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A Review of the Problem of Lead Poisoning in Waterfowl

Discussion


Most of the waterfowl that die from lead poisoning do so after the hunting season. The history of mass die-offs (Bellrose 1959: Table 1) as well as the chronology of specimens sent to the National Wildlife Health Laboratory (Table 11) substantiate this conclusion. Without the activities of hunters to drive them from the feed beds and with a steadily diminishing food supply, waterfowl are attracted back to shotover areas, where spent shot is abundant. 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 current diet of the bird is the single most important factor: the intake of protein, calcium, and phosphorus reduce lead toxicosis. 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. Consequently, breeding females and young ducks may have a greater resistance to lead poisoning.

In addition to the outright lethality of lead, its sublethal effects have only recently become known. Dieter and Finley (1979) found that biochemical lesions in the brains of mallards dosed with one No. 4 lead shot pellet occurred earlier than did such external evidence of the disease as wing droop and vent staining.

In establishing the role that steel shot might play in the welfare of waterfowl populations, we need to evaluate further the differential effect of lead toxicosis on the sexes and the effect on population dynamics of seasonal losses.

Two hypotheses have been advanced regarding the effect of gender on lead poisoning in ducks. In the first, lead has no differential effect on males and females. In the second, lead has a greater effect upon females. Evidence supporting the first hypothesis comes from White and Stendell (1977:474), who found no significant difference between lead levels in male and female wing bones of mallards, black ducks, and pintails. Support for the assumption that females suffer more from the effects of lead than males is found in the higher mortality rate (approximately 25 percent higher) of banded female mallards dosed with one shot compared with similarly dosed males (Fig. 6). Moreover, weight loss among penned game-farm female mallards dosed with lead was greater than that among drakes (Fig. 5). In a series of experiments with penned mallards, Jordan and Bellrose (1951:21) found a mortality rate twice as high among hens as drakes except during spring. Only during a brief period from late February through March were mallard females less susceptible to lead than males (Bellrose 1959:276).

As previously noted, losses due to crippling occur throughout the hunting season, but losses from lead poisoning are most extensive after hunting ceases. Of the many die-offs of waterfowl cited by Bellrose (1959: Table 1), only two occurred late in the hunting season; the others were reported still later in the winter and spring. Moreover, the chronology of lead-poisoned specimens received at the National Wildlife Health Laboratory, Madison, Wisconsin, from throughout the nation documents that losses largely occur from December through February (Table 11). An examination of waterfowl population dynamics suggests that the late winter, early spring lead poisoning losses have a more important influence on potential production than do the crippling losses that occur during the hunting season. Birds lost from one factor, hunting, for example, are replaced by birds that survived the hunting season and continue to resist natural causes of death because more habitat niches were made available to them. The earlier in the fall that mortality occurs, the better the opportunity for the remaining birds to survive and breed. The nearer to the breeding season that a bird survives, the more likely it will achieve breeding status. Because most losses from lead poisoning occur just prior to the breeding season, they affect the breeding population more severely than an equal number of crippling losses during the previous fall.

The potential impact of lead poisoning on fall duck populations is related to the relative abundance of the most vulnerable species (Table 12): mallard, black duck, mottled duck, pintail, canvasback, redhead, and ring-necked duck. Together, these species make up 43 percent of the continental game duck population and, ironically, are the species that have been of most concern to conservationists in the past decade.

Table 12- Estimated fall duck populations and relative abundance of the principal species in recent years based on breeding populations and number of young equal to number of adults.

Species
Fall Population 1970-79a
Percent
Mallard
21,200,000
21.3
Black Duck
2,800,000
2.8
Mottled Duckb
200,000
0.2
Pintail
14,000,000
14.1
Wood Duck
6,400,000
6.4
Wigeon
7,000,000
7.0
Gadwall
4,000,000
4.0
Green-winged Teal
6,300,000
6.3
Blue-winged Teal
12,300,000
12.4
Shoveler
4,400,000
4.4
Canvasback
1,200,000
1.2
Redhead
1,700,000
1.7
Ring-necked Duck
1,900,000
1.9
Lesser Scaup
16,000,000
16.1
Total
99,400,000
99.8
aContinental estimate of breeding ducks from draft copy of North American waterfowl management plan, November 1985 (Canadian Wildlife Service and U.S. Fish and Wildlife Service: Table 1).
bData for the mottled duck from U.S. Fish and Wildlife Service winter inventories, 1980-1985, x 2.

We believe more is to be gained by the judicious use of steel shot than by the continued blanket shooting of lead shot. Although we recognize that the use of steel shot would have limited impact in some habitats and on certain species of waterfowl, its use would be of considerable benefit in many habitats and on several species. In addition, we recognize that in some areas botulism, fowl cholera, and duck virus emeritus cause extensive waterfowl mortality; however, merely because large numbers of ducks in some areas die from other diseases does not justify ignoring losses due to lead. Deaths due to lead poisoning, unlike deaths caused by other diseases, can be eliminated by regulation. If other diseases could be eliminated by regulations, we would urge their implementation.

The partial changeover from lead to steel that has occurred has already had a measurable impact on lead poisoning. Although the use of steel shot has been limited in time and place, a surprisingly large proportion of analyzed gizzards show steel replacing lead in shot ingestion (Table 13). Thus, the regulated use of steel shot would appear to have the potential for promptly reducing mortality due to lead poisoning.

Table 13 - Comparative ingestion of lead and steel shot by waterfowl in states and other regions, 1974-1984.

Flyway/State
Years
Investigator
Lead
Steel
Both
% with lead onlya
No.
%
No.
%
No.
%
Atlantic
No record
 
Mississippi
Michigan
1977-79
Nelson & Johnson 1980
416
3.4
416
3.4
14
0.1
49.2
Ohio
1977-79
Bednarik & Shieldcastle 1980
230
2.7
61
0.7
-b
-
79.0
Indiana
1977-80
Sporre & Belvins 1981
150
6.7
50
2.2
15
0.7
69.8
Missouri
1977-81
Hamburg & Babcock 1982
719
3.8
319
1.7
56
0.3
65.7
Illinois
1979-82
Anderson 1982
525
5.1
108
1.0
23
1.2
80.0
Central
Texas
1981-83
Texas Parks & Wildl. Dept. 1982,1983
753
6.1
395
3.2
288
2.3
52.4
Pacific
Oregonc
1974-83
Vendshus n.d.
127
16.9
52
6.9
55
7.3
54.3
California
1979-80
Moore & King 1980
726
9.0
34
0.4
-b
-
95.5
United States
1974-75
White & Stendell 1977
244
8.9
74
2.7
15
0.5
73.3
aPercent of gizzards with lead pellets only.
bNot classified.
cSauvie Island only.

Although the extent of opposition to steel shot by waterfowl hunters has not been documented, strong and well-organized opposition continues (Arnett 1985:7; Kendzie 1985). Recently, Feierabend (1985) summarized the legal challenges to nontoxic (steel) shot regulations and found that all decisions, including a recent case in Federal Court (National Wildlife Federation 1985a), have gone in favor of steel shot. On the other hand, as late as the 1981 waterfowl hunting season, 53.9 percent of Illinois waterfowl hunters replying to a questionnaire had never used steel shot for hunting waterfowl; however, 47.5 percent of these hunters believed that lead poisoning was a problem "on some, many, or all areas" and 51.9 percent responded that they would voluntarily use steel shot in some areas if asked to do so by the Illinois Department of Conservation (Anderson 1983: 1).

Although disagreement continues regarding the extent of lead poisoning in waterfowl, most biologists, wildlife managers and administrators, and waterfowl hunters agree that appreciable mortality results. With the continuing decline in quality and quantity of nest habitat for waterfowl and the consequent declines in continental waterfowl populations (the length of the 1985 waterfowl hunting seasons were reduced by the U.S. Fish and Wildlife Service in response to lower populations), a conservative approach to the problem of lead poisoning in waterfowl seems prudent. It sometimes seems as if advocates of steel shot are being asked to demonstrate that steel is "better" than lead before its use is acceptable. Instead, we should focus on the effects of the use of lead and steel shot on ducks and geese -- the mortality rate from lead poisoning and crippling by lead shot versus the mortality rate from crippling by steel shot.


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