The impacts of lead contamination on wildlife have been documented since the 1800's. Traditionally, the most recognized exposure pathways for wildlife was through the ingestion of spent lead shot pellets in wetlands where hunters had discharged high volumes of shot. However, today, wildlife biologists understand a number of exposure pathways in which wildlife can ingest lead and become sick, and in many cases die and are working with hunters and anglers to reduce these unnatural mortalities through use of non-lead alternatives.
Finding lead in the environment
Wildlife do not encounter lead from natural sources in the wild and rather gain exposure only from what remains from human activities. This exposure can come from many sources, including paint from old buildings, mine tailings, industrial or domestic uses of lead like wheel weights, and contaminated sediment or water. However, the general consensus among wildlife biologists is that the primary sources of lead contamination in wildlife in Iowa and throughout the upper Midwest is through direct consumption of lead from spent ammunition or discarded fishing tackle.
Lead is not known to biomagnify, meaning consumption of plants or animals that have absorbed lead is not thought to be an exposure pathway affecting wildlife. Rather, lead contamination results from direct exposure to lead, almost exclusively occurring when an animal consumes it, either accidentally in association with other food items or intentionally after mistaking it for a natural substance.
Table 1. Common sources of lead consumption among wildlife.
|Source||Vulnerable species or groups||Intervening factors|
Lost fishing tackle
Waterbirds including loons, ducks, geese, and swans
Water depth, water chemistry, vegetation, movement of sediments to bury lead.
Spent shot pellets on the ground
Ground-foraging birds, including mourning doves
Most problematic in areas of high concentrated shooting like hunting preserves, managed hunting fields, or trap ranges.
Spent shot pellets in water
Waterbirds, namely ducks, geese, and swans
Water depth, water chemistry, vegetation, movement of sediments to bury lead.
Shot pellets in unrecovered game (crippling loss, wounding loss)
Predatory animals (hawks, owls, eagles)
Concentration of prey (e.g., historically around high waterfowl harvest areas pre-1991 ban on lead ammunition in waterfowl hunting).
Bullet fragments in:
Scavenging animals including hawks, vultures, eagles, crows, and some mammals.
Temperatures and decomposition rates, snow cover, alternative food source availability, timing of year.
Lead bullets, like the .50 caliber muzzleloader bullet pictured here at left, fragment upon impact, leaving behind small pieces that can injure or kill wildlife that eat them. Non-toxic bullets, like the copper one pictured here at right, are a safe and effective alternative. Copper casing around lead bullets, as shown to the left, do not reduce the rates of lead fragmentation on impact.
Unique threats to birds
Most research on wildlife and lead contamination has been conducted on birds, which are uniquely vulnerable to lead contamination because of morphological adaptions they have for digestion. Most species, including waterfowl (ducks, geese, and swans) and upland game birds (including pheasants, quail, and doves) have muscular gizzards which create unique opportunities for contamination in two ways. First they naturally consume hard substances (namely small rocks) for ‘grit’ which aids in digestion, and many species have been documented to consume spent shot or fishing tackle from fields or from water bodies, presumably mistaking the pellets for suitable grit. Once ingested, these species are further vulnerable to contamination from ingested lead because rather than swiftly passing ingested lead as mammals would, birds can retain hard substances in their gizzard for periods of multiple days to weeks, leading to chronic absorption of lead from a single exposure event. More carnivorous birds do not have a well-developed muscular gizzard, but some species, including Bald Eagles, are thought to be uniquely vulnerable to lead contamination because of the highly acidic nature of their stomach, which breaks down lead and makes them more vulnerable to exposure, despite possibly passing or egesting lead quicker than other birds. These physiological characteristics of birds along with their foraging habits make birds uniquely vulnerable to lead contamination.
Table 2: Midwestern birds documented to have been poisoned by lead ammunition or fishing tackle.
|Black-crowned night heron||X|
|Lesser snow goose||X|
|Greater white-fronted goose||X|
|Great horned owl||X|
|Dark eyed junco||X|
Note: This list may not be comprehensive and is based on published research findings. Some species are routinely found to have ingested lead whereas others may represent rare documented instances. If you are aware of omissions from this table from published literature, please contact us to make the amendment.
Learn more about efforts to encourage hunters and anglers to voluntarily switch to non-lead ammunition and tackle through Blank Park Zoo’s collaborative LEADing The Way Campaign.
Technical resources to help hunters make the switch to non-lead ammunition can be found at HuntingWithNon-lead.org
Learn more about Lead Poisoning in Wild Birds with this article from the National Wildlife Health Center
For a more technical overview of the science of lead contamination in wildlife download the Technical Review on Sources and Implications of Lead Ammunition and Fishing Tackle on Natural Resources by The Wildlife Society.
Article updated July 2018
Sources used for the article
Bellrose, F. C. 1959. Lead poisoning as a mortality factor in waterfowl populations. Illinois Natural History Survey Bulletin 27:235-288.
Fisher, I. J., D. J. Pain, and V. G. Thomas. 2006. A review of lead poisoning from ammunition sources in terrestrial birds. Biological Conservation 131:421-432.
Franson, J. C., S. P. Hansen, T. E. Creekmore, C. J. Brand, D. C. Evers, A. E. Duerr, and S. DeStefano. 2003. Lead fishing weights and other fishing tackle in selected waterbirds. Waterbirds 26:345-352.
Friend, M. and J. C. Franson. 1999. Field manual of wildlife diseases. General field procedures and diseases of birds (No. ITR-1999-001). Geological Survey Madison Wi Biological Resources Div. Chapter 43: Lead
Golden, N. H., S. E. Warner, and M. J. Coffey. 2016. A review and assessment of spent lead ammunition and its exposure and effects to scavenging birds in the United States. Pages 123-191 in W. P. de Voogt, editor. Reviews of Environmental Contamination and Toxicology Volume 237. Springer International Publishing, Cham.
Haig, S. M., J. D'Elia, C. Eagles-Smith, J. M. Fair, J. Gervais, G. Herring, J. W. Rivers, and J. H. Schulz. 2014. The persistent problem of lead poisoning in birds from ammunition and fishing tackle. The Condor 116:408-428.
Lindblom, R. A., L. M. Reichart, B. A. Mandernack, M. Solensky, C. W. Schoenebeck, and P. T. Redig. 2017. Influence of snowfall on blood lead levels of free-flying bald eagles (Haliaeetus leucocephalus) in the Upper Mississippi River Valley. Journal of Wildlife Diseases 53:816-823.
Pattee, O.H. and Pain, D.J., 2003. Lead in the environment. Handbook of ecotoxicology, 2, pp.373-399. (link to online copy)
Perroy, R. L., C. S. Belby, and C. J. Mertens. 2014. Mapping and modeling three dimensional lead contamination in the wetland sediments of a former trap-shooting range. Science of the Total Environment 487:72-81.
Reiter-Marolf, W. J., S. J. Dinsmore, and J. A. Blanchong. 2017. Environmental contaminants in excrement of Iowa's nesting and wintering bald eagles (Haliaeetus leucocephalus). The Wilson Journal of Ornithology 129:148-154.
Runia, T. J., and A. J. Solem. 2016. Spent lead shot availability and ingestion by ring‐necked pheasants in South Dakota. Wildlife Society Bulletin 40:477-486.
Schulz, J. H., J. J. Millspaugh, B. E. Washburn, G. R. Wester, J. T. Lanigan, and J. C. Franson. 2002. Spent-shot availability and ingestion on areas managed for mourning doves. Wildlife Society Bulletin 30:112-120.
Warner, S. E., E. E. Britton, D. N. Becker, and M. J. Coffey. 2014. Bald eagle lead exposure in the Upper Midwest. Journal of Fish and Wildlife Management 5:208-216.
Yaw, T., K. Neumann, L. Bernard, J. Cancilla, T. Evans, A. Martin-Schwarze, and B. Zaffarano. 2017. Lead poisoning in bald eagles admitted to wildlife rehabilitation facilities in Iowa, 2004-2014. Journal of Fish and Wildlife Management 8:465-473.