Lead shot is used by hunters and is scattered over moorlands, marshes, wet fields and estuary mudflats, when it fails to kill the hunted wildfowl. Large numbers of birds are dying after they ingest discarded shot together with grit during feeding. Stomach acids in the bird's gizzard and the grinding action of the grit produce high concentrations of toxic lead salts in the intestine, which is the absorbed into the blood stream and deposited in the tissues of vital organs. Birds ingesting greater than around ten shot may die within a few days. A smaller number of shot are sufficient to cause poisoning which may lead to death within weeks (from emaciation, weakness and malfunction of the digestive organs). Poisoned birds tend to die slowly and are rarely seen because they secrete themselves and their corpses are consumed by predators or scavengers. Not all birds die, but those that survive have internal damage and are more sensitive to further exposure. Contaminated migrating waterfowl contaminate other waterfowl areas where hunting is not allowed. Predators of waterfowl are also affected -- over 144 bald eagles have died of lead poisoning in lower USA since the 1960's (this endangered species numbers some 3000 individuals in total). Raptors in the Mediterranean may also be at risk.
The use of lead shot for wildfowl hunting became illegal in the USA prior to the 1991/92 hunting season. In Denmark, lead shot is banned on wetlands of international importance for wildfowl (Ramsar Convention sites), and selective restrictions also apply in Canada and some other countries. However, lead shot remains intact in most soils for many years.
It is conservatively estimated that wildfowl hunters alone use 1,000 to 2,000 tonnes of shot annually in Europe, and 2,000 to 4,000 tonnes annually in the USA. Research in the USA and the UK indicate that 2 to 3% of the autumn population of ducks dies each year from lead poisoning. This means up to 2.4 million birds a year in the USA.
Birds are most likely to ingest shot in areas with high shot densities and little grit, a situation that exists in many of the heavily hunted river deltas of the Mediterranean, where shot densities of to 2 million pellets per hectare accumulate in popular spots such as the Carmargue and Evros deltas. Acid moorlands and the acidic conditions at the bottom of many lakes render lead most biologically available, to the degree where it is concentrated enough to affect other aquatic wildlife, such as fish and plants; shot from a clay pigeon shoot in the UK has killed plants over 6 hectares and resulted in lead concentrations in the remaining vegetation of 250 times those regarded as acceptable for grazing animals. Species which most frequently ingest shot are not necessarily the most susceptible to lead poisoning (species with diets rich in protein, calcium and phosphorous reduce the toxic effects of lead), but diving ducks and swans are especially affected because of the bottom-feeding habits and also because they ingest anglers' split-lead shot used to weight lines.