Lead is a toxic metal with no known physiological functions in the bodies of animals. In fact, it is known to have negative effects on most body systems. Once absorbed, usually through the gut after ingestion, it passes into the blood stream and then into soft tissues, such as kidneys and liver, and into bone. Although the levels in the soft tissues may decrease over time, lead poisoning remains in the bones of an animal for its entire lifetime.
Humans have a long history of lead use but, because of the known risks posed by this metal to human health, most of its uses are now strictly regulated in Europe and most other developed countries. However, this is not true of the use of lead in the manufacture of ammunition (shotgun pellets, slugs and rifle bullets). And although it has been shown that lead poisoning contributed to the population declines of iconic birds such as the American bald eagle and the Californian condor, bans on the use of lead ammunition have met with little success in Europe.
Birds that scavenge carrion, such as vultures, may ingest lead pellets and fragments of ammunition in the flesh of animals that have been shot by hunters and then lost or left behind in the field or forest. Birds of prey (raptors) may catch live prey that have previously been shot by hunters but have survived. This prey may have lead pellets embedded in their flesh that have not killed them. Raptors will also scavenge when the opportunity arises and may ingest lead ammunition in this way as well, which results in poisoning.
Until recently, there was not enough data on concentrations of lead in the tissues of raptors in Europe to assess the impacts of lead poisoning on populations of these birds. However, a recent review of the topic has stimulated more interest. Although the review study documented tissue lead concentrations, it did not attempt to assess the numbers of raptors killed by lead poisoning throughout Europe.
A new study by scientists from the University of Cambridge, UK, has now compiled updated information on the levels of lead poisoning in 22 raptor species across Europe and used these data to estimate the impacts on populations of some of these species. The experts hypothesized that variation in lead poisoning between species and between countries is related to the intensity of hunting activities.
The researchers used measurements of lead concentration in the livers of over 3,000 raptors found dead or dying in the wild in 13 countries. This data had been collected by scientists in the various countries since the 1970s. They considered birds to have died from lead poisoning if they had 20 ppm (dry weight), or more, of lead in their tissues.
Their results, published today in the journal Science of The Total Environment, showed that 10 species of raptors are particularly affected by lead poisoning. The worst affected are species like eagles that are naturally long-lived, rear few young per year and breed later in life.
However, even populations of species familiar to bird-watchers across the different countries, such as the common buzzard and red kite, were affected. The researchers estimate that the overall European populations of these 10 raptor species are at least six percent smaller than they should be, due solely to the effects of lead poisoning from gun ammunition.
Europe’s white-tailed eagle population is estimated to be 14 percent smaller than it would have been in the absence of more than a century of exposure to lethal levels of lead in some of its food. Golden eagle and griffon vulture populations are 13 and 12 percent smaller, respectively, and numbers of northern goshawks are six percent lower than they should be. Red kites and western marsh harriers were estimated to have populations with three percent fewer individuals than they should have.
And even though common buzzard populations are only 1.5 percent smaller than they would be in the absence of lead poisoning, this still equates to almost 22,000 fewer adults of this widespread species, say the researchers.
A range of alternatives to lead shot and rifle bullets are widely available to hunters and some countries have had great success legislating for the use of these alternatives in hunting. Currently, only two European nations – Denmark and the Netherlands – have banned lead shot. Denmark plans to follow this up with a ban on lead rifle bullets. Both the European Union and the UK are considering legal bans on all lead ammunition due to its effects on wildlife and the health of human consumers of game meat. However, many hunting groups oppose this change, according to researchers.
The same authors published results of a previous study just last month which showed that over 99 percent of pheasants killed in the UK are still shot with lead, despite hunting groups urging members to switch to non-toxic gunshot in 2020. In addition, X-ray studies of wild ducks in the UK have shown that about a quarter of live birds currently carry lead shot in their bodies.
“The continued blanket use of lead ammunition means that hunting as a pastime simply cannot be considered sustainable unless things change,” said study lead author and conservation scientist Professor Rhys Green. “Unfortunately, efforts to encourage voluntary shifts away from lead shot have been completely ineffective so far,” he said.
“The kinds of reductions in raptor populations suggested by our study would be considered worthy of strong action, including legislation, if caused by habitat destruction or deliberate poisoning.”
“It’s taken decades for researchers from across Europe to amass sufficient data to enable us to calculate the impacts of lead poisoning on raptor populations,” explained study co-author Professor Debbie Pain. “We can now see just how substantial population impacts can be for some of our most charismatic and vulnerable species – species that are protected by EU Regulation and the UK Wildlife & Countryside Act.”
“The avoidable suffering and death of numerous individual raptors from lead poisoning should be sufficient to require the use of non-toxic alternatives. These population-level impacts make this both doubly important and urgent,” she emphasized.
The researchers also investigated the association between the number of lead poisonings in each country and the “hunter density. ” This was taken to be the average numbers of hunters per square kilometer in each country, using data from the European Federation for Hunting and Conservation. Not surprisingly, the incidences of lead poisoning were positively correlated with the density of hunters. In addition, countries with no hunters using lead ammunition had virtually no lead-poisoned raptors.
In conclusion, the researchers stress the importance of introducing and enforcing laws to ban the hunting of animals using lead ammunition. They expect that the populations of bird species currently most impacted by lead poisoning would be substantially larger if their exposure to dietary lead could be reduced to a low level.