Lead Contamination of mid-Atlantic Songbirds
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.
Lead in Songbirds
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.
What did Karin find?
There are strong differences between urban and rural areas in soil lead concentrations (see chart below).
Soil Lead Content (parts per million)
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.
How do these data compare to accepted tolerance levels of lead in the environment?
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 scientific paper:
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.
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