Vol. 25, No. 7 
Page 5 
respectively. The Seneca-Spring Valley segment hosted 44 additional eagles in 
1931. Areas of eagle concentration in the Illinois Valley included Plum Island 
below Starved Rock Lock and Dam, Goose Pond (Putnam County), Meyer's Ditch near 
Lake Chautauqua, Grand Island near Bath, the Sangamon River mouth area, and the 
Six-mile Island area of Swan Lake. 
In the Mississippi River valley, bald eagles increased gradually until late 
December 1931 when their numbers reached the following peaks: 262 birds from 
Moline to Alton during the 1st week of March, 197 birds from Dubuque to Moline in 
February, and 135 birds from St. Louis to Cairo in January. Major concentrations 
of the birds occurred below dams, particularly the Oquawka and Keokuk dams, where 
the water remained open for fishing during cold weather. Many birds were also 
observed in the vicinity of the Keithsburg, Louisa, Batchtown, and Shanks refuges 
and the Chester-Cairo area. A few individuals of the 1931-82 winter population 
also dispersed to cooling lakes and reservoirs throughout the state at an average 
density of 2 eagles per lake. Strip mine lakes in Fulton and Knox counties 
hosted additional individuals with a peak of 14 birds in early January. 
Immature bald eagles 1-4 years of age can readily be identified by their 
mottled brown plumage, which contrasts greatly with the adult's conspicuous white 
head and tail. Approximately 35% of all eagles observed during the winter of 
1981-82 were immatures as compared with 30% for the winters of 1976 through 1973. 
The actual percentage of immature eagles in the population is probably higher 
because the dull-colored immatures are more easily overlooked than adults when 
perched in shoreline trees. 
In early March this spring, the bald eagles began leaving for their breeding 
grounds. The exodus was completed rather abruptly by the last week of March. 
