Herons in Everglades National Park

Frohring, Paula

A report on a study exploring the hypothesis that hydrological conditions correlate with the nesting success of four wading birds: Great Egret, Snowy Egret, Tricolored Heron and Little Blue Heron. Several species of wading birds, including herons, are thought to have decreased in population in southern Florida over the several decades since the dedication of Everglades National Park in 1947 (Robertson and Kushlan 1974). Although definitive historical data are limited or lacking, all available evidence suggests that substantial decreases have occurred, at least in the past two decades following the institution of management of the surface water flowing into Everlgades National Park. Such a decline has been documented best in the Wood Stork (Mycteria americana) (Kushlan and Frohring 1986, Ogden et al. 1987). The collapse of the southern Florida Wood Stork population is clearly related to the effects of drainage and water management (Kushlan et al. 1975, Ogden et al. 1978, Kushlan and Frohring 1986, Ogden et al. 1987). The changing population status of other wading birds is less well documentable, but it seems likely that other species nesting in the normal spring dry season might also be affected by changes in the hydrological regime that determines habitat availability and foraging effectiveness. The focus of the current study was to determine patterns of habitat use, prey consumption, and nesting success of four species of herons and to seek relationships between nesting success and environmental conditions. Heron species studied were the Great Egret (Casmerodius alba), Snowy Egret (Egretta thula), Tricolored Heron (Egretta t tricolor), and the Little Blue Heron (Egretta caerulea). All are long legged aquatic birds that nest colonially (see Hancock and Kushlan 1984 for an account of their general ecology). In the southern Florida, they often nest together and feed in the interior and coastal wetlands, including the Everglades marsh. Our study was conducted in southern Florida in 1977-1981. The study concentrated on birds that nested in the spring drying and fed in the interior Everglades of Everglades National Park. For comparisons we also studied colony sites in the Everglades of Conservation Area 3, the'Big Cypress Swamp, and Florida Bay in some years.