The threat of global mercury pollution to bird migration: potential mechanisms and current evidence

Abstract

Mercury is a global pollutant that has been widely shown to adversely affect reproduction and other endpoints related to fitness and health in birds, but almost nothing is known about its effects on migration relative to other life cycle processes. Here I consider the physiological and histological effects that mercury is known to have on non-migrating birds and non-avian vertebrates to identify potential mechanisms by which mercury might hinder migration performance. I posit that the broad ability of mercury to inactivate enzymes and compromise the function of other proteins is a single mechanism by which mercury has strong potential to disrupt many of the physiological processes that make long-distance migration possible. In just this way alone, there is reason to expect mercury to interfere with navigation, flight endurance, oxidative balance, and stopover refueling. Navigation and flight could be further affected by neurotoxic effects of mercury on the brain regions that process geomagnetic information from the visual system and control biomechanics, respectively. Interference with photochemical reactions in the retina and decreases in scotopic vision sensitivity caused by mercury also have the potential to disrupt visual-based magnetic navigation. Finally, migration performance and possibly survival might be limited by the immunosuppressive effects of mercury on birds at a time when exposure to novel pathogens and parasites is great. I conclude that mercury pollution is likely to be further challenging what is already often the most difficult and perilous phase of a migratory bird's annual cycle, potentially contributing to global declines in migratory bird populations.

Differential reliance on aquatic prey subsidies influences mercury exposure in riparian arachnids and songbirds

“…et al, 2016). Contrary to these benefits, our study also shows that reliance on aquatic prey increases Hg exposure, which can interfere with migration (Seewagen, 2018), reproduction (Jackson, Evers, Etterson, et al, 2011;Varian-Ramos et al, 2014), and survival (Ma et al, 2018) at environmentally relevant exposures. The varying reliance on aquatic prey among species and individuals complicates the calculation of Hg risk to riparian communities near contaminated water bodies because a more detailed understanding of foraging ecology (beyond broad classification of granivore, omnivore, or insectivore) is needed to assess risk.…”

“…Other contaminants, such as Hg, are not known to affect aquatic insect survival in Hg contaminated areas; therefore, high insect emergence rates can move large amounts of Hg into riparian zones (Tweedy et al, 2013). Mercury is of particular concern for songbirds as it continues to be found in the environment at high concentrations across North America (Cristol & Evers, 2020), and Hg exposure negatively impacts many critical aspects of the songbird life cycle, including reproduction Jackson, Evers, Etterson, et al, 2011) and migration (Seewagen, 2018).…”

The impact of mercury on North American songbirds: effects, trends, and predictive factors

“…High body burdens of Hg may be exacerbated by migration, and we predict that the greater the distance the more chance that Hg will interact negatively with the demands of migration (Seewagen 2020). While the initial Hg body burden of a neotropical or palaeotropical migrant is important for a songbird's fitness level for completing its migration, the levels of MeHg bioavailability at the destination habitat are also important within a bird's annual cycle.…”

“…In addition, several studies have correlated avian health with MeHg exposure at non-pointsource sites (Ackerman et al 2016;Whitney and Cristol 2017a;Evers 2018). Many songbirds exhibit phenomenal feats of navigation and endurance during their migratory flights, making them particularly vulnerable to the effects of Hg during this taxing part of the life cycle (Seewagen et al 2020).…”

Hyper- and Hypo-Osmoregulatory Performance of Atlantic Salmon (Salmo salar) Smolts Infected With Pomphorhynchus tereticollis (Acanthocephala)

“…Physiological processes (e.g., metabolic and neuroendocrine pathways) drive how animals translate environmental cues into movement decisions (Goossens et al, 2020) and also mediate fitness costs of these decisions (Alerstam et al, 2003;Brownscombe et al, 2017) and consequent life-history trade-offs (Ricklefs and Wikelski, 2002). While the performance of migratory aquatic species has been clearly linked to a range of abiotic drivers such as temperature (e.g., Crossin et al, 2008;Gilbert and Tierney, 2018), oxygen limitation (e.g., Rosa and Seibel, 2010), water flow (e.g., Swanson et al, 2004), water chemistry (e.g., Borges et al, 2019), and pollution (e.g., Seewagen, 2020), the role of biotic factors in the ecophysiology of animal migration is arguably less well studied. In particular, parasites and pathogens have received relatively little attention, but are critical components of any ecosystem and are likely to exert a range of physiologically mediated influences on migration decisions and fitness outcomes (Piersma, 1997;Altizer et al, 2000;Gylfe et al, 2000;Norris and Evans, 2000;Møller and Szép, 2011).…”