. Distribution and abundance of winter populations of bald eagles in Illinois. Bald eagle. Illinois Natural History Survey Biological Notes have the greatest influence on the accuracy of bald eagle counts. Fog, rain, snow, and wind not only make it difficult for observers to see and age eagles but such conditions also may influence the activity of eagles, thereby making them less visible. Difficulties in obtaining accurate determinations of eagle numbers in Illinois do not eliminate the value of such counts. The counts can identify important winter areas and document trends in bald eagle popu-
. Distribution and abundance of winter populations of bald eagles in Illinois. Bald eagle. Illinois Natural History Survey Biological Notes have the greatest influence on the accuracy of bald eagle counts. Fog, rain, snow, and wind not only make it difficult for observers to see and age eagles but such conditions also may influence the activity of eagles, thereby making them less visible. Difficulties in obtaining accurate determinations of eagle numbers in Illinois do not eliminate the value of such counts. The counts can identify important winter areas and document trends in bald eagle popu- lations. The bald eagle inventories organized by Elton Fawks, for example, were prompted by concern over the low proportion of immatures seen in winter popu- lations during the 1950s (Fawks 1983). After quickly increasing from in 1961 to in 1965, the proportion of immatures sighted on Fawks' counts has fluctuated slightly around a value of 30%. As with any migratory species, suitable winter habitat is essen- tial to the survival of individuals and the subsequent reproductive success of the population. NIGHT ROOSTS Night roosts generally offered seclusion from human disturbance and protection from cold winds. At many of the roost sites, a windbreak was provided by the walls of a ravine or glen. On level areas, eagles selected trees for roosting that were protected by other trees (Fig. 28) (Fischer & Osterfeld 1981, Sabine & Klimstra 1985). Cotton woods (Populus deltoides) were the preferred roost tree in some areas, and it appeared that size and configuration of available trees were im- portant. Bald eagles generally selected trees that were taller than surrounding ones and had large, horizontal branches (Sabine & Klimstra 1985). Although bald eagles perch and forage along major stretches of the Illinois and Mississippi rivers, relatively few areas have been identified and reported as night roosts (Fig. 29). Other undiscovered or unre- ported night roost
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