Effects of lead on wildlife and wildfowl

Lead is toxic to all life with no safe threshold for exposure. It is a general toxin that affects virtually all systems in the body such as the nervous and reproductive systems1.

Can lead be dangerous to wildlife?
Mallard duckYes. When any bird or mammal ingests spent lead ammunition by mistaking it for grit or foodstuffs, or by scavenging unretrieved shot quarry, it can result in lead poisoning and death2. In addition, animals that are shot but not killed may carry lead shot in their bodies and this adversely affects their wellbeing3.

Are there also sub-lethal effects?
Yes. Although these are difficult to measure there is increasing evidence of welfare impacts and behavioural change, including in reproduction, predator avoidance, foraging ability and in avian flight4-8.

What species are susceptible to lead poisoning?
Effects of lead poisoning have been documented extensively in wildfowl9-14, and also in terrestrial birds including game and predatory species15-18. In some species present in the UK – namely mallards, whooper swans and golden eagles – recent studies elsewhere have shown effects at lower blood concentrations than previously reported19-21.

Does lead shot have an impact on the wider environment?
Lead is recognised as a highly toxic substance to both humans and wildlife. Because of this, it is near-inevitable that it will have a negative impact where it occurs22.

Shooting sports in the UK release 5,000-6,000 tonnes of lead ammunition into the environment every year. Some 2,040 tonnes of this ammunition is released into the rural environment22,23. There are no precise estimates of exactly where this lead shot is dispersed, this is because there are no official estimates of the number of animals shot, cartridges fired, or shoot participants23.

In addition to wildfowl, other animal groups are affected by lead shot poisoning. Raptors and scavengers can become poisoned when they consume carcasses or live prey that contain lead shot, and lead shot may also present a risk to foraging game birds1,22.

There is as yet little evidence of the impact of lead on other species of wildlife22. There is also not much knowledge about how lead shot interacts with the environment as it degrades. But it is widely accepted that the effect of lead increases with the dose. Some research shows that lead can have varying effects on over 60 non-wildfowl species, including passerine birds, mammals, and amphibians. There is also some evidence that lead can be absorbed by plants and soil micro-organisms22.

There are several routes in which lead can reach wildlife and cause poisoning, including22:

  • Direct ingestion of spent lead shot from the environment
  • Indirect ingestion of spent lead shot by predators/scavengers in the bodies of their prey
  • Movement of spent lead shot via plants
  • Movement of spent lead shot via soil organisms/invertebrates
  • Movement of spent lead shot from embedded shot/bullets into body tissues

Is there any evidence that lead shot exposure is having an impact on game birds?
A GWCT study published in 2005 found that 4.5% of discovered dead birds contained lead shot in their gizzards and estimated that 1.2% of living wild grey partridges contained ingested lead shot at any one time16. Other UK studies report similar findings in pheasants15 and red-legged partridge24 but do not record impacts on bird health and welfare. A Canadian study found elevated levels of lead in American woodcock that were traced back to lead shot ingestion25.

Is there any evidence that lead shot exposure is having an impact on other terrestrial wildlife?
Yes. Although little evidence is available from the UK, an increasing number of studies worldwide have shown that predatory birds suffer from lead poisoning through ingestion of spent lead ammunition while scavenging carcasses of unretrieved quarry or discarded offal (“grallochs”)26-29. There is also some evidence that suggests passerine birds, mammals, and amphibians may be affected22.

The most famous example is that of the California Condor, which was driven to the brink of extinction by lead shot poisoning. It was saved by captive breeding and reintroduction to the wild combined with a ban on the use of lead ammunition, initially across the reintroduction zone and since 2019 throughout California30.

How big is the problem?
At present, there are only formal estimates of the impact of lead ammunition on wildfowl, some birds of prey, and gamebirds such as grey partridge22,31.

Is this likely to result in population-level effects in any species?
Computer modelling of bird populations and correlative studies suggest that lead poisoning may be affecting population growth rates and sizes in a number of bird species including grey partridges31, red kites31 and both dabbling and diving ducks32 in the UK, and common buzzards in Germany31.

How long does lead remain available in the environment for ingestion by wildlife?
Lead is a stable metal and degrades very slowly, so can persist in the environment for a long time. It can take 10s or 100s of years to fully break down depending on the conditions12,33. Lead pellets can sink into water bodies, soils, and sediments as well as remaining available in bird carcasses, which can have a knock-on effect for animals higher up the food chain34.

What about ‘legacy’ gunshot?
Legacy gunshot is ammunition that has been spent or used. There is always a chance of wildlife consuming old lead pellets, but it is much more likely for an animal to consume recently spent pellets12. That said, historical pellets can accumulate in the environment. Spent lead pellets pose a persistent and significant threat to wildlife35.

Is lead shot the only remaining source of lead exposure for wildlife?
No. Lead occurs naturally in the environment and as a result, some areas of the UK have high levels of naturally occurring lead minerals on the ground, but these are localised17,36, whereas throughout Europe exposure to lead ammunition ground sources is much more widespread.

Other sources of lead include pollution from industry and agriculture into water and soils37. Illegal disposal of items like lead fishing weights under 1oz also add to this but have been banned since 198712,33,38

Effects of lead on wildfowl

Lead ammunition and wildfowl
First reported in the 1870s33, lead ammunition – or lead shot – poisoning has been widely recognised as a threat since the mid-1900s following wildfowl deaths in the USA, France, Italy, Britain and some Scandinavian countries39. Lead poisoning is now a well-known issue for humans, domestic animals, and wildlife alike, with lead shot posing a particular threat to wildfowl12. Wildfowl are birds such as ducks, geese, swans, moorhens, and coots.

Why is lead dangerous to wildfowl?
Lead is a widely used material in shooting sports. Despite restrictions, spent lead pellets can become available in habitats used by wildfowl12. These birds accidentally ingest spent lead pellets when feeding by mistaking the pellets for items of food or grit, which they use to grind down food40. Lead pellets are then directly ingested or are slowly ground down in the gizzard of the bird. This leads to varying degrees of acute or chronic lead poisoning41.

What is the effect of lead on wildfowl?
Lead has significant harmful impacts on wildfowl. Symptoms of lead shot poisoning vary because lead is a ‘non-specific’ toxin, meaning it can affect the whole body1. Characteristic symptoms include1,33,42:

  • Weight loss and muscle wastage
  • Anaemia, weakness, and lethargy
  • Loss of vision and depth perception
  • Seizures, convulsions, and paralysis of limbs
  • Green diarrhoea
  • Wing drooping
  • Lack of coordination, balance, and mobility

Two of the main symptoms are muscle wastage and paralysis, which can affect the throat, gizzard, and digestive system of poisoned birds. This means that eating becomes very difficult, leading to extreme weight loss and starvation20. Research in laboratories and on wild birds has shown that poisoned birds have problems with growth, development, and reproduction43. Poisoned birds often exhibit changes in their behaviour, having more accidents22and failing to avoid predators22,34,41. They also become more vulnerable to disease and parasites1,43.

Birds suffering from lead shot poisoning often die. This could either be directly because of levels of lead in their body, or because of related issues caused by increased lead levels such as starvation, predation, or accidents12. Trials in mallard ducks show that the chance of a duck surviving the month decreases by 19% after ingestion of a single lead pellet3. In cases of high exposure, birds might not display any symptoms before dying1,20.

Where birds repeatedly consume lead pellets, levels of lead have the potential to build up in the body33. Consumed pellets erode in the stomach and gizzard, after which toxic lead salts enter the bloodstream and can be deposited in the kidneys, liver, bones, and feathers1,44. Stored lead can also leach out of bone in female birds during egg production when the bird requires more calcium than normal1,45. Levels of lead can remain high in the blood and tissues for months at a time, persisting in bone for much longer46.

How quickly does lead poisoning happen in wildfowl?
When wildfowl ingest lead pellets, they fully erode and absorb into the body within 2-3 weeks22,33. Blood lead levels generally peak 2 days after ingestion and take up to 36 days to return to normal47. This means that in a single year, a bird can consume multiple loads of lead pellets and experience several episodes of poisoning. This results in prolonged sub-lethal effects, suffering, and potential death22,47.

Why are wildfowl susceptible to the harmful effects of lead?
X-ray showing lead pellets collected in the gizzard of a swan (Photo by Lamiot CC BY-SA)Wildfowl find food by diving or dabbling in water - looking for insects, molluscs, seeds, vegetation, roots, and other food48. Because of this wildfowl are likely to ingest pellets - by accident - that have fallen in and around bodies of water. Lead pellets are mistaken for food or grit, which birds use to grind up food in the gizzard49. This means that pellets are directly consumed or eroded in the gizzard34.

Are some species more at risk?
Any duck, goose, swan, coot or moorhen that feeds in an area where lead shot is used is at risk of lead poisoning43,47.

Studies suggest that swans (particularly Bewick’s and whooper swans) and geese are more susceptible to lead shot poisoning. Swans and geese commonly forage on agricultural land over which it is often legal to shoot with lead shot (depending on UK country specific legislation)12. Swans may also require particularly large quantities of grit when consuming more indigestible foods such as potatoes, corn, and barley and so are more likely to ingest spent shot12.

Diving ducks – such as tufted ducks, pochard, and goldeneye – are very likely to ingest lead pellets instead of grit, because they eat larger seeds40. They are exposed to lead shot when they gather food from the bottom of ponds and lakes, where the sediment is too compact for lead shot to sink out of reach12. Dabbling ducks – such as mallards, wigeon and gadwall – mainly eat plant leaves and so are generally thought to be less at risk43.

In recent years scientists have found that ducks and swans can suffer lead shot poisoning at lower levels than previously realised46.

How big is the problem?
Scientists estimate that millions of birds suffer from sub-lethal effects of lead shot every year throughout Europe34. Research estimates suggest that between 30-60,00022 and 50-100,0001 birds are likely to perish in the UK each winter as a direct result of lead shot poisoning. Long-term monitoring found that 8.1% of birds found dead between 2000-2010 had died from ingesting lead pellets12,23. Some animals had ingested hundreds of pellets34.

Records of autopsied birds from 1971-2010 showed that 1 in 4 migratory swans and 1 in 10 wildfowl exhibited lead shot poisoning as the cause of death12,22. A total of 42% of whooper swans that underwent blood tests in winters between 2010 and 2014 also showed high levels of lead in their blood12,20.

Scientists estimate that 1.5-3.0% of wildfowl overwintering in the UK each year die of lead shot poisoning1.

Lead shot poisoning is difficult to quantify primarily due to the likelihood of under-estimation, although some over-estimation is possible. Lead ingestion could be more common than thought because pellets are only present for a short time before they absorb into the body. Lead shot poisoning can also present subtle, sub-lethal effects that are hard to notice43 and result in wildfowl deaths being attributed to other factors12. Birds may also consume lead pellets from other countries when they travel to and from their overwintering sites23. However, research has shown that migratory wildfowl have high blood lead levels in mid-late winter when they are most likely to have been in the UK for several weeks. Given that blood lead concentrations tend to reflect exposures within 35–40 days of testing, it is therefore probable that most will have ingested lead shot in the UK12.

It is important to remember that regardless of the number of wildfowl or other wildlife affected, lead is a harmful toxin that can cause great suffering and death. Any lack of studies providing hard data on lead shot poisoning does not mean that lead is not a serious and noteworthy issue for wildlife22.

Where dead birds are found, what signs of lead poisoning are found in a post-mortem?
When scientists discover dead birds, they will often carry out an autopsy to try to determine the cause of death. Scientists look for specific signs that ingestion of lead pellets was the cause of death, including12,23,40,42:

  • Lead pellets in the crop, gizzard, stomach, or intestines
  • High levels of lead in the blood
  • High levels of lead in the liver, kidneys, bones, or feathers
  • Low body weight
  • Wastage of the gizzard and liver
  • Reduced levels of d-ALAD protein (needed to make blood)

It can sometimes be difficult to find the source of lead in the body. If birds have lead in their blood or organs, as well as in their gizzards or stomachs, it is safe to assume the lead was ingested47. Scientists can also carry out tests for elevated blood levels or lead shot ingestion on live birds through x-rays and blood samples40.

How are the estimates for the number of wildfowl that die from lead poisoning generated?
Scientists can use published, long-term research on live and deceased birds to estimate the number of birds poisoned by lead ammunition. Lead shot poisoning in birds can be confirmed through post-mortem analysis, and in live birds, through taking blood samples whereby elevated blood lead levels are indicative of poisoning12,44. The most recent estimates look at the use of lead shot and the scale of lead poisoning in ducks between 1999 and 2020. These estimates compare data from the GWCT National Gamebag Census50 to shoot surveys and poisoning data1,51.

Various sources of information are needed to estimate annual mortality from lead shot poisoning. This includes (i) the average proportion of wildfowl with ingested shot, derived from UK studies of hunter-shot birds and birds found dead and autopsied and (ii) the estimated British wintering population of wildfowl1. Adjustments can be made to account for the turnover of lead in the ecosystem and for hunting bias, as birds that have ingested lead shot are more likely to be shot by hunters because of their weakened state1.

If mortality estimates are relatively high, why are so few dead birds found?
Birds suffering from lead shot poisoning tend to die few at a time and will crawl away, hiding in vegetation away from their flock before they die. This results in less-conspicuous deaths rather than larger die-off events12,52. There is also likely to be a bias towards finding larger birds such as geese or swans, due to their more obvious size and colour33.

Scientists have observed that predators and scavengers often remove poisoned birds before people find them. Trials show that predators remove dead birds in 24-72 hours on average, and 48% of carcasses are completely scavenged within 24 hours52.

Is this likely to result in population-level effects in any species?
Scientists know that lead shot poisoning can affect birth, death, and survival rates in wildfowl. Because of that, lead shot poisoning is capable of changing population sizes, growth rates, and demographics22,33,43.

Lead ammunition has been used in the UK for so long that before and after data is not available for scientists to study. There is also a lack of UK-based studies providing data to determine if population-level impacts are occurring22,33. However, more recent research on eight duck populations has shown correlations between population growth rates and lead ingestion rates, suggesting that lead shot poisoning has the potential to limit duck populations43.

Population-level impacts are near inevitable for both common and threatened species34. This is because many wildfowl species are migratory and so face many pressures throughout their range1.

Is this why lead ammunition was banned over wetlands across Europe?
In January 2021, European Union (EU) Member States voted to introduce a ban on the use of lead shot in wetlands across the EU. The ban will apply from February 2023 in all EU states, harmonising and extending existing wetland-related legislation of 22 EU states53. It will introduce a ban on lead shot over wetlands for the first time in Poland, Ireland, Romania, Slovenia and Malta54.

The ban was introduced under the Registration, Evaluation, Authorisation, and Restriction of Chemicals (REACH) framework. This followed an investigation by the European Chemicals Agency into the risks of lead to human and environmental health53.


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