Historically, bald eagles were becoming secondarily lead poisoned by scavenging on lead poisoned or lead shot-crippled waterfowl. TRC’s research on lead poisoning in bald eagles back then along with collaborative work with the Minnesota Department of Natural Resources, built an important portion of the cumulative data that led to the passing of a 1991 Federal law banning the use of lead ammunition for waterfowl hunting.
In 1997, TRC undertook another study to evaluate if banning of lead shot for waterfowl hunting had reduced the number of lead-poisoned eagles. Surprisingly, it showed the prevalence of poisoned eagles didn’t change even with good hunter compliance. This suggested that eagles were being poisoned from another source of lead, deer gutpiles left in the field by hunters.
Therefore, TRC's conducted a 13-year (1996-2009) retrospective study of lead poisoning in bald eagles (Haliaeetus leucocephalus) to test the hypothesis that spent lead from ammunition, present in the carcasses and gutpiles of white-tailed deer, represents an important source of lead exposure.
We analyzed four epidemiological parameters: 1) seasonal prevalence and relationship with deer hunting season onset in Minnesota, Wisconsin and Iowa; 2) correlation of the animal recovery location with deer hunting zones; 3) lead isotope ratio analysis of metal fragments recovered from the gastrointestinal tract of lead-poisoned bald eagles and of whole blood; and, 4) comparison of kidney copper concentration from lead-exposed vs. non-exposed eagles.
A statistically significant seasonal and geographical association (p<0.01) was established between deer hunting season onset and hunting zones, with the incidence of eagle poisoning. The majority of cases occurred during late fall and early winter, with significantly higher number of poisoned bald eagles recovered from the deer hunting rifle zone. The lead isotope ratio analysis revealed: 1) most of the paired blood-fragment samples have a closely matched isotopic signature; and, 2) the majority of the blood and fragment samples from lead exposed eagles were 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.
The results from these four epidemiological parameters strongly support the hypothesis that spent lead from ammunition is an important source of lead exposure for bald eagles.