Waterfowl undoubtedly ingest more shot after than during the hunting season. As a result of this seasonal variation in ingestion rates, calculations based on gizzards from ducks shot by hunters underestimate the incidence of shot ingestion. Moreover, lead-poisoned birds are more likely to be killed by hunters because these birds are in an already weakened condition. After the hunting season, such birds would, in all probability, fall victim to predators other than man, but in either case their deaths are at least indirectly a result of lead poisoning. The proclivity of a given waterfowl species to ingest shot pellets is based on its feeding habits and habitats.
Some species consistently ingest more pellets than others.
After a shot has been ingested, however, many factors determine its lethality. The volume of food consumed and its rate of passage through the gastrointestinal tract help to prevent the absorption of intestinal lead. Similarly, the volume of grit ingested and its rate of passage play a role in the elimination of lead. These variables are compounded by those introduced by the deposition and withdrawal of lead from the skeletal system. The more active the calcium metabolism of the bird, the more active the deposition or transportation of lead, or both.