Eagles and Lead
SOAR's Eagles and Lead Research
SOAR began gathering data in 2004 on the admissions of bald eagles to wildlife rehabilitators in Iowa. Eagle data includes: date of admission, circumstance of admission, gender, age - adult/juvenile determined by plumage, county found, lead levels from blood sample or liver or bone biopsy, and if euthanized, died, or released.
SOAR has also looked for the source of the lead, as able, through x-ray, CT scan, or fluoroscope image of eagles and other raptors that show symptoms of lead, and deer carcass and packaged venison. SOAR has documented eagles and other scavengers feeding on carcasses.
Signs of toxic lead levels in raptors include ataxia (muscle weakness and an inability to control voluntary muscle movement due to central nervous system being affected), lime-green feces, vomiting, seizures, partial or full paralysis of the wings and legs, impaired vision, and death.
How do we test for lead?
Testing for lead can be done two ways; blood samples and liver or bone samples. While the bird is still alive (antemortem), blood samples can be taken and analyzed in-house using our LeadCare II diagnostic machine. Liver and bone biopsies are completed only after the bird dies (postmortem). Test results can be expressed in parts-per-million (ppm) or micrograms per deciliter (µg/dL). For reference, 0.1 ppm = 10 µg/dL.
Blood lead level test results and what they mean:
- Under 0.2 ppm (<20 µg/dL) is background and is considered normal.
- Between 0.2-0.6 ppm (20-60 µg/dL) is considered a subclinical lead level and indicates exposure to lead, but may not be lethal by itself and may or may not require treatment, based on symptoms.
- 0.6 ppm and above (>60 µg/dL) is considered a clinical lead level and most likely will chelate
Liver biopsy results and what they mean:
- Less than 2.0 ppm is background and is considered normal.
- Between 2.0 ppm and 6 ppm is considered subclinical exposure to lead.
- 6 ppm and greater is considered a clinical lead level.
Exposure to lead can cause the animal to be impaired and not be able to catch prey, avoid collisions, etc. so exposure levels are still potentially lethal indirectly.
Cumulative Bald Eagle Data 2004-2022
Nineteen years (2004-2022) of bald eagle (Haliaeetus leucocephalus) data from state and federally permitted wildlife rehabilitators in Iowa were assessed for elevated lead levels in blood or tissue samples. In total, 781 bald eagles were admitted; 720 were tested for lead exposure. Of the eagles tested 448 (62%) had background/normal (<0.2 ppm in blood or <2.0 ppm in liver samples), 68 (9%) had subclinical lead levels (>0.2 ppm in blood or >2.0 ppm in liver samples), and 204 (29%) had clinical (lethal without treatment) lead levels (>0.6 ppm or >6.0 ppm in liver samples). Overall, 38% of bald eagles admitted were lead effected.
Chart 1 These data were analyzed over time and show an increase in the number of bald eagles admitted to rehabilitation facilities and little change in the prevalence or number of lead poisoning cases.
Chart 2 The incidence or rate of lead poisoning cases shows a decrease and leveling off over time. This could be a result in the decrease in availability to scavengers of potentially lead-containing carcasses on the landscape. According to Iowa Department of Natural Resources data the number of deer harvested in Iowa has decreased from approximately 200,000 (2004) to 100,000 (2021) animals. The number of pheasants harvested has decreased from approximately 1,000,000 (2004) to 400,000 (2021) animals.
Chart 3 In an effort to determine the source of lead poisoning for bald eagles radiographs were taken of 416 eagles (2004-2022). These eagles were DOA, died, or were euthanized. Necropsy results show an ammunition source; 42% were found to have ingested shot and 58% ingested bullet or slug fragments (two of these having entire expanded bullets in their stomachs). This work also showed that eagles are still being illegally shot (7% of sample); 64% of these showed shot from a shotgun and 36% showed shrapnel from a rifle bullet.
Cumulative Bald Eagle Data 2004-2014
SOAR compiled the first 11 years of collected data and submitted a research paper to the Journal of Fish and Wildlife Management for publication. In the 11 years of data, a total of 322 bald eagles were admitted.
What did the data tell us about the lead exposure to bald eagles in Iowa?
- The data gathered by wildlife rehabilitators in Iowa on lead exposure in bald eagles are consistent with datasets from many other rehabilitation centers.
- Adult bald eagles had higher lead concentrations than immature or juvenile bald eagles, consistent with data on rehabilitating bald eagles (Cruz-Martinez et al. 2012 PDF) and data from bald eagles found dead (Franson and Russell 2014 PDF).
- Lead concentrations were highest across all age classes during the autumn (October–January), which coincides with the hunting and trapping seasons in Iowa.
- Wildlife rehabilitators routinely collect blood samples from bald eagles admitted. Prioritizing blood collection for a blood lead analysis in bald eagles can help direct care.
Bald eagles and other large raptors are not only predators, but they also are opportunistic scavengers. What does this mean? Simply, an opportunistic scavenger is not likely to ignore a large carcass laying in a field or alongside the road. Field observations of eagles confirm their behavior of seeking out and feeding from large carcasses for several days at a time.
Yes, this is an eagle feeding station -- SOAR used salvaged roadkill deer (with the necessary permit from an Iowa DNR conservation officer to pick up) and carp placed in a field in Carroll County, along with a motion-activated trail camera. The dates on the photos are correct, but the time stamp is wrong. SOAR is, of course, very careful to examine carcasses to make sure there is no chance of lead. Please do not use butcher scraps from lead-shot animals or leave animals shot with any type of lead ammunition out for predators or scavengers to eat.
Where can lead be found in the environment?
Lead is a naturally occurring metal found in the earth's crust. Lead can be found many places, much because of human activity through burning fossil fuels, mining, and manufacturing. Water can be contaminated with lead from mines, waste dumps, and industrial plants. House paint and gasoline were once manufactured with lead. Fishing sinkers and jigs are often made from lead. Most firearms ammunition contain lead including pellets, shot, slugs, round balls, and bullets.
By the mid-1990s, lead had been removed from the manufacturing process in many products in the U.S., including paint and gasoline.
How does lead poison?
Lead is a potentially deadly toxin that damages internal organs of the body and can impact all animals, including humans. For both birds and people, lead must be eaten (ingested) or lead particles or fumes inhaled to elevate lead levels to cause poisoning. Examples of how people can ingest lead include eating paint chips, inhaling paint fumes and paint dust, eating wildlife harvested with lead shot or lead slugs. Stomach acid breaks down lead and then lead is absorbed into the blood stream. Fine particles and fumes that are inhaled are absorbed into the bloodstream through the lungs. Learn more about the physiology of lead from the Agency for Toxic Substances & Disease Registry, a part of the Centers for Disease Control (CDC), in this section of a medical education course! (343 KB PDF)
Primary lead exposure in animals is caused by the animal eating lead directly, mistaking it for food or grit. Secondary lead exposure in animals comes from the animal eating another animal that contains lead. This prey animal either swallowed lead or has lead shot/slug embedded in it.
What are the effects of lead poisoning?
Lead mimics calcium in an animal's body, it impacts nervous tissue and is stored in bone. Blood lead levels (BLL) are measured in micrograms per deciliter. At a BLL below 10 micrograms per deciliter (10 µg/dL), this lead can cross the placenta and can be found in breast milk.
Young children and pregnant women absorb more lead than do adult males. In children, even a small amount of lead can cause learning delays, decreased intelligence, shortened attention span, and very high lead levels may cause brain damage or even death. Lead exposure in children may also result in expression of antisocial behaviors.
Very low lead levels increase an adult males’ risk of stroke and heart attack and can decrease sperm count.
Later in life, lead can re-emerge from bone tissue causing high blood pressure, kidney failure, and Parkinson’s Disease.
High BLL in birds (from loons, doves, cranes, swans; to vultures, eagles, crows, and other scavenging birds) impacts the nervous and circulatory systems and the kidneys. The weakened bird has trouble flying, hunting/feeding, is much more susceptible to infection, and often starves. In female birds with low BLL, reproduction is impacted.
What are the exposure thresholds for humans and wildlife?
The CDC uses a blood lead reference value of 3.5 µg/dL to identify children with blood lead levels that are higher than most children's levels in the U.S. A blood lead level of 3.5 or greater in a child is a trigger for continued monitoring by healthcare providers. This level changed in May 2021 in response to the Lead Exposure Prevention and Advisory Committee from 5 to 3.5 µg/dL. However, there are no safe levels of lead in the body. Even low levels of lead in the blood can negatively impact a child's health and viewed as a concern.
There are no established exposure levels for wildlife as it is different for each species. In clinical trials, 200 milligrams of lead (one #4 size piece of shot or the size of a grain of rice) was found to be the lethal dose for a bald eagle.
For more information about lead, visit:
- Environmental Protection Agency
- Centers for Disease Control
- Department of Health and Human Services, Agency for Toxic Substances and Disease Registry
Articles about the impacts of lead:
- Neurotoxicity and aggressiveness triggered by low-level lead in children: a review (135 KB PDF)
- Whole-Body Lifetime Occupational Lead Exposure and Risk of Parkinson’s Disease (506 KB PDF)
- Health Risks from Lead-Based Ammunition in the Environment: A Consensus Statement of Scientists (190 KB PDF)
- Health Risks from Lead-Based Ammunition in the Environment An editorial in Environmental Health Perspectives, June 2013 (107 KB PDF)