Evidence of negative seasonal carry‐over effects of breeding ground mercury exposure on survival of migratory songbirds

Abstract

Mercury (Hg) is a well‐known global contaminant that persists in the environment. The organic form, methylmercury (MeHg) has been shown to adversely affect bird immune function, foraging behavior, navigation, and flight ability, which individually or together could reduce migration performance, and ultimately survival. Nestlings grow feathers at their natal site, and in North America many adult passerines undergo a complete feather molt prior to autumn migration at or near their breeding location. Body Hg is redistributed into growing feathers, and remains stable following feather growth. As flight feathers are retained in most species over the non‐breeding season until molt in the following summer, tail feathers can be used at other times and places as indicators of Hg body burden on the breeding grounds. In five migratory passerine species, we compared Hg concentrations in tail feathers that were grown prior to autumn migration and retained until the following spring. We predicted that we would observe a shift in the distribution of species‐specific feather Hg values towards lower means in the spring if Hg reduced survival over the migration and winter periods. We found reductions in mean feather Hg between autumn and spring in two long‐distance migratory insectivores (blackpoll warbler Setophaga striata; American redstart Setophaga ruticilla). Most significantly, spring‐returning blackpoll warblers, a species that undertakes long non‐stop flights to South America during autumn migration, had nearly 50 percent lower Hg concentrations than those that departed in the autumn. Our finding suggests that Hg exposure on the breeding areas could have a carry‐over effect to influence migration success and survival of insectivorous songbirds that undergo extensive and demanding migratory journeys. More investigation is needed to fully understand the relationships among Hg exposure, migration performance, and survival of songbirds.

Mercury toxicity risk and corticosterone levels across the breeding range of the Yellow-breasted Chat

“…This trend has been reported elsewhere for other songbirds (Keller et al 2014;Ma et al 2018). One purported explanation for this phenomenon is that nestling birds have high energetic demands, especially for protein-rich foods, and, therefore, may be consuming higher volumes of insects and spiders, compared to older birds who may be eating proportionally more plant-based foods (Warner et al 2012;Ma et al 2018). This is likely true for chats because nestling chats are fed predominantly adult and larval insects (Schadd and Ritchison 1998) and occasionally berries (McKibbin and Bishop 2008) while adult chats consume roughly equal proportions of invertebrates and fruit (Howell 1932;Eckerle and Thompson 2020).…”

“…Hg levels in chat feathers were relatively low, 0.30 ± 0.02 μg/g dw. The average level of Hg in tail feathers for chats in our study is slightly lower than the 0.4 – 0.7 μg/g dw averages reported for other songbirds in terrestrial habitats without a known point source of contamination (Óvári et al 2018 ; Ma et al 2018 ; Stenhouse et al 2020 ). The two chats that fell within the low-risk category for blood equivalent levels (0.2–1.0 μg/g ww) were at the low end of this range with equivalent blood Hg levels of less than 0.3 μg/g ww.…”

“…Hg levels in chat feathers were relatively low, 0.30 ± 0.02 μg/g dw. The average level of Hg in tail feathers for chats in our study is slightly lower than the 0.4 -0.7 μg/g dw averages reported for other songbirds in terrestrial habitats without a known point source of contamination (Óvári et al 2018;Ma et al 2018;Stenhouse et al 2020).…”

“…Early developmental exposure to Hg can affect survival, reproduction, behaviour, and neuroanatomical development (Yu et al 2017 ; Paris et al 2018 ; Heddle et al 2020 ). Moreover, Hg may negatively affect migration and overwinter survival, further contributing to the decline of migratory songbirds (Ma et al 2018 ; Seewagen 2020 ).…”

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.…”