Lead Contamination in Wildlife

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. 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. All exposure is 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 highly 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:

  • Gut piles or discarded carcasses (primarily from furbearers and deer)
  • Unrecovered game (crippling loss, discarded carcasses)
  • Animals shot and disposed of for wildlife management
  • Euthanized animals (primarily livestock and wounded deer from vehicle incidents)
  • Illegally shot and discarded animals

Scavenging animals, including hawks, vultures, eagles, crows, and some mammals.

Temperatures and decomposition rates, snow cover, alternative food source availability, time of year.

Lead bullets
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. 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.

Species Shot Bullet fragments Tackle
Common loon     X
Double-crested cormorant     X
Black-crowned night heron     X
Great blue-heron     X
Trumpeter swan X   X
Lesser snow goose X    
Greater white-fronted goose X    
Canada goose X   X
Mallard X   X
Green-winged teal X    
Northern shoveler X    
American wigeon X    
Gadwall X    
Black duck X    
Northern pintail X    
Wood duck X    
Common goldeneye X    
Ring-necked duck X    
Canvasback X   X
Redhead X   X
Lesser scaup     X
Greater scaup X   X
Ruddy duck X    
Common merganser     X
Bald eagle X X X
Golden eagle   X  
Northern harrier X    
Sharp-shinned hawk X    
Cooper’s hawk X    
Red-tailed hawk X    
Prairie falcon X    
Peregrine falcon X    
Turkey vulture X X  
Gray partridge X    
Ring-necked pheasant X    
Northern bobwhite X    
Ruffed grouse X    
Wild turkey X    
Sandhill crane X   X
American coot X    
Virginia rail X    
Sora X    
American woodcock X    
Herring gull X    
Mourning dove X    
Great horned owl X    
Snowy owl X X  
Long-eared owl X    
Blue-headed vireo X    
American crow   X  
Brown thrasher X    
Yellow-rumped warbler X    
Dark eyed junco X    
Brown-headed cowbird 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.

Learning more

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.