When people in the Washington, D.C., area, hear about lead, it's typically in reference to levels in the drinking water. But lead, a heavy metal, persists in the environment in many places today because of the past use of lead-based paint and leaded gasoline.
Although these products have not been used in the United States for many years, lead can still be found at high levels in the soil of urban and suburban environments.
Lead poisoning in wildlife has been well-investigated in sport fish and waterfowl, however its effects on urban wildlife has received little attention.
Birds take up lead into their tissues through their diet and inadvertent soil consumption. The presence of lead in blood can have serious health consequences for birds, including reduced weight gain for nestlings, reduced organ growth, and a reduced ability to sustain necessary metabolic functions.
At elevated levels, lead contamination can result in nestling and adult death. As reported in the scientific literature, scientists have determined that in certain bird species negative physiological effects begin occurring at a blood lead level of just over 0.2 parts per million (ppm).
Lethal contamination levels for many bird species are unknown. Different species tolerate lead to varying degrees and research in this field is lacking. But one study reported a lethal level of 1.2 ppm for waterfowl.
Karin Roux, a graduate student from George Mason University and a former research assistant with Neighborhood Nestwatch, began studying the levels of lead in the blood of Nestwatch birds in 2003.
Her study species included the robin, catbird, cardinal, and song sparrow, all of which are found in the backyards of Nestwatch participants. She also correlated growth rates of nestlings to local soil lead levels to determine the potentially negative impacts of lead on nestling condition.
There are strong differences between urban and rural areas in soil lead concentrations (see chart below).
Soil lead content in urban and suburban backyards average nearly nine times higher than that of rural backyards—275 ppm compared to 25 ppm.
Furthermore, adults and nestlings of all species inhabiting lead-contaminated environments had much higher blood lead levels than individuals inhabiting relatively uncontaminated environments in rural areas.
Well, actual tolerance levels do not exist as yet for wild birds specifically, but the Environmental Protection Agency's "Action Level" for soil content in children's play areas is 300 ppm.
Most of our sites are below this but are close.
In addition, The Centers for Disease Control holds a blood level of 0.1 ppm as being cause for medical attention in human children. Nestling blood levels in some suburban and urban sites are above this threshold.
To date, few studies have examined the impact of lead in urban environments on adult bird survival and nestling growth rates. Karin's research on lead as part of Nestwatch has highlighted one of the many challenges birds must face in urbanized environments.
The results of this research will offer insight into how an increasingly urban world will affect bird populations and other wildlife.
This article summarizes the information in this publication:
Roux, K. and Marra, Peter P. 2007. The presence and impact of environmental lead in passerine birds along an urban to rural land-use gradient. Archives of Environmental Contamination and Toxicology, 53: 261-268.
Contamination of wetlands by lead shot and lead fishing weights has generated a tremendous amount of research into the impact of lead poisoning on wildlife. Less well known are the potential threats to wildlife posed by lead contaminants still prevalent in urban environments. Despite a U.S. federal ban on lead-based paint and gasoline in 1978 and 1986, respectively, lead residue is still prevalent at hazardous levels in urban and suburban environments and may present a health concern for people and wildlife, particularly birds. We quantified soil lead content in residential properties across a rural-to-urban land-use gradient in the metropolitan Washington, D.C. area and then assessed the impact of lead contamination on body condition in adult and nestling passerine birds at the same sites. Soil lead concentration was significantly higher in urban sites compared to rural sites. Accordingly, adult and nestling birds captured in urban sites had significantly higher blood lead concentrations than their rural counterparts. However, only gray catbird nestlings exhibited lower body condition as a result of lead contamination. Birds continue to breed in urban habitats despite numerous negative attributes to these environments including light, noise, pedestrian and toxic contaminants, such as lead. These sites often contain habitat that appears suitable for roosting, nesting, and foraging and thus may act as an ecological trap for breeding birds because breeding success is often negatively associated with increasing urbanization. Lead contamination is one more feature of urbanization that birds and other wildlife must face in an increasingly developed world.
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