Relative risk of waterbird species on the Berkeley Pit: a mortality assessment of 18 taxa

Abstract Acute and chronic health effects of chemical contaminants to wildlife are well‐documented across laboratory and field studies. For waterbirds, impacts of acid metalliferous water (AMW) are a particular focus in which dermal exposure and ingestion have been identified to cause direct mortality and potentially reduce long‐term fitness. Acid mine waterbodies, or pit lakes, are a result of open‐pit mining and can increase exposure to metals and other contaminants for waterbirds that use them, especially during migration. The Berkeley Pit, in Butte, Montana, has unique geochemistry and its geographic location overlapping the Pacific and Central Flyways makes it a field‐based laboratory to quantify waterbird population demographics and evaluate management strategies. Using systematic waterbird observations on the Berkeley Pit over a 6‐year period (2018–2023), we calculated relative risk (RR) for 54 observed waterbird taxa exposed to AMW. Of these 54, 17 known waterbird species and 1 unknown duck (n = 18 taxa total) had at least 1 known mortality. Five species of the 18 taxa had RR ratios in which their respective 95% confidence intervals did not overlap 1, suggesting an increased (RR > 1) or decreased (RR < 1) risk of mortality associated with the Berkeley Pit relative to all other species with an observed mortality. Four species had RR ratios that were > 1 (canvasback [ Aythya valisineria ], common loon [ Gavia immer ], mallard [ Anas platyrhynchos ], and snow goose [ Anser caerulescens ]), suggesting that these species had an increased risk of mortality associated with the Berkeley Pit. One species had a RR ratio that was significantly < 1 (eared grebe [ Podiceps nigricollis ]) suggesting this species has a decreased risk of mortality associated with the Berkeley Pit relative to other species. Our data provided a comprehensive assessment of which species are at higher risk when exposed to AMW. Our results provide baseline information for effective management and conservation of species exposed to anthropogenic landscapes and associated contaminants.