MALLARD 



7,200P00 



6,000,000 



4,800,000- 



3,600,000 



2,400,000- 



1,200,000- 



ILLINOIS RIVER VALLEY 

 MISSISSIPPI RIVER VALLEY 



1946 10 1954 



1955 to 1964 



Fig. 15. — The mallard duck feeds primarily on plant foods, largely seeds of moist-soil plant.s and waste crop grains, so its pop- 

 ulation was not affected by the loss in mollusca and aquatic vegetation. The population decline in Illinois reflects the continental 

 trend for this species. (Illinois Natural History Survey data.) 



particular have escaped severe drought losses. In spite 

 of their greater productivity during this drought period, 

 the lesser scaups and ring-necked ducks, as well as other 

 diving ducks in the Illinois River valley, declined in num- 

 bers proportionately much more than the mallard. 



The difference in the geographic and yearly popula- 

 tion change between the mallard and the diving ducks 

 fortifies our belief that the post- 19.55 diving duck popula- 

 tion loss is directly related to the loss in food resources 

 resulting from silting, and from inban and industrial 

 pollution of the Illinois Ri\er and bottomland lakes. 



To generalize, it would appear that recent environ- 

 mental changes in the Illinois River, due to activities of 

 an enlarging human population, have produced dis- 

 astrous consecjuences on food resources for diving ducks 

 as wtII as for some dabblers. From the mid-1950's to 

 the present, a combination of soil pollution jjIus indus- 

 trial and domestic pollution appears to ha\c eliminated 

 as a functional ])art of the (•n\ir()nmcnt tin- important 

 aquatic plant and animal life necessary for the support 

 of populations of many species of ducks. 'I'hesc may not 

 be the only factors involved; we do know that in certain 

 [jlaces the raising of water levels, for examjjle, has con- 

 tributed to the decline of a(|uatic vegetation. 



OTHER BIRDS 



A study of any group of species of birds will show 

 fluctuations in numbers from year to year. Some of these 

 changes in bird populations in the Illinois River valley, 



although rons])icuous. cannot be tiarcd to the deteriora- 



tion of water ciuality in the river. For example, the pro- 

 thonotaiy warbler (Protonolaria citrca) was abundant 

 near Chautauqua Lake 15 years ago and is now moder- 

 ately rare. It is a cavity-nesting, insectivorous species, 

 and at the time of its abundance there were many dead 

 willows along the edges of the lake. The warblers used 

 holes in the trees for nesting. As the trees decayed and 

 fell, the numbers of the birds decreased, and the logical 

 explanation for this population reduction is the disap- 

 pearance of nesting sites. 



Changes in some other sjJecies may have a moie di- 

 rect relationshi]) to changes in the river. 



Cormorants 



Each autumn during the 1940's and the early I950's 

 there was a large flight of double-crested cormorants 

 (Phalacrocorax auritus) down the Illinois River valley. 

 The migrants usually arrived between October 5 and 9. 

 Many thousands remained until early November, con- 

 ducting fishing drives in the larger bottomland lakes. 

 On October 16, 1950, we estimated that there were 

 15,000 cormorants on the lakes in the valley between 

 Spring Vallev and Meiedosia. 



The largest single flight of cormorants was ob.served 

 on October 7, 1940, when an estimated 12,000 passed 

 Havana. Another flight of approximately 9.500 |jassecl 

 that city on October 9, 1949. 



A rapid decline in the numbers of cormorants visit- 

 ing the Illinois River valley occurred after 1950. The 

 largest passage in 1955 occurred on October 14, w^hen 

 4,000 were estimated. By 1958 the great passage of cor- 

 morants had dwindled to onlv SOO which were observed 



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