Lead Exposure and the Effects on Scavenging Birds

California condor.
Photo credit: Scott Flaherty/USFWS

When avian predators and scavengers consume the remains of big game in the field or animals that were shot with a lead bullet and not retrieved, the bullet or its fragments may be ingested and can result in lead poisoning.  A bird with lead poisoning could die simply from the toxicity of the lead, or they could exhibit physical and behavioral changes, including muscle loss leading to lack of balance, anemia, gasping, tremors, and impaired ability to fly. A weakened bird is more vulnerable to predators, or may have trouble feeding, mating, nesting, and caring for its young.

California condors, bald eagles, golden eagles, turkey vultures and black vultures are scavenging birds that may be particularly vulnerable to the exposure and effects of lead from ammunition due to their foraging strategies and food preferences, physiological processes that facilitate the absorption of lead, and demographic traits.  To better understand the potential effects to these species and assess the existing science on this issue, we, the U.S. Fish and Wildlife Service, conducted a critical review of the published literature to evaluate three objectives:

Turkey vultures.
Photo credit: J.N. Stuart, Creative Commons

• To ascertain the contribution of ammunition as a source of lead exposure and its contribution to effects, including mortality, to scavenging birds.
• To examine whether there are viable pathways of exposure for scavenging birds to other environmental sources of lead.
• To assess the toxicity to birds of alternate metals used in ammunition.

After examining multiple lines of evidence, scientists concluded that lead ammunition is the most frequent cause of lead exposure and poisoning in scavenging birds in the United States. The suite of evidence to support this conclusion includes:

• the extent of lead ammunition currently used in hunting and its tendency  to fragment,
• the behavioral ecology and physiology of scavenging birds,
• scavenging birds’ susceptibility to lead as exhibited in controlled dosing studies,
• the diagnosis of lead poisoning in birds by well-established tissue thresholds and clinical signs,
• the recovery of ingested lead fragments or pellets from exposed birds,
• observations of birds feeding on contaminated carcasses,
• isotopic analyses relating tissue concentrations to ammunition,
• patterns of mortality coincident with hunting seasons, and
• the lack of substantial evidence for other sources of lead.

While the Service will not, as of now, be using this information to create any new policies, we did review the toxicity of alternate metals that can replace lead, and found that they are currently available and present limited environmental threats.

The full report appears in the peer-reviewed journal Reviews of Environmental Toxicology and Chemistry.

This article was first published by the US Fish and Wildlife Services 4 January 2016