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Foraging Ecology of Wading Birds Using an Altered Landscape in Central Florida


We conducted a field study during 2 consecutive breeding seasons (1988 and 1989) on the foraging ecology of wading birds (Ciconiiformes) in a highly disturbed landscape. In particular, we examined the foraging ecology of wading birds using Lake Hancock, a hypereutrophic lake. Our efforts also focused on the relative use of various wetland types, particularly phosphate mines and other altered, nutrient-enriched ecosystems, by Snowy Egrets. In 1988 and 1989, we conducted boat surveys around the lakeshore quantifying the number of foraging wading birds. We also measured several water parameters. Numbers of wading birds foraging at Lake Hancock increased during both years when water levels were seasonally low. In 1989, the drier of the two years, the number of wading birds was inversely correlated with water level. Furthermore, the total number of wading birds sighted in 1989 was correlated positively with water temperature and secchi depth, and negatively with dissolved oxygen.

Twelve species of wading birds were observed foraging at the lake during the two years. Both Great and Snowy egrets exhibited an irregular, but similar pattern of lake use during both years. As most of the lake was too deep to wade into, foraging birds took advantage of numerous perch substrates: this included floating mats of red maple and willow, dense cattails, and fishing nets. They also obtained fish from deep water areas by foraging aerially. The effects of poor water transparency and a limited littoral zone may be offset by the substantial fish population and limited human disturbance at the lake.

Through radio-tracking, we identified the foraging areas used by Snowy Egrets nesting at the colonies at Lake Hancock. We located 17 radio-tagged Snowy Egrets 567 times at 98 different foraging sites during the 2 breeding seasons. During the 1988 and 1989 breeding seasons, radio-tagged Snowy Egrets foraged more often, foraged in larger groups, and flew farther to feed in artificial habitats associated with phosphate mining than they did in natural habitats. The use of these artificial sites is likely due both to their temporal and spatial availability and their high biological productivity.

The patterns of use of this hypereutrophic lake and nearby phosphate mines by foraging wading birds dispels some of the popular concepts about the foraging habitat requirements of wading birds. Both areas are non-pristine, deep water systems. In both situations, aerial foraging, an unusual and energetically expensive foraging behavior, was frequent. The results of this study demonstrate the importance of understanding the function of altered and artificially nutrient-enriched wetlands as alternatives to lost natural wetlands in a growth state such as Florida.