Author Archives: laycrose

The Lead Stops Here

The use of lead shot for hunting and sport shooting has caused major problems for upland foraging birds (Raptors). Raptors are predatory birds that use their feet and talons to catch and kill their prey. Recently, I started volunteering at the Rocky Mountain Raptor Program (RMRP), which is a non-profit organization that provides medical care and rehabilitation to these birds. In a two month period I witnessed three Raptors who were brought to our facility suffering from lead poisoning, of which only one survived and was able to be released after undergoing extensive chelation treatment. After seeing the effect of lead first hand, I wanted to know more about how these birds were becoming poisoned and how can we prevent this from happening in the future.

Animals can be exposed to lead in many ways: mines, smelting plants, harbors, industrial areas, urban areas and leaded petrol combustion [3]. Hunting with lead shot is the primary cause of lead toxicity in raptors due to their foraging behavior. Raptors are obligate carnivores who are able to get all of their water and nutrients from meat. They also exhibit generalist and opportunistic feeding behavior so when they see a gut pile left behind by a hunter they will take full advantage of this tasty treat. This type of foraging behavior causes raptors to inevitably ingest lead that is left behind in gut piles, lost or un-retrieved game, and when capturing wounded animals who have shot embedded in their tissues [2]. Once in the system the lead is rapidly dissolved by the low pH of the stomach and absorbed into the bloodstream [4]. Lead accumulates in the kidneys, liver, brain and most of all in the bones due to its long biological half-life. Lead also has the ability to disguise itself as other metals and be taken up into the cells more readily. Lead disrupts important biological processes such as DNA transcription and enzymatic pathways [1]. High accumulation of lead in  bone is a result of exposure to low concentrations over long periods of time, which can cause mineralization of the bones in adult birds [5].

Symptoms of lead poisoning can be seen immediately in raptors and include distension of the proventriculus, green watery feces, weight loss, anemia and drooping posture [4]. It can also affect the nervous system and become detrimental to biochemical functions causing blindness, depression of certain blood enzymes, reduced reproduction, and impaired immune function [4]. Furthermore, it can result in behavioral changes that make raptors prone to predation, which results in higher mortality rates [4]. This is of direct conservation concern because many raptors are long-lived, slow breeding species with small populations like the California condor. In a study done by Clark and Scheuhammer on lead poisoning in upland foraging birds in Canada, four deaths of reintroduced California condors were directly attributed to leftover lead shot in deer carcasses [3].

Size, age, and proximity to human activity have monumental influences on the distribution of lead in a bird’s tissue. The bigger the bird the more energy it needs to take in and, therefore, they ingest large amounts of meat, which gives them a higher probability of ingesting lead than smaller raptors [3]. Figure 1 on the left is taken from a study done on lead exposure in four different raptor species from Spain [3]. It shows that lead concentrations have a direct correlation to the size of the bird, which are listed as follows from largest to smallest; Eagle Owl > Buzzard > Kestrel > Little Owl. The reason for the low concentrations found in the buzzard compared to its size, as opposed to the smaller two raptors, can be attributed to the fact that the European buzzard inhabits places with low human activity.

Hunting season also plays a factor in lead exposure in large birds of prey [1]. Human activity is the main source of environmental lead and is why these birds are becoming poisoned; the closer raptors are to human activity (hunting, sport shooting, etc.) the higher the lead concentrations found in the bloodstream [4]. With the human population growing at an exponential rate, small towns will expand into urban and industrial areas. This will increase human contact with wildlife populations which, in turn will increase raptor exposure to lead shot.

Lead has been shown to cause long term detrimental effects on humans and mammals [5]. It is a highly toxic heavy metal that can be released into the environment in many ways. Acute exposure (high concentrations over short time) in vertebrates can lead to death, while chronic exposure can affect reproductive success, behavior, physiological and immune responses [5]. Lead may be a cheaper alternative for ammunition but the effects it has on wildlife is very expensive. It’s not just raptors that are of concern here, other species of birds have also been known to suffer from lead poisoning due to lead shot left behind from sport shooting or hunting. These species of birds mistakenly pick up lead pellets along with grit for their gizzards. This has been seen in species like whooping cranes, sandhill cranes, Virginia rails and even a wild turkey [4]. In 1991, the use of lead shot on waterfowl was made illegal in the United States. Since then, alternative forms of non-toxic shot have been introduced and approved for waterfowl that can also be used on upland game [4].

The most obvious solution for this growing problem would be to ban the use of lead bullets and replace them with a form of non-toxic pellets. We could also follow the lead of Britain, Australia, Sweden, Denmark, Norway, Finland and the Netherlands in establishing restrictions on the use of lead for hunting. Lead free zones can be established in places like breeding grounds and migratory rest stops that are important to wildlife. All of these alternatives are feasible and can be implemented with very little repercussion to us. For the future of our raptors and other bird species, something needs to be done soon before it is too late.

Works Cited:

[1] Clark, A. J., and A. M. Scherhammer. “Lead Poisoning in Upland-foraging Birds of Prey in Canada.” Ecotoxicology 4th ser. 12.1 (2003): 23-30. Web of Science. Web. 20 Mar. 2015.

[2] Custer, Thomas W., J. Christian Franson, and Oliver H. Pattee. “Tissue Lead Distribution and Hematologic Effects In American Kestrels (Falco Sparverius L.) Fed Biologically Incorporated Lead.” Journal of Wildlife Diseases 20.1 (1984): 39-43. Web of Science. Web. 20 Mar. 2015.

[3] Fisher, Ian J., Deborah J. Pain, and Vernon G. Thomas. “A Review of Lead Poisoning from Ammunition Sources in Terrestrial Birds.” Biological Conservation 131.3 (2006): 421-32. Web of Science. Web. 20 Mar. 2015.

[4] Gangoso, Laura, Pedro Álvarez-Lloret, Alejandro. A.B. Rodríguez-Navarro, Rafael Mateo, Fernando Hiraldo, and José Antonio Donázar. “Long-term Effects of Lead Poisoning on Bone Mineralization in Vultures Exposed to Ammunition Sources.” Environmental Pollution 157.2 (2009): 569-74. Web of Science. Web. 20 Mar. 2015.

[5] García-Fernández, A. J., M. Motas-Guzmán, I. Navas, P. María-Mojica, A. Luna, and J. A. Sánchez-García. “Environmental Exposure and Distribution of Lead in Four Species of Raptors in Southeastern Spain.” Archives of Environmental Contamination and Toxicology 33.1 (1997): 76-82. Web of Science. Web. 20 Mar. 2015.