North Grafton, Massachusetts – By the time the veterinarian saw the Canada goose, it was starving. Lumpy bulges ran the length of its neck, from its white chinstrap to its shrunken breast. It was too weak to squabble – so sluggish, in fact, that the veterinarian could scoop up the goose and move it to the stainless steel table without throwing a blanket over it.
A team of four rushed in to treat the goose, flushing a bucketful of sand from its esophagus. But X-rays of its digestive tract bore out another problem – tiny flecks in the sand. A blood test confirmed the veterinarian’s suspicions: lead poisoning. The goose had eaten sand laced with lead at a pond near Boston.
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It’s well-known that high levels of lead kill birds. But now it’s becoming clear that amounts commonly encountered by waterfowl and raptors can mess up their digestion, brains, hearts, vision and other body processes critical for their survival in the wild.
Fledglings exposed to low levels may wander from nests and stumble around, while their parents may be unable to maneuver around power lines or swerve out of oncoming traffic.
“It seems like a silly question to think about a bird’s IQ, but it’s not, really. If these animals are not perfect mentally, natural selection will pick them off,” said Dr. Mark Pokras, a wildlife veterinarian at the Cummings School of Veterinary Medicine at Tufts University.
No one knows how many birds die of lead poisoning and how many more are contaminated with lower doses. But some studies of scavengers such as condors and eagles have suggested that more than 90 percent have detectable lead in their blood.
Despite the emergency treatment at the Tufts clinic, the goose had to be euthanized because the damage to its digestive tract was too severe. Lead scrambles the muscle contractions that move food through the esophagus into organs that can absorb nutrients.
“The bird was literally starving to death on a full stomach,” Pokras said.
Lead Still Ubiquitous
It’s not clear where the lead that poisoned the goose might have come from. Scrapings of old house paint, fragments of lead shot or shavings of scrap metal were a few of Pokras’ guesses. Urban soils and waterways across the country are littered with lead, deposited there by centuries of human activity, that pose a health risk to people as well.
While lead has been removed from gasoline and most paints, today’s sources include hunting ammunition, fishing tackle, abandoned smelters, old bridge paints and car batteries.
“It surprises some people, because we think we’ve solved the lead problem, but it remains a serious problem in the environment,” said Joanna Burger, a biologist at Rutgers University in New Jersey.
Despite the ubiquity of lead and the constant exposures to wildlife, most of what scientists know about the way lead harms the basic functions of life comes from studying humans.
“We often think of animals as sentinels for human health, but with lead, humans have been the ‘canary in the coalmine’,” Pokras said.
Lead is one of the most-studied environmental toxicants. In ancient Rome, people were poisoned by lead pots used to store wine. But it wasn’t until centuries later that people started to link the symptoms to the source. In the 1700s, an English physician showed that severe abdominal cramps commonly experienced by cider drinkers were caused by lead leaching from the presses used to crush the apples.
In birds, the research came much later.
Scientists working on Midway Island in the 1980s described a “droop wing” in albatross that had eaten paint chips near an abandoned building. Their wings hung flaccidly by their sides. For centuries, physicians had observed a similar problem with wrist nerves in lead-poisoned people.
Researchers began to realize that many of the health problems caused by lead in the environment were not specific to humans. The gastrointestinal symptoms found in England’s early cider drinkers, for instance, are similar to the signs found in birds.
In the clinic, Pokras is performing a necropsy on the dead goose, whittling with his scalpel through stretchy, clear sheets of connective tissue that cling to skin and muscle on its breast. Pushing aside the ropey neck muscle, he points to what looks like a strand of dental floss running from the goose’s head down to the pocket of organs in its belly. It’s the vagus nerve – the pathway that carries signals from the brain to the muscles of the digestive tract to contract and move food into the stomach.
“Lead literally impacts every system of the body,” he said.
Its chemical structure is very similar to calcium, so the body confuses the neurotoxic metal for the vital nutrient. Humans, birds and just about every other living thing on the planet need calcium to send brain signals from cell to cell. Important connections are lost when lead disrupts those pathways.
noticed their chicks acting weirdly. Some would wander from the nest into neighboring gull territories, where they were killed. “It was happening at some nests and not others. I was seeing these behavioral abnormalities that I just couldn’t account for,” she said.In the late 1980s, Burger was studying herring gulls on Long Island when she
Burger collected feathers and found that some had elevated lead levels. She had a hunch that the young birds, like children, suffered from neurological problems when exposed to lead. Over the next two decades, Burger and her husband Dr. Michael Gochfeld, a human physician and lead expert at the University of Medicine and Dentistry of New Jersey, performed a series of experiments to test that hypothesis.
They found that lead-exposed chicks stumbled more when they walked, often missed the mark when pecking at their parents’ bills for food and were slower to recognize their siblings, parents and nest site in the wild or their caretaker in captivity. All of these are important learning tasks for chicks that may be killed if they wander away from the nest and approach the wrong adult.
“Almost every test we did, lead impaired them,” Burger said.
Researchers have surmised that if lead impairs cognition and neurological pathways, birds with lead in their bodies may be more likely to collide with objects they wouldn’t normally hit. In California, three endangered condors died in 2011 after hitting power lines.
“Can lead compromise a bird to the point that it collides with power lines? It’s a logical pathway, but it’s more than we can prove scientifically,” said Chris Parish, project supervisor for The Peregrine Fund’s condor field project in Arizona.
For years, scientists have been trying to understand whether lead-poisoned birds are more likely to fly into power lines. Some studies have found a link while others have not. One reason for the inconsistency may be the amount of lead in the birds’ blood.
British researchers studying mute swans found that those with moderately-elevated lead levels were more likely to suffer injuries from collisions with power lines than birds with very low lead levels or birds with very high levels. Birds with extremely high lead levels may be too weak to fly at all.
“We know what lead can do at really high levels, but we don’t really know what impacts it is having at low levels. We often get birds in for trauma and they have lead in their system, but we can only speculate,” said Dr. Julia Ponder, executive director of the Raptor Center, the University of Minnesota’s rehabilitation facility for wild birds.
For condors, the survival of the species may hinge on reducing lead exposures. In the 1980s, only 22 remained alive in the wild. Intensive conservation programs have brought the population up to around 400. In certain parts of the birds’ range – in northern Arizona and Utah – scientists have struggled to reintroduce the condor. Lead poisoning is the major culprit.
“Lead is the number one problem. It accounts for about 50 percent of the deaths in our [condor] program. Without getting lead out of the environment, we have very little hope of recovering the species in all parts of its range,” Parish said.
Unlike urban birds such as the Canada goose – whose exposure is largely an artifact of old lead sources – scavengers such as condors, vultures and eagles often are poisoned by lead shot during big game hunting seasons.
Carrion-eaters can ingest lead fragments from carcasses of deer and other big game that have been dressed in the field. One carcass can poison several birds. When lead bullets hit the animal they shatter: A Minnesota study found small lead particles can travel up to two feet from the wound site. And the danger isn’t just to birds: Lead ammo can taint meat for hunters and their families, too. Some hunters are making the switch to copper slugs and bullets.
“Lead is a very, very severe poison. It’s silly that in the 21st century we still allow this,” said John Fitzpatrick, director of the Cornell Lab of Ornithology.
Roughly 98.5 percent of the eagles admitted to Minnesota’s Raptor Center had measurable levels of lead in their blood. In Maine, about one-third of the eagles treated over the past decade at Avian Haven, a rehabilitation center, had blood lead levels above 10 micrograms per deciliter, according to executive director Diane Winn. Out of those 44 eagles, 28 died either from a lethal dose of lead or from an injury to which their lead exposures may well have contributed.
It’s not clear what amount of lead is deadly to birds. Lethal exposures seem to vary a great deal between species, and even among individuals. Geese and ducks apparently tolerate higher levels of lead than eagles or condors. Effects may start to appear at lead levels between 20 and 60 micrograms per deciliter. With treatment, prognosis for survival is generally good below 100 micrograms, according to Dr. Pat Redig, a wildlife veterinarian at the University of Minnesota. Many rehabilitators begin treatments to remove the toxic metal from the blood around 20 micrograms.
In comparison, for lead-exposed children, the Centers for Disease Control and Prevention recommends action for levels higher than five micrograms per deciliter, although it adds that “no safe level has been identified.”
Far less is known about the subtle effects of lead on the animal brain. A handful of studies have documented aggressive behaviors in dogs and cats as well as in rodents and songbirds.
“In humans we’ve observed that chronic low levels of lead exposure in early childhood may have a greater effect on cognition than higher levels of exposure, but we don’t know whether that holds true for birds,” said Dr. Bruce Lanphear, a professor at Simon Fraser University in British Columbia who specializes in children’s health.
For birds and other wildlife, the research has a long way to go to catch up.
“The veterinary literature on lead poisoning today looks a lot like the human literature of 100 years ago,” Pokras said.
Even with treatment, the road to recovery – and release – can be an uncertain one. Many birds treated at clinics and wildlife centers for lead poisoning are left with permanent brain damage.
Sometimes the birds “show signs of mental illness,” Redig said. “A healthy eagle should hiss at you, open its wings or show some kind of fear response. Many times when you approach these birds, it’s like nobody’s home.”
It may be a problem with the part of the brain that processes visual information, or it may be that the birds can see just fine but the connections in the brain that convert visual images into actions have been lost. In children, lead exposure slows decision-making abilities and reaction times and interferes with the processing of visual images.
Other birds are permanently weakened because lead interferes with heart function. “We’ll try to exercise a bird and discover it has no stamina,” Redig said.
In severe cases of lead poisoning, lesions are found in parts of the brain that control the body’s auto-pilot processes, such as heart rate and breathing. Deformities also are found in the ventricles that pump blood in the heart.
In less severe cases, the damage can be harder to spot, but the deficits are often lasting. One bald eagle was thin and dehydrated when it arrived at SOAR Raptor Rehab center in Dedham, Iowa in 2011. A blood test showed lead levels exceeding 20 micrograms per deciliter. The eagle went through chelation therapy to clear the toxic metal from its blood. Over the next couple of weeks, she gained weight steadily, but she navigated clumsily about the flight pen.
“Her vision was permanently impaired. We couldn’t release her,” said Kay Neumann, executive director at SOAR.
Although the eagle survived, Neumann counts her as a “wild mortality.” She will never return to the wilderness to hunt, nest or reproduce, and she won’t pass her genes along to the next generation of Iowa’s rebounding eagle population.
Back in Massachusetts, Pokras whisks small flecks of liver tissue from the Canada goose into a small plastic specimen jar. A pathologist will examine the tissue under a microscope for signs of any underlying diseases that could have exacerbated the effects of lead. Its absorption can be increased by conditions that stress the body.
“As veterinarians, we like to flip the paradigm and see what we can learn from the human literature,” Pokras said.
Hundreds of years of human study have guided the scientific foray into the effects of lead in wildlife. A better understanding of exposure in birds – the effects and routes – can inform strategies to reduce lead poisoning across all species, including humans. While a Canada goose doesn’t carry the conservation significance of a bald eagle or a condor, Pokras believes it is important to attempt to save them.
“We learn from them,” he said. “Each case can help give us a sense of what’s going on out there in the greater environment – and how we should respond.”