. Clarksvllle (12) Horseshoe Lake (10) \© Fig. ;)cali<)iis of essential bald eagle wiiilei Kliriiilied by Southcin cl al. 1985. Numbers inditale iin|joilaiin- rankiiiH- linois, bald eagle census data for Illinois should be considered anestimateor index of actual pojjulalions. For a variety of identified and imdeteriTiined reasons, counts of bald eagles made on a specific area are subject to extreme short-term variations. This vari- ation is well demonstrated by data from Fischer (1982). In his study, two or three ground counts often were made on the same day, and those co
. Clarksvllle (12) Horseshoe Lake (10) \© Fig. ;)cali<)iis of essential bald eagle wiiilei Kliriiilied by Southcin cl al. 1985. Numbers inditale iin|joilaiin- rankiiiH- linois, bald eagle census data for Illinois should be considered anestimateor index of actual pojjulalions. For a variety of identified and imdeteriTiined reasons, counts of bald eagles made on a specific area are subject to extreme short-term variations. This vari- ation is well demonstrated by data from Fischer (1982). In his study, two or three ground counts often were made on the same day, and those counts differed by as much as 35% or 150 eagles. However, variations in the numbers of eagles seen were not consistent. Numbers of eagles sometimes increased or decreased during the same day. I'he lack of sinuiltaneous counts over a wide area and the mobility of bald eagles make it diffic ult tf) determine whether daily flue luations in numbers were c aiised by lo( al movements or bv shifts in populations over larger geograjjhic aieas. Dunstan and Fawks (19HI) stated that food avail- ability is the primary reason (or the southwaid move- ment of bald eagles into Illinois during wintei. Keister et al. (1987) reported that the interaction of food avail- ability and proximity of loosting habilal appealed to be a primary factor in the behavior of wintering bald eagles. Structures such as locks and dams or activities that maintain open water in winter and, therefore, the availability of fish serve as an attractant to bald eagles, partic ularly if suitable daytime perches and night roost areas are nearby. As noted by Steenhof (1978), the abundance and distribution of wintering bald eagles have been greatly af fee ted by construe tion of dams and impoundments. Fischer (1982) found that the extent of ice cover on the Mississippi River influenced the local distribution of eagles. As the area of open water changed, the eagles shifted their pat- terns of use to take advantage of available f
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