Martell, M.S., J.B. Nibe, P.T. Redig, and G.M. Buhl. 1991. A study of bald eagles wintering along the Mississippi river between St. Paul and Red Wing, Min., and the St. Croix river south of Hudson, Wis., 1987-1990. Final report submitted to the Nongame Wildlife Program, Minnesota Department of Natural Resources. 53 pp.
Bald Eagles (Halieetus leucocephalus) were studied during winter along portions of the St. Croix River, from Hudson Wis. to its confluence with the Mississippi River at Prescott Wis., and along the Mississippi River from Prescott Wis. to the north end of Lake Pepin. Literature searches, interviews with wildlife managers, biologists, and local residents were used along with ground searches to locate winter use areas. At these areas counts were made of eagle foraging and night roosting. The study encompassed three field seasons; the winters of 1987-88, 1988-89, and 1989-90.
Nine winter use locations were identified and monitored. Pigs Eye Island (PEI) (river mile (rm) 836-832) was the northernmost area studied. PEI was used during this study for feeding (high of 13 birds), and nesting. No roosting was observed although other studies have documented this behavior at PEI. Grey Cloud Island (rm 828-817) was used only during spring and fall for both feeding and roosting. Prescott, Wis. (rm 812-808) was used on a regular basis during the study as a feeding site by up to 19 eagles. Eagle Point/Big River Coulee (rm 807-805) and North Lake (rm 804-802) were used as roosts with feeding occurring nearby along the Mississippi River. Both of these areas had fluctuating use with numbers at the Eagle Point area declining during the study, possibly due to residential development in the area. Two other areas along the Mississippi River were also studied; Daimond Bluff (rm 801-800) and Prairie Island Lock and Dam #3 (rm 797). Both of these areas were used for feeding, but accurate counts were difficult to obtain from the ground.
Eagle congregated along the St. Croix River before it froze at Lake Mallalieu in Hudson, Wis. in large numbers (up to 77 birds) when fish die-offs occurred. A few birds were seen all winter at the Kinnickinic delta.
The availability of a food source and open water seemed to be the most important factors in determining where eagles congregated. Adults typically out-numbered immatures during the winter. Large movements of eagles did not take place except during migration. Night roosting seems to occur in trees along the river as well as at communal sites in more protected areas away from the river. Protection of feeding and roosting sites is important in maintaining healthy Bald Eagle populations in the North-Central United States.