Lead Exposure Makes Mockingbirds Aggressive

Analysis by Dr. Karen Shaw Becker

lead linked to aggression in mockingbirds

Story at-a-glance -

  • Researchers used mockingbirds to determine how lead contamination in soil affects urban wildlife, noting that songbirds aren’t the only wildlife exposed to lead in urban areas
  • Simulated territorial intrusion scenarios were created by placing a stuffed mockingbird near mockingbird nests in metropolitan areas of New Orleans known to have high or low levels of lead in the soil
  • In areas with low lead levels in the soil, the mockingbirds’ response was “conservative,” with increased vocalizations, wing flapping and “fly bys” meant to intimidate
  • Where lead contamination was high, the mockingbirds were much more aggressive, attacking the perceived intruder and ripping out its feathers
  • Wildlife is adversely affected by lead levels in soil, as well as in ammunition, fishing tackle and even drinking water, but so are pets and humans, which makes alternatives and heightened awareness doubly important

“Sub-lethal” is a term scientists use to describe dangerous, but not fatal, levels of lead in soil and other sources, posing a threat to wildlife. For mockingbirds used in a scientific experiment, if the potentially toxic substance doesn’t actually kill them, it certainly doesn’t make them stronger; it makes them more aggressive.

Researchers at Tulane University in New Orleans conducted a study and reported the possibility that mockingbirds aren’t the only wildlife exposed to lead in urban areas.

Their findings, published in Science of the Total Environment,1 involved “simulated territorial intrusion” by placing a stuffed mockingbird in close proximity to dozens of northern mockingbirds in metropolitan areas of New Orleans known to have high or low levels of lead in the soil.

Jordan Karubian, Ph.D., associate professor at Tulane’s Department of Ecology and Evolutionary Biology, said placing the mock-up bird within 25 feet of nests being built by pairs of the songbirds created a situation in which the birds felt threatened enough to respond territorially. The researchers also played recordings of male mockingbirds singing to make the scenario seem more real.

It worked, but it exhibited a marked difference depending on where the situations were created. In areas with low lead levels in the soil, the mockingbirds’ response was described as “conservative.” They increased vocalizations, flapped their wings and engaged in “fly bys” meant to intimidate. On the other hand:

“In high-lead neighborhoods, though, the mockingbirds responded far more aggressively, attacking the perceived intruder and even ripping out its feathers. Researchers learned quickly they had to place the fake bird in a cage to continue the study and protect it from damage.”2

Mockingbird Aggression Due to Lead Exposure the ‘Tip of the Iceberg’

According to Karubian, “There’s considerable lead contamination in soils around the world, which means literally billions of animals, both urban wildlife and pets, are likely exposed at sub-lethal levels.” While it’s not killing them, it’s evident that it can affect not just their behavior but their physiology. Increased aggression among mockingbirds may “just be the tip of the iceberg.”3

Mockingbirds were used in the research because they’re common in both urban and outlying residential areas. Their territorial behavior during breeding season is the perfect time to observe their protective behavior and compare areas where the soil is known to be more or less compromised by lead toxicity.

Dr. Kelly Diehl, interim vice president of scientific programs at the Morris Animal Foundation, believes the problem deserves much more attention because lead in soil is common, but it’s still present even though steps have been taken to remove lead content from paint and other products.

What Lead Exposure Can Do to Wildlife — and Humans

Penn State Extension notes that lead occurs naturally in soils, usually at concentrations ranging from 10 to 50 mg/kg (milligrams of lead per kilogram of soil, aka equivalent to parts of lead per million or ppm). However:

“Because of the widespread use of leaded paint before the mid-1970s, and leaded gasoline before the mid-1980s, as well as contamination from various industrial sources, urban soils often have lead concentrations much greater than normal background levels.

These concentrations frequently range from 150 mg/kg to as high as 10,000 mg/kg at the base of a home painted with lead-based paint. Lead does not biodegrade, or disappear over time, but remains in soils for thousands of years.”4

The Soil Science Society of America5 says chipped lead paint is dangerous because overexposure can lead to reduced brain function, developmental delays and altered motor skills in children, especially because they’re much quicker to absorb lead than adults. So what does that mean for animals, including pets? Experts agree it’s still a major problem, and solutions are being sought in several areas.

Hunting and fishing are a potential threat many don’t think about. Not only is lead shot a source of lead contamination for animals and birds, such as waterfowl, but also eagles, hawks and other raptors. Lead ammunition used in deer hunting is another concern because the bullets fragment into tiny pieces, says the National Park Service.6

National park officials maintain that lead poisoning is the most serious threat facing the California condor because they feed solely on dead animals, so they’re more frequently exposed to this hazard than most other wildlife. Even small amounts of lead can sicken or kill them.

But lead contamination is just as serious for humans. Sub-lethal exposure “has been linked to increased aggression, with people who grow up in high-lead neighborhoods having a higher probability to be incarcerated for violent crime. They also tend to score lower in standardized tests.”7

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What You Can Do to Help Protect Wildlife and Pets From Overexposure

The problem of lead contamination for animals is widespread. For instance, residual dust remains in soil, and earthworms and insects are eaten by birds. Experts aren’t sure what the implications are for dogs, cats and other pets.

Sinkers used in fishing may dissolve into groundwater, which poses a danger to people, animals and plants alike. They can be swallowed intact, even by animals not being hunted. Further, “Metallic lead pellets deposited onto soils and aquatic sediments are not chemically or environmentally inert, although tens or hundreds of years may be required for total breakdown and dissolution of pellets.”8

There are many ammunition and tackle alternatives available for people who hunt and fish. Nontoxic tackle could be made of tin, bismuth, steel and tungsten-nickel alloy.9 Non-lead ammunition could be made of steel, copper or bismuth.10 The California Department of Fish and Wildlife has adopted regulations requiring the use of alternative non-lead ammunition when taking any wildlife with a firearm in California. The law goes into effect on July 1, 2019,11 and other states are passing similar legislation.

Drinking water is another area pet owners should be aware of, because just like people, pets and wildlife also require water to live. Lead, arsenic, mercury and cadmium were even found in several tested pet foods, with contaminants found in nearly every one of them.12

As an example of how serious this threat is, an assistant professor at the Michigan State University College of Veterinary Medicine tested the water consumed by dogs following the Flint, Michigan crisis in 2016. He found the lead concentration in the dogs in Flint to be four times the median concentration in control populations of dogs.

Here’s the sobering part: This was after the contamination was a known factor and most people with dogs had already started providing them with water from other sources. The authors of this study concluded:

“Results highlighted the importance of maintaining awareness of lead exposure and considering both human and animal well-being in cases of environmental toxicant exposures.”13

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