Northern Prairie Wildlife Research Center
Because of the widespread distribution of lead shot from the northern breeding grounds to the southern wintering grounds, it is available fall through spring to waterfowl feeding on areas that have been hunted. As a result, mortality accrues on a day-to-day basis. These losses, however, are usually overlooked because predators quickly dispose of moribund birds. Studies in Missouri and Texas, for example, revealed that predators rapidly removed waterfowl carcasses placed by biologists in wetland habitats. Moreover, dead ducks in natural settings are difficult to find, and freshly planted carcasses in marsh vegetation were largely overlooked by searchers employed to find them. Only when massive die-offs of waterfowl occur in a limited area do losses from lead poisoning attract public attention. Such die-offs are the result of unusually high rates of shot ingestion; however, nutrition and low temperatures may be ancillary causes. Under these conditions, waterfowl die in numbers that exceed the ability of predators to consume them and to keep the environment tidy. Most die-offs from lead toxicosis occur after the hunting season - in winter and early spring.
The potential impact of lead poisoning on waterfowl populations has been ascertained from diverse sources of information. Almost 200,000 gizzards from more than 16 species of waterfowl in a number of geographical regions have been examined for lead shot during hunting seasons in fall and early winter. Scores of experiments with penned wild and gamefarm waterfowl have been conducted by numerous investigators to evaluate the effects of shot dose, nutrition, age, and sex and to study the physiological manifestations of lead toxicosis. A pertinent finding of nutrition studies was that protein, calcium, and phosphorus play an important role in determining the lethality of lead.
Species of waterfowl vary in their proclivity to ingest shot and, because of differing food habits, in their susceptibility to ingested lead. Lead toxicosis poses the greatest threat to mallards, followed in lessening degrees by black ducks, mottled ducks, pintails, canvasbacks, redheads, and ring-necked ducks. The potential for lead poisoning in other duck species is low. At times swans and geese become victims in numbers sufficiently large to cause concern.
The use of steel shot as a substitute for lead shot in waterfowl hunting is the only currently feasible solution to the problem of lead poisoning. Steel shot is less dense than lead shot but produces a tighter pattern and shorter shot string. The lower density of steel shot can be compensated for by increasing shot size and velocity, thus delivering similar levels of energy to the target. No significant differences in crippling rates were found in all but 3 of 15 tests comparing the effects of steel and lead shot. In only 1 of those 3 tests did steel shot cripple more ducks than the lead shot being tested. Although not significantly different, rates of ducks lost from crippling wounds with steel shot were 5.3 percent lower in a Michigan study, 4.9 percent lower to 7.3 percent higher in a Missouri study, and 14.3 percent higher in a Louisiana study. The marshes hunted in Missouri and Michigan, however, are more typical of the vast majority of hunting habitats than the marsh hunted in Louisiana.
Crippling losses to waterfowl populations from steel shot are less harmful than crippling losses plus lead toxicosis from lead shot. Several related points merit consideration. Lead poisoning causes important losses to the most abundant species of waterfowl. The sublethal effects of lead poisoning are recognized but have not been quantified. Except for a brief period in spring, lead may affect females more adversely than males. Finally, seasonal differences in the time of losses are important. A cripple lost during the hunting season has less impact on the breeding population than a lead-poisoned duck lost during the winter or spring.