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A Review of the Problem of Lead Poisoning in Waterfowl

Susceptibility to Lead Poisoning

Susceptibility by Species

The susceptibility of a waterfowl species to lead poisoning depends on its tendency to ingest lead shot and its food habits relating to the intake of protein, calcium, and phosphorus. Variability among species in shot ingestion has been established through the analysis of more than 200,000 gizzards collected from waterfowl across the United States. Food habits are more difficult to evaluate.

Necropsies of waterfowl sent to the National Wildlife Health Laboratory, Madison, Wisconsin, 1939-1984, help us to understand the susceptibility of various species (Table 4). Obviously, the carcasses of large species, such as swans and geese, are more noticeable than those of smaller species, and people tend to attach more importance to reporting them. The data in Table 4, therefore, are undoubtedly biased toward large species, particularly swans and geese. In addition, the data were recorded by number of lead poisoning episodes that were investigated, not by total number of birds involved. Even with these limitations, mallards were involved in 25.7 percent of the episodes in which lead was determined to be the cause of death. The pintail, involved in 8.7 percent of the episodes, was the second most important duck species. Redhead, canvasback, and lesser scaup were the next most important species. Other duck species were reported less frequently.

On the basis of tendency to ingest shot, level of lead in wing bones, and food habits, we attempted to rate the susceptibility of species to lead toxicosis. All data point to the mallard as highly vulnerable. This susceptibility results from its moderately high tendency to ingest shot (8 percent of the sample; Fig. 1, Table 1) and from its food habits. More than any other duck, mallards feed extensively upon cereal grains, followed by weed seeds; only in restricted areas do they make use of aquatic plants, animal life, or both (Bellrose 1976:242, 243). Consequently, in most areas of the United States, mallard diets are high in carbohydrates and low in protein.

Black ducks ingest lead at about the same rate as mallards. On coastal marshes their diet is higher in invertebrates and aquatic plants than is the diet of mallards (Bellrose 1976:260, 261). In the Midwest, black ducks feed extensively on waste corn along with mallards. Perhaps because of their high protein intake in the Northeast, the proportion of black duck wing bones with lead levels over 20 ppm was lower than that of mallards (Stendell et al. 1979 and Table 5). In Maine, however, the shot ingestion rate of black ducks was higher than that of mallards (Longcore et al. 1982).

Mottled ducks have the highest rate of shot ingestion of any species (Table 1). The percentage of wing bones in juvenile mottled ducks with lead over 20 ppm is three times as high as the percentage in juvenile mallards; the percentage of wing bones with lead over 20 ppm in adult mottled ducks, however, is only slightly more than twice as high as the percentage in adult mallards (Table 5). The food of mottled ducks in fall, based on an analysis of 1,105 gizzards by Stutzenbaker (1984: Table 11), is composed of 30 percent spikerush (Eleocharis cellulosa). This and other green vegetation consumed by mottled ducks provide a high level of protein to complement their 15 percent use of rice, which is high in carbohydrates. The protein level of their diet probably mitigates against the catastrophic losses that might otherwise occur in this species.

Table 5 - Percentage of several duck species with lead >20.0 ppm in their wing bones (from Stendell et al. 1979).

Combined Mean
Black Duck
Mottled Duck
Lesser Scaup

Pintails have a shot ingestion rate slightly higher than that of mallards (Fig. 1, Table 1). Because of a more favorable protein diet, especially in California, their losses from lead poisoning probably are not proportionally as large as those of mallards even though shot ingestion rates for pintails are average in California. Only about half as many pintails from the Pacific Flyway as mallards in that flyway had lead concentrations in their wing bones greater than 20 ppm (Table 5). The large losses of pintails from lead toxicosis at Catahoula Lake, Louisiana, however, confirm that this species can be highly susceptible (Bellrose 1959: Table 1; Wills and Glasgow 1964; Zwank et al. 1985). Among the lead-poisoned dead ducks picked up in California, the pintail has been the most numerous, a finding to be expected because pintails comprise almost half (44 percent) of the fall and winter duck population in that area.

The remaining dabbling ducks (wood duck, gadwall, wigeon, green-winged teal, blue-winged teal, and shoveler) are involved in relatively fewer episodes of lead poisoning (Table 4), but their losses are difficult to estimate. Shot ingestion rates are low for these species (Fig. 1, Table 1), and no analyses of lead levels in wing bones, blood, or liver have been undertaken.

Lead ingestion rates in the bay diving ducks are appreciably higher than those in mallards and pintails (Fig. 1, Table 1). A large proportion of the wing bones from canvasbacks and redheads had lead levels over 20 ppm (Table 5). Although the aquatic plants and animals in the diets of canvasbacks and redheads (Bellrose 1976:312-313,324) would tend to reduce toxicity to levels below those suffered by mallards, the large proportion of their wing bones with lead concentrations of over 20 ppm suggests that the beneficial aspects of their diet are overwhelmed by the large amount of lead these birds ingest. Less than 1 percent of the wing bones of the lesser scaup, however, which also has a high rate of shot ingestion (Fig. 1, Table 1), had lead levels over 20 ppm. Its extensive diet of mollusca, a rich source of protein and calcium (Bellrose 1976:354), may inhibit the absorption of lead into the blood. Ring-necked ducks also ingest large amounts of lead (Fig. 1, Table 1) and have diets primarily composed of vegetable matter (Bellrose 1976: 334). Thus, we anticipate that the incidence of lead poisoning among ring-necked ducks should be comparable to that of redheads. On the basis of lead shot ingestion and levels of lead in wing bones, we estimate that losses of canvasbacks, redheads, and ring-necked ducks are on the same order of magnitude in their populations as is the case in mallards.

Tundra (Cygnus columbianus) and trumpeter swans (C. buccinator) and Canada and snow geese collected from a sizable number of lead poisoning episodes were sent to the National Health Laboratory (Table 4); the rates of shot ingestion by Canada and snow geese (Table 3) are generally lower than those of mallards. The diet of Canada geese focuses on grains, which are high in carbohydrates, and green forage of cereal grains, pasture grasses, and legumes, which are all high in protein (Bellrose 1976: 164). Like Canada geese, snow geese have increasingly deserted feeding on marsh plants to feed on agricultural crops (Bellrose 1976: 123).

As long as geese utilize appreciable amounts of green forage, the amount of lead they normally ingest seldom leads to death. However, when green forage is unavailable or in short supply (often because of weather), lead poisoning may become an important factor in mortality. During the late winter of 1977, for example, an estimated 3,500 Canada geese died in southern Illinois from the effects of lead poisoning (Illinois Department of Conservation 1977). From late January through early April 1974, an estimated 925 Canada geese were victims of lead poisoning at Turk's Pond in southeastern Colorado (Szymczak and Adrian 1978). On the Missouri River in South Dakota during the winter of 1978-79, a minimum of 3,665 Canada geese and 1,091 ducks lost to lead poisoning were counted and as many as 8,000 Canada geese were estimated to have been lost (South Dakota press release from Chuck Post). A winter inventory on four areas with relatively deep water in east-central Wisconsin in December 1980 indicated the presence of 66,900 Canada geese (Amundson in press). Severe cold weather hit the area on 3 January 1981, and the water on the four areas froze. Many of the geese moved to the relatively shallow Lake Puckaway and to Grand River, where water was less than 2 feet deep and shot deposition was as high as 12,000 lead pellets per acre. Nearly 3,200 Canada geese dead from lead poisoning were picked up in and around Lake Puckaway from 14 January through March 1981. These birds amounted to more than 20 percent of the entire wintering flock of Canada geese in east-central Wisconsin. In January-February 1984 an estimated 431 waterfowl died from lead poisoning on the Suter National Wildlife Refuge, California. Of these, 346 (80.3 percent) were snow geese. Necropsy of sick and dead snow geese indicated lead poisoning in 79 percent of the immature birds and 21 percent of the adults. Avian cholera accounted for 11 percent of the immature and 38 percent of the adult snow geese and avian botulism for 0.4 percent of the immatures. Rice, moist soil plants, millet, and other foods for waterfowl are produced on the refuge. Of the lead-poisoned snow geese with shot in their gizzards, the gizzards of immature geese contained an average of 9.4 pellets; those of adults contained an average of 8.0 (U.S. Fish and Wildlife Service 1984).

Mitigating Effects of Diet

Over a span of 35 years, we and others have conducted dozens of experiments with penned ducks in an effort to evaluate the importance of lead shot, to screen potential substitutes for lead shot, to study the physiology of lead poisoning, and to relate our findings to the results of similar studies. We found that weight change between the control and experimental birds was one of the best criteria for assessing levels of lead toxicity. Mortality differences among the dosed and undosed groups provided a more tangible but less precise measurement of toxicosis. These studies reveal that many variables affect toxicity levels. The important variables that have been identified are type and amount of food consumed, amount of soil taken into the digestive tract, age, sex, and size of the bird, amount of lead shot ingested, and season.

Probably the single most important factor controlling the level of toxicity of ingested lead is the type of food consumed (Jordan and Bellrose 1951). In numerous experiments with various kinds of foods, the highest levels of lead toxicity occurred with a diet of corn and the lowest with commercial duck pellets that were high in protein. Of the aquatic plants tested, the green foliage of the following was found to be the most effective in suppressing the toxic effects of lead: coontail (Ceratophyllum demersum), sago pondweed (Potamogeton pectinatus), duckweed (Spirodela polyrhiza, Lemna minor), and chara (Chara sp.). Animal food in the form of quahog clam meat (Mercenaria mercenaria) and oyster shells fed to lesser scaups also suppressed toxic symptoms. On the other hand, in addition to corn, toxic effects increased when penned ducks were fed diets of weed seeds, wheat, rice, or other small grains (Fig. 3).

GIF-- Body Weight Loss With Shot Pellets (PIC)
Figure 3 - Average daily net percentage body weight loss among ducks on various diets and dosed with one or two No. 4 or No. 6 lead shot. Data from Illinois Natural History Survey files.

Foods most successful in alleviating lead toxicity were those high in protein. The crude protein content of several foods was determined by the Department of Animal Nutrition, University of Illinois at Urbana-Champaign: large seed smartweed (Polygonum pensylvanicum), 7.6 percent; corn, 9.3 percent; duck millet (Echinochloa crusgalli), 9.5 percent; coontail, 12.3 percent; mixed small grains, 13.5 percent; duckweed, 18.3 percent; and commercial duck pellets, 18.9 percent. Increasing protein in the diet of penned mallards by adding egg albumen to corn reduced weight loss by 47 percent and increased survival by 71 percent. Similar results were achieved by adding oyster shell, calcium carbonate, and phosphorus to corn (Jordan and Bellrose 1951). Godin (1967) and Longcore et al. (1974) demonstrated that oyster shell grit, high in calcium content, reduced lead toxicosis in mallards.

Comparisons of lead-dosed mallards on a high protein-calcium diet of turkey mash with lead-dosed mallards on a low protein-calcium diet of hen scratch indicated the importance of calcium and protein in mitigating the effects of ingested lead (Koranda et al. 1979). Although the function of protein could not be determined from the data in this study, its presence undoubtedly lowered body burdens from ingested lead and prevented lethality in ducks that had received 3-6 pellets. Koranda and his colleagues considered the value of a diet high in calcium and protein to be twofold: (1) it reduces the absorption of lead from the gastrointestinal tract, and (2) it lowers the general body burden of lead in the bird. Longcore et al. (1974: 10) also concluded that lead storage in animal tissues is decreased by a high calcium diet and increased by a low calcium diet.

A comparison of lead erosion rates with lead excretion rates was made by Irwin (1977:287) for mallards on corn and corn-soybean diets. The lead excretion rate of ducks fed corn and soybeans was similar to the lead erosion rate, a finding that indicates that most of the lead was not absorbed into the blood stream but passed out through the gastrointestinal tract. Carcasses of ducks solely on corn diets had lead concentrations of less than 5 percent of the total dissolved lead, evidence that much of the apparent retention of lead was caused by an accumulation of lead in the gastrointestinal tract. Irwin concluded that no dietary component in the corn-soybean diet explained its antagonism to lead, but as the composition of nutrients added to the corn diet approached those in the corn-soybean diet, lead toxicosis was further abated.

The importance of diet in mitigating the effects of lead toxicosis is emphasized by Stendell et al. (1979:9), who concluded, "These studies show that the uptake of lead in mallard wing bones can be rapid, but that the diet - here corn vs. a commercial mash - has an important influence on rate of uptake of lead by bone."

Another example of low uptake of lead probably related to diet is shown in the low percentages of juvenile lesser scaup (0.4 percent) and adults (0.1 percent) with more than 20.0 ppm in their wing bones (Stendell et al. 1979:7). On the other hand, 20.0 percent of juvenile redheads and 35.3 percent of adults and 7.3 percent of juvenile canvasbacks and 26.8 percent of adults had levels higher than 20.0 ppm. All three species of these bay diving ducks have similar high rates of shot ingestion. Lesser scaups usually feed most extensively on animal life, followed by canvasbacks utilizing more plant material, and redheads feeding still more heavily upon vegetative matter (Bellrose 1976:312, 313, 324, 354). A correlation between the frequency of high levels of lead in the wing bones of these three species of bay diving ducks and their food habits appears to exist. The large losses of lesser scaup from lead poisoning at Rice Lake, Illinois, during the spring of 1972 appear to have occurred because the birds fed largely upon smartweed seeds rather than on mollusca (W.L. Anderson 1975; Bellrose, personal observation).

Much but not all reduction of lead toxicosis in waterfowl by diets high in protein and certain minerals appears to take place in the digestive tract. In addition, protein and minerals may reduce absorption of lead into the blood stream. If lead were absorbed into the blood stream, higher lead levels in the wing bones of the lesser scaups analyzed by Stendell et al. (1979) would be anticipated. Calcium has long been known to reduce the toxicity to aquatic organisms of some metal ions (cations). According to Skidmore (1964:233), this reduced toxicity may occur because calcium antagonizes heavy metal (lead) ions through reduction in the permeability of cell membranes and thereby reduces the speed by which lead penetrates tissues.

Findings by Sanderson and Irwin (1976:57), however, indicate that the effects of diet on lead poisoning may go beyond the digestive tract. They held gamefarm mallards in individual pens and dosed each duck with five No. 4 lead shot. Diets were corn, corn plus 10 g of soil daily, duck pellets, and duck pellets plus 10 g of soil daily. The daily erosion rate of lead in the gizzard and the daily excretion rate of lead in the feces were measured for each bird. Ducks on corn alone eroded the least lead on a daily and total basis, excreted the least lead on a daily and total basis, and retained the least lead on a total basis; these ducks, however, were most severely affected by lead poisoning. Ducks on corn alone retained more lead on a daily basis than ducks on corn plus soil and ducks on a pellet diet; ducks on pellets and soil retained only slightly more lead on a daily basis than ducks on corn. Ducks on pellets and soil had the highest total and daily erosion rates of lead, the second highest total and daily excretion rates of lead, and retained the most lead on a daily and total basis; these ducks, however, showed the least adverse effects of lead poisoning. Tests were not run for lead residues in the ducks, but a reasonable assumption is that more lead entered the bloodstreams of ducks on pellets and soil than entered the bloodstreams of ducks on a corn diet with no soil. In this study, diet appears to provide protection from lead even after it was absorbed from the digestive tract; diet also seemed to account for a higher excretion rate of lead.

Differences in Susceptibility Attributable to Sex

Experiments with male and female domestic and wild mallards demonstrated differences between the sexes in the manifestations of lead poisoning. Females were affected to a greater extent than males, except during the spring. The increased resistance of females to lead poisoning in spring during the pre-breeding and breeding periods appears to be related to a high metabolic rate and to the mobilization of energy resources for egg-laying (Finley and Dieter 1978). In addition, spring was the only season in which food consumption by females exceeded that of males (Jordan and Bellrose 1951:21).

According to White and Stendell (1977:474), the frequency of shot ingestion by males and females among mallards and black ducks was similar. Among pintails, males ingested shot at a significantly higher rate than females. White and Stendell also reported that both sexes of these three species had similar proportions of lead (over 20 ppm) in their wing bones. Among five species of trapped ducks, Bellrose (1959:256) found that a larger percentage of female (16.0%) than male (9.8%) mallards ingested shot. Little difference was found, however, between the sexes in pintail, blue-winged teal, and wood duck (8.9% for males and 7.4% for females); more lesser scaup males (9.0%) ingested shot than did females (4.5%).

Differences in Susceptibility Attributable to Age

Several experiments compared survival and weight loss between adult and immature ducks. Up to about 7 months of age, lead had less effect on younger birds under laboratory conditions. After late December little difference was found between age groups. We surmise that because lead salts follow the same pathways in the blood stream as calcium, a high proportion of the lead was deposited in the skeletons of the maturing young mallards. This deposition would remove circulating lead from the blood stream and help to reduce lead toxicosis in vital organs. As the skeleton (particularly the sternum) of the young ducks became increasingly ossified, less lead, we postulate, was deposited in bone structures. As a result, after about 7 months higher levels were found in the blood. Nevertheless, lead is deposited in wing bones almost immediately after exposure in both juveniles and adults (Stendell et al. 1979:9).

At Catahoula Lake, Louisiana, Shealy and his colleagues (1982:43) found that lead ingestion occurred more frequently in adults than in juveniles in both mallards and pintails. Lead content of wing bones examined by Stendell et al. (1979:6,7) showed slightly higher concentrations of lead in adults and a higher proportion of adults with >/=20 ppm lead among mallards, mottled ducks, pintails, canvasbacks, redheads, and lesser scaups. Only juvenile black ducks had slightly higher levels of lead in their wing bones than the adults of their species. These data suggest that juvenile ducks of these species ingest fewer shot than adults or that the ossifying sternum of juveniles takes up a greater share of blood lead. The latter assumption seems more likely.

Differences in Susceptibility Attributable to Size

As might be expected, the larger the waterfowl, the less effect a given amount of ingested lead has. Under controlled experiments, Canada geese (Branta canadensis interior) showed the least effects of lead toxicosis followed in ascending order by mallards, pintails, and blue-winged teal. All tests were not made at the same time or with the same foods and shot doses, but studies pairing mallards with each of the other species provided a basis for comparison.
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