The case of the loon is perhaps the most telling in North America, and responsible for much of the push for legislation banning lead fishing sinkers. According to reference 5:
It turns out that the form of wildlife primarily at risk from lead poisoning due to fish sinkers is waterfowl, not fish. This stems from the birds' habit of eating smaller sinkers (especially the "split-shot" type) to use as grit in their gizzards, where they pulverize hard-to-digest items such as seeds. As the sinkers are ground along with sand and rocks, the lead is released into their bodies in concentrated form, leading to debilitation and death. It's estimated that perhaps 2% of all waterfowl die per year as a result of ingesting lead shot and other lead objects. In England, the mute swan suffers greatly from ingestion of lost fishing sinkers--one study in 1982 reported that half of all mute swan deaths in England were a result of lead poisoning. Many varieties of geese, ducks, and cranes as well as non-waterfowl such as the mourning dove and others have been studied and shown to have suffered from lead poisoning due to fishing sinkers as well.
A 2.5 year study of mortalities of common loons in New England found that lead toxicity from ingested fishing sinkers was the most common cause of death in adult breeding birds ... The study reported that 64 percent of adult common loons (Gavia immer) received for analysis from New Hampshire, and 44 percent of adults received from Maine, had ingested fishing sinkers. Thirty-one adults were examined, and of these, 16 (52 percent) were shown to have died from lead poisoning ... Levels of lead found in the blood of loons that had ingested sinkers averaged 1.4 ppm. The study indicated that scientists consider 0.35 to 0.60 ppm lead in the blood to be indicative of lead poisoning in many species. Levels of lead in the livers of 4 loons that had lead sinkers in their gizzards ranged from 5.03 to 18.0 ppm, while levels in 10 loons that did not have fishing sinkers in their gizzards ranged from
Sadly, the potential damage to wildlife from environmental lead doesn't end there. Raptors such as the American bald eagle, Andean condor, honey buzzard, king vulture, and California condor can suffer from secondary lead poisoning after eating contaminated fish or waterfowl. Most secondary contamination of predators, it should be said, derives from ingestion of lead shot; many of the lead pellets and pieces from lead sinkers pass harmlessly through the digestive systems of predators or are regurgitated. However, documented cases of poisoning from lead fishing sinkers do exist (reference 1).