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• 6 The time required and distance traveled to reach a feeding site were correlated, but each metric provides slightly different information (Fig. 2 - 11). The time spent is most useful in determining energetic costs, whereas, distance is more useful in quantifying various habitat parameters. The maximum distance traveled to a feeding site was between 26 and 43 km (Fig. 7 - 11). The mode was always less than 1 km in all cases. The median distance traveled was similar between great blue herons and great egrets at Smith Slough, 7.4 and 7.0 km, but was different between the species at Peltier Lake. At Peltier Lake, great blue herons traveled a median distance of 2.7 km compared to 11.8 km for great egrets. The median distance would be even greater at Peltier Lake, 115 km, if the 6 great egrets that flew to a roost site on the shore of Peltier Lake were disregarded. The reason for this difference in distance flown between great blue herons and great egrets may be accounted for by the habitat preference noted above in Table 2. The rookery is located on an island in Peltier Lake, which is 1 of several large lakes in the area. There are few small ponds or marshes in the immediate vicinity of the rookery, hence the great egrets had to fly farther to find preferable feeding habitat than did the great blue herons. The median distance flown to feeding sites at tie _V rmillion River rookery was smaller than at the other rookeries, however, because only nights were made. The median distance would have been greater had more flights been made.A The spatial distribution of heron and egret feeding sites varied:by_rookery location (Figs. ° 12 - 17). The Peltier Lake birds flew fairly uniformly out in all directions from the rookery (Fig. 12 - 13). The Mississippi River rookeries, Smith Slough (Fig. 14 - 15), Vermillion River (Fig. . 16), and Pigs Eye (Fig. 17), however, had feeding flight lines constrained by the course of the upper Mississippi River. The birds that were lost en route, square yellow boxes on figures 12 - 17, were lost for several reasons. Those at Pigs Eye were lost mainly because of interference with other airplanes or lack of access to restricted air space. At the other rookeries, however, losses were mainly do to the difficulty of following birds that began to circle and gain altitude rapidly. These were often the birds that were leaving the Mississippi River to go inland to feed. They apparently found that it was energetically beneficial to gain altitude by riding thermals and then to coast to distant feeding sites. The distance flown, time to fly to feeding sites, and feeding habitat characteristics at Vermillion River and Smith Slough rookeries were similar to other rookeries on the upper Mississippi River that the author has worked during the past 3 years. Herons and egrets utilize shallow water, braided channel habitat near their rookery, with some off -river feeding. The use of off river feeding habitat is dependent on water levels on the upper Mississippi River for both species. Water level on the upper Mississippi River seems to be more important for great egrets than for great blue herons, however. When the river stage is high, especially in the spring, many to most great egrets may leave the upper Mississippi River to feed because there is little shallow water within the flood plain. As noted above, great egrets land in water and then proceed to feed. They seem to rarely feed from perches such as logs and rocks. Great blue herons, on the other hand, will feed from perches, so they can use deeper water habitats and hence still feed on the upper Mississippi River even during higher water levels. In 1998, the water levels on the upper Mississippi River were generally lower than normal; the spring floods were of less magnitude. IP Because of this, there was more on -river feeding by Smith Slough herons and egrets than has been seen at other rookeries in other years.