Historically the Illinois River's backwater lakes have been among the most important migration areas for several species of ducks, including mallards. Since 1948, however, significantly fewer migrating mallards have alighted in the Illinois valley each fall. The trend is thought to reflect the general decline in mallard numbers across North America, but there is evidence that local conditions also have deteriorated.
Mallard Ducks Along Major Illinois Waterways 1948-1991*
Source: Ecological Resources, Illinois Natural History Survey, 1994
The Illinois River's complex system of wetlands has long attracted more mallards than that of the Mississippi River. (In the late 1940s, mallards along the Illinois outnumbered those sojourning on the larger river nine to one.) The post-1948 decline in mallard counts along the Illinois has been relatively steeper than that recorded on the Mississippi, where sedimentation has not caused such drastic reductions in the amount and variety of natural plant foods available to migrating flocks. In addition, tillage of increased acreage of harvested corn fields in central Illinois during fall sharply decreased the waste grain available to the field-feeding mallards.
Two other duck species--the lesser scaup and the canvasback--have suffered more drastic population crashes on the Illinois River. Lesser scaups were abundant in the Illinois valley before the 1950s, especially on Upper Peoria Lake. More than 585,000 ducks were counted on one stretch of the river in 1954; three years later the number was around 10,000. Similar trends were recorded in populations of canvasbacks. During the 1952 migration more than 105,000 birds were counted along the Illinois River north of Peoria; in 1971, only 120 were seen.
The cause of the decline in numbers of lesser scaup and canvasback ducks in the Illinois River valley is a scarcity of food. The rafts of aquatic vegetation that used to sustain the canvasback flocks on the Upper Peoria Lake have disappeared as a result of sedimentation (which causes turbid water and flocculent lakebeds) and changes in seasonal water level cycles. Also the benthic macroinvertebrate community--the small clams and other bottom dwelling creatures especially crucial to diving ducks like the lesser scaup--was likely affected by sedimentation and pollution from various domestic, industrial, and agricultural sources.
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