Lead Levels in Wildlife
For the past two decades, biologists have been working to recover the California condor. The condor reached the brink of extinction in the 1970s, but with an intensive captive breeding program, the species has been re-introduced to the wild and the population is growing again in its native habitat. This recovery effort has now encountered a new hurdle: lead poisoning. Each year, many condors are clinically treated for lead poisoning. Condors are scavengers, meaning they don’t kill their prey, but instead eat meat from animals that they find already dead. The Peregrine Fund researchers found that carcasses of shot deer contained small lead particles, and that this food source was being utilized by endangered condors.
While work with condors continues, Craighead Beringia South, through discussions with The Peregrine Fund, began examining this issue in the broader context of scavenger communities. Craighead Beringia South has been monitoring the raven population in the Jackson Hole area for nearly eight years, continuing the long-term data set begun by John and Frank Craighead more than 50 years ago. These studies indicate the raven population in the area has more than tripled during this time. Additionally, this raven population overlaps an area of intensive big game hunting. Thus, Craighead Beringia South is in an ideal position to investigate the relationship between scavengers, their access to discarded game meat, and the presence of lead. In 2004, in a three-phase approach to investigate this relationship, Craighead Beringia South began seeking answers to questions about lead and scavengers.
Initially, we sought to document the presence of lead. As part of our 2004-05 banding program, we began taking a small amount of blood from each raven captured. These samples were tested immediately for the presence of lead. At the same time, we tested for lead in shot carcasses, and began compiling estimates of the number of gut piles – or hunter-discarded meat and tissue – in the field.
Blood lead levels that are higher than the normal baseline indicate exposure to lead within the previous two weeks. And, if our hypothesis of the scavenger-hunting connection holds true, we should see a spike in blood lead levels in the fall, the time they have the greatest opportunity to visit gut piles. That’s exactly what we found (Figure 1). Of the 220 ravens tested during a complete year, we found no lead exposure in ravens tested outside of the hunting season. The first raven exhibiting clinical levels of lead exposure was tested the opening week of hunting season; and, 56% of ravens tested during the hunting season had acute lead exposure. In addition, x-rays of shot carcasses (Figure 2) and gut piles documented multiple fragments of lead. Thus, the first phase of our project found a high correlation between the presence of lead in ravens, and the birds’ access to lead in the form of bullet fragments.
After documenting the role of gut piles as sources of lead for ravens, we expanded the investigation to document how widespread the exposure to lead is, and to investigate some of the potential effects. Specifically, we investigated more potential scavengers of gut piles and tested their blood lead levels throughout the fall and winter to determine exposure. We sought samples from bald eagles, golden eagles, wolves, bears, coyotes, and cougars. Three of the six species targeted for testing are listed as endangered or threatened species, making this information potentially critical to recovery efforts. Further, we sought to develop new, non-invasive methodologies for quantifying past lead exposure in wildlife. To date, live animals can only be tested for acute exposure (within the past two weeks); understanding and quantifying past exposure rates would greatly enhance our understanding of this issue. Past exposure in breeding adults may have strong impacts on their offspring. Similar to lead exposure in children, data suggest that lead exposure in developing birds is of more serious and immediate consequence to survival than lead exposure in adults.
While understanding the presence and effects of lead exposure in wildlife populations is very important, the system effects on natural processes is the logical and critical next step in developing action strategies. Ultimately, we will use the information from Phase I and II to map the flow of lead exposure and its impacts on natural systems. Of immediate concern are the impacts on protected systems such as Grand Teton National Park. Such areas are primarily established to preserve intact examples of our national heritage. If these systems are unnaturally impacted by environmental toxins, can they really be considered protected? And, how can we mitigate or eliminate such impacts? Additionally, what are the direct impacts on people? What is the extent of lead in wild meat obtained through hunts? What are the risks to hunters and their families?
Through all three phases of our lead levels research, our goal is to obtain a greater understanding of the natural world, human impacts on the natural world, and our role in conserving the ecological and evolutionary integrity of natural ecosystems.
Craighead, D.C. and B. Bedroisan. 2008. A Relationship Between Blood Lead Levels of Common Ravens and the Hunting Season in the Southern Yellowstone Ecosystem. Proceedings from Boise Conference. (PDF)
Craighead, D.C. and B. Bedroisan. 2007. Blood Lead Levels of Common Ravens With Access to Big-Game Offal. J. Wildlife Management, Vol. 72, pp. 240-245. (PDF)