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• • • 7 The only other data on feeding habitat on the upper Mississippi River were done in 1978 (Thompson 1978b). Thirty five herons and egrets were followed by airplane from the Dairy land Power rookery at the north end of Pool 9 right below Lock and Dam 8 (Thompson 1978b). The mean distance flown, 6.5 km for great blue herons and 8.6 km for great egrets, was similar to the median distance of 7.4 and 7.0 km in this study from the Smith Slough rookery. Thompson (1978b) also found that most herons and egrets fed within the flood plain corridor. The maximum distance traveled, 20.4 km for a great blue heron and 35.2 km for a great egret, was less than in this study, but the fewer number of birds followed, probably accounted for that. In both Thompson's (1978b) and this study great egrets flew farther than great blue herons. This may be due to the more restricted feeding habitat of great egrets. Management Implications The first step in preserving and improving heron and egret feeding habitat is knowing what constitutes good habitat. Both herons and egrets seem to require shallow- water, protected-areas, which contain food, within 7 - 12 km of the rookery. Ideally,good feeding habitat should also be within 1 km rookery. Because great blue herons can feed from perches, such as logs and rocks, they can utilize a slightly broader array of habitats than can great egrets, which mainly feed by wading. When given a choice great_ egrets prefer smaller ponds and marshes and avoid large, lakes,, probably because most of the surface area of large lakes is not available to great egrets for feeding. Both species can fly long distances (> 40 km) to find feeding habitat, but most feed close to their rookery. Each species may fly long distances when temporary conditions, such as spring floods, reduce feeding habitat nearby. Because it is energetically costly to fly long distances to feed, they probably can not do this, however, on a long term basis or during periods, such as when young are rapidly growing, which have high energetic costs. References Custer, T. W., and C. Bunck. 1992. Feeding flights of breeding double- crested cormorants at two Wisconsin colonies. J. Field Ornithol. 63:203 -211. Custer, T. W., R. K. Hines, and C. M. Custer. 1996. Nest initiation and clutch size of great blue herons on the Mississippi River in relation to the 1993 flood. Condor 98:181 -188. Custer, T. W., and R. G. Osborn. 1978. Feeding habitat used by colonially- breeding herons, egrets and ibises in North Carolina. Auk 95:733 -743. Gibbs, J. P. 1991. Spatial relationships between nesting colonies and foraging areas of great blue — herons. Auk 108:764 -770. Krebs, J. R. 1974. Colonial nesting and social feeding as strategies for exploiting food resources in the Great blue Heron (Ardea herodias). Behaviour 51:99 -134. Miller, R. F. 1943. The great blue heron. The breeding birds of the Philadelphia region (Part II). Cassinia 33:1 -23. McNeil, R., R. Benoit, and J -L. Desgranges. 1993. Daytime and nighttime activity at a breeding