Faber, R.A. 1986. Preliminary assessment of sources of lead contamination in tundra swans frequenting the weaver bottoms. Final report submitted to the Nongame Wildlife Program, Minnesota Department of Natural Resources. 16 pp.
Sources of lead contamination for tundra swans were studied in the Weaver Bottoms, a backwater area of the Mississippi River in southeastern Minnesota in 1985 and 1986. Densities of lead shot in the top 15-20 cm of sediment within 200 m of sites used by waterfowl hunters were 27.6 shot/square meter in an area heavily used for waterfowling and 13.1 shot/square meter in an area less heavily used. Sampling in two areas closed to hunting yielded no shot in any sample. Other lead objects, such as fishing sinkers, were not encountered in any sample. Tubers of the arrowhead plant, Sagittaria latifolia, which are used by tundra swans for food were analyzed for lead content. The mean level of contamination among four sampling areas was 0.35 ppm wet-weight (1.87 ppm dry weight). No significant trends were noted between sites near or far from a major highway and those open or closed to hunting. Daily intake of arrowhead tubers by swans was estimated by determining the tuber population in an area where swans concentrate during the fall migration before and after the migration to determine total loss of tubers (185,034 kg) and dividing this by the total swan-days of use of the area (23,537). The total loss of tubers/swan-day of use was 7,860 g. This is regarded as an overestimate of swan intake, however, because other species steal tubers from the swans as they feed. Even if all of the tubers were taken by swans and all of the lead from the tubers were transferred to the swans' bloodstream, it would take 46 days of feeding to ingest the equivalent of one #6 lead shot from this area. Since most swans spend no more than three weeks in the area, it is unlikely that the lead from arrowhead tubers contributes significantly to lead poisoning in tundra swans in this area. Two swans found dead or moribund in the area were x-rayed for shot and the lives analyzed for lead. No shot were found and the lead levels were low. Death was attributed to unknown factors.