;Heinz et al, 2009Heinz et al, , 2011Kenow et al, 2011).…”