WATERFOWL 



Our emphasis, earlier in this report, on the great 

 reduction in the quantity of vascular water plants has a 

 special application here. These organisms form the base 

 of the food pyramid upon which many other kinds of 

 life depend. Crustaceans and aquatic insects occur abun- 

 dantly on water plants. Such animals, as well as the 

 plants themselves, form a part of the diet of various 

 aquatic birds. The absence of these and other aquatic 

 organisms can become limiting factors for some water- 

 fowl populations. The disappearance of the fingernail 

 clams and other bottom fauna created a drastic loss in 

 the food supply of most diving ducks that inhabited the 

 Illinois River valley. 



Anderson (1959:316) found that mollusca made up 

 more than 85 percent of the diet of lesser scaup ducks 

 [Aythya affinis) . The ring-necked ducks [Aythya col- 

 laris) made mollusca about 25 percent of their diet, and 

 the food of canvasbacks {Aythya vallisincria) was made 

 up of about 9 percent mollusca. 



The combined loss of aquatic plants and bottom 

 animals has drastically affected the numbers of diving 

 ducks that use the Illinois during their migrations. Just 

 as the loss of mollusca apparently caused problems with 

 the lesser scaup, the loss of vegetation in Spring Lake, 

 near Manito, has affected other species. Coontail pro- 

 vided food for several thousand redheads [Aythya ameri- 

 cana) during the spring migration. Now the redheads 

 are forced to seek food elsewhere. Peoria Lake, once the 



scene of the greatest fall concentration of diving ducks 

 in Illinois, has suffered an almost complete loss of these 

 birds. 



Many dabbling duck species as well, such as the 

 widgeon (Marcca amcricana) and the gadwall (Anas 

 strepera), are well known to be dependent on water 

 plants for food. 



Let us briefly examine some f)opulation statistics for 

 a few important duck species. 



Lesser Scaup 



Fig. 1 1 shows the yearly change in lesssr scaup pop- 

 ulations in both the Illinois and Mississippi river valleys 

 from 1946 to 1964. Both rivers are included to indicate 

 the possibility of change of duck populations from one 

 valley to the other. Prior to 1955 the bulk of the fall 

 population of this species was concentrated in the Illinois 

 River valley. A tremendous decline occurred among the 

 lesser scaups stopping along the Illinois in 1955, and 

 numbers have remained insignificant since then. This 

 reduction is synchronous with the disappearance of 

 fingernail clams. It would appear that, after 1956. some 

 elements of the Illinois River scaup population gradually 

 shifted to the Mississippi. Although this shift could ac- 

 count for a part of the previous Illinois River popula- 

 tion, Fig. 1 1 indicates that the total population for the 

 state has been substantially reduced. ^Ve might infer 

 that the Mississippi Ri\ er docs not contain enough food 

 to support a population of lesser scaup such as was 



LESSER SCAUP 



2,520,000-1 



ILLINOIS RIVER VALLEY 

 MISSISSIPPI RIVER VALLEY 



1,200,000 



> 1, 000,000 



800,000 



600,000 



K 

 lij 

 SI 



400,000 



< 200,000- 



1946 '47 



1946 to 1954 



1955 to 1964 



Fig. 11. — Changes in Irsscr scaup duck populations on the Illinois and Mississippi ii\er valleys from 19-lti to 196.1. The reduced 

 populations after 1954 coincide with the virtual disappearance of the fingernail clams from the river above Bcardstown. (Illinois 

 Natural History Survey data.) 



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