For the past 40 years lead exposure and lead poisoning have been major health issues for bald eagles received by or admitted to our clinic.
The statistics are alarming:
- 90% of the bald eagles received each year (120-130) for all types of problems have elevated lead residues in their blood
- 20–25% percent of these eagles have sufficiently high levels to cause clinical lead poisoning. Most of these birds die or are euthanized.
- In the last 24 years, over 500 eagles received or admitted to our clinic have either died or had to be euthanized due to lead poisoning.
- Data on location of origin and seasonal timing of lead poisoning events in eagles clearly indicate that spent lead ammunition from both shotguns and rifles is the source of lead exposure.
Banning of lead ammunition for waterfowl hunting not enough
Our research on lead exposure and lead poisoning in eagles began 40 years ago. In the 1970s and 1980s, when bald eagles (Haliaeetus leucocephalus) were at their population lows from DDT poisoning, lead poisoning in eagles appeared to be related to ingestion of lead-poisoned waterfowl. Our research on lead poisoning in bald eagles at that time, along with our collaborative work with the Minnesota Department of Natural Resources, built much of the cumulative data that led to the passing of a 1986 federal law that banned the use of lead ammunition for waterfowl hunting.
Nearly a decade later in 1997, we undertook a major analysis of eagle admissions to determine whether banning lead shot for waterfowl hunting had reduced the number of lead-poisoned eagles (Kramer and Redig 1977). Surprisingly, that study showed:
- The prevalence of poisoned eagles didn’t change even though there was documented evidence of good hunter compliance with the ban.
- This suggested that eagles were being poisoned from another source of lead.
- Further analysis of the seasonality of the major influxes of lead-poisoned eagles revealed notable spikes in admission beginning in mid-November and continuing through winter.
- This pattern led to the hypothesis that wounded deer and deer gut piles left in the field by hunters could be the source of lead; eagles are known to scavenge heavily on deer remains in winter.
Study points to spent lead from deer hunting as primary source of lead poisoning
We then conducted another 13-year (1996-2009) retrospective study of lead poisoning in bald eagles to test the hypothesis that spent lead from ammunition in the carcasses and gut piles of white-tailed deer is an important source of lead exposure. In this study, we analyzed four epidemiological parameters:
- Seasonal prevalence and relationship with the onset of deer hunting season in Minnesota, Wisconsin, and Iowa
- Correlation of animal recovery locations within deer-hunting zones
- Lead isotope ratio analysis of metal fragments recovered from the gastrointestinal tract of lead-poisoned bald eagles and lead levels in their blood
- Comparison of kidney copper concentration in lead-exposed and non-exposed eagles
The results of our 13-year retrospective study showed:
- A statistically significant seasonal and geographical association (p<0.01) between the incidence of eagle poisoning and the onset of deer hunting season and hunting zones was present.
- A majority of cases occurred during late fall and early winter, with a significantly higher number of poisoned bald eagles recovered from the deer hunting rifle zone
- Most of the paired blood-fragment samples have a closely matched isotopic signature, thereby demonstrating that ingested lead was the source of lead found in the bloodstream.
- The majority of the blood and fragment samples collected from lead-exposed eagles was within the isotope ratio from ammunition samples reported by Church et al. (2006).
- The kidney copper concentration was significantly higher in lead-exposed eagles (p=0.002), implying the ingestion of fragments from copper-jacketed lead bullets.
- About 75% of the lead-poisoned eagles were adult, or breeding-age, birds.
While there is conclusive evidence that spent ammunition in deer remains is a significant if not primary source of toxicity, there is direct evidence also that some cases of poisoning are due to shotgun pellets that may be embedded in small, upland game such as pheasants, squirrels, and rabbits that may be wounded and subsequently consumed by eagles. There is also anecdotal evidence that lead residues left in the carcasses of coyotes may be yet another source.
These results strongly support the hypothesis that spent lead from ammunition is an important, if not the primary, source of lead exposure for bald eagles. Armed with these findings, we have partnered with The Wildlife Society to promote education among deer hunters and encourage voluntary adoption of non-toxic copper ammunition. The Minnesota Deer Hunter’s Association has been receptive to this approach and has published several articles on the subject in Whitetails magazine.
While there is growing understanding in the deer-hunting community as well as many anecdotal reports from hunters who have switched to non-toxic ammunition due to this five-year educational effort, the overall admission rate of lead poisoned eagles is unchanged.
Much more work is needed to effectively remove this toxin from the food source of eagles.
Cruz-Martinez, L, PT Redig, and J Deen. 2012. Lead from spent ammunition: a source of exposure and poisoning in bald eagles. Human-Wildlife Interactions 6(1): 94-104.
Kramer, J. and P T Redig. 1997. Sixteen years of lead poisoning in eagles, 1980-1995: An epizootiologic view. J. Raptor Research 31(4):327-332.
Church, ME, R Gwiazda, RW Risebrough et al. 2006. Ammunition is the principal source of lead accumulated by California condors re-introduced to the wild. Environ. Sci. Technol. 40(19):6143-50.