I 



present on the combined rivers from 1946 lo 1954, for 

 iheir decline was greater than that for the flyway as a 

 whole. 



Ring-necked Duck 



Ring-necked ducks were at a peak in numbers in 

 1949 when aquatic plants reached maximum abundance 

 in Peoria Lake (Fig. 12). With the decline in abun- 

 dance of both aquatic plants and mollusca in the Illinois 

 River valley, populations of these ducks declined. In the 

 1955-1964 period the number of ring-necked ducks did 

 not decline proportionately so much as did the lesser 

 scaups. The ringneck seemed to be better able to use 

 the flooded moist-soil plants for food. Moreover there 

 was some decline in the populations of this species in 

 the Mississippi area for several years after 1955. The 

 decline probably reflects a known decrease in flyway 

 population from out-of-state causes, for there was no 

 commensurate deterioration in habitat conditions in the 

 Mississippi valley. 



Canvasback 



Populations of this species slumped as badly as did 

 those of the lesser scaup following the loss of plant and 

 animal food resources from the Illinois River valley 

 (Fig. 13). The canvasback population in the Mississippi 

 River valley increased during the 1955-1964 period, fol- 

 lowing the great disappearance of these birds from the 

 Illinois system, but the total population for the state is 

 still far below the pre-1955 level. Canvasbacks in the 

 Mississippi River section of Illinois are forced to feed 

 almost entirely on animal life because of the scarcity of 



aquatic vegetation, yet food-habit studies (Anderson 

 1959; Cottam 1939) show their preference for aquatic 

 plant foods. 



The paucity of plant-food resources in the Mississippi 

 River section probably limits the numbers of canvas- 

 backs to a level far below that accommodated by both 

 river valleys in the early 1950's. 



Ruddy Duck 



The ruddy duck {Oxyura jamaicensis) declined in 

 numbers with the decline in aquatic plants and mol- 

 lusca (Fig. 14), but this decline was proportionately less 

 than in other diving ducks, possibly because of this spe- 

 cies' propensity to feed on aquatic insect larvae (Ander- 

 son, loc. cit.). Insects have not as a group suffered 

 catyclismic losses as have the mollusca in bottomland 

 lakes. Therefore more animal food to the liking of ruddy 

 ducks remains available in this valley. The Mississippi 

 River seems to have absorbed a part of the Illinois River 

 population, as in the case of other diving ducks, but it 

 appears that the "carrying capacity" of the Mississippi 

 may not be sufficient to overcome the loss of the food 

 resources in the Illinois. 



Mallard 



Populations of this common duck (Anas plalyrhyn- 

 chos) in the Illinois and Mississippi river vallevs do not 

 show the same trends as do the dixing duck pojjulations, 

 or those of the dabblers dependent on aquatic vegeta- 

 tion. Although mallard populations declined after 1955, 

 this decline occurred almost equally in both valleys; no 



RINGNECK 



900,000 



nn] = ILLINOIS RIVER VALLEY 

 ■ = MISSISSIPPI RIVER VALLEY 



150,000 



1946 47 



360,000-. 



^ 300,000 



,240,000 



z 



o 180,000 



120,000- 



60,000 



1946 10 1954 



1955 to 1964 



Fig. 12. — The ring-ncrkrd durk drrlinrd in abund;inrr in the Illinois Ri\rr following thr loss of the- molUisi- food rcsourci- and 

 tlic reduction in aquatic vegetation. (Illinois .Natural History Survey data.) 



