Distribution of Wading Birds Relative to Vegetation and Water Depths in the Northern Everglades of Florida, USA

Abstract

The response of Great Blue Herons (Ardea herodias), Great Egrets (Casmerodius albus), Wood Storks (Mycteria americana), and White Ibises (Eudocimus albus) to water level (index of depth) and vegetation in the northern Everglades of Florida was studied in two years, each with dissimilar water levels. A regression model was constructed for each species in an average year (1988) and a dry year (1989) to examine the relationship of bird abundance to water depth and area of eight vegetation classes. The analyses showed that bird abundance is related to both water level and the vegetation community, but water level generally had the greatest effect. Models showed that in the average year (1988), there was a water level threshold, above which bird abundance was predicted to decline. The level threshold varied among species and may have reflected species-specific foraging constraints. However, in the dry year (1989), the relationship between bird abundance and water level was positive and linear, indicating that few places had water deeper than the thresholds observed in the average year. Overall, the area of slough vegetation appeared to have the second greatest effect on bird abundance. Generally, all models had low coefficients of determination (R2 range 0.06-0.42) suggesting that factors other than water level and vegetation were important, or birds were responding to variables in the model, but at different spatial scales than that which the data were collected. Models for Great Blue Herons and Great Egrets had higher coefficients of determination than models for Wood Storks and White Ibises. The more solitary feeding behavior of the herons and egrets resulted in a more even distribution across the marsh than for storks and ibises, which were usually found in flocks. Our study suggests that if restoration of the Everglades results in more natural hydrologic cycles, an increase in the amount of slough habitat, and a decrease in the proportion of cattails, foraging conditions for wading birds may improve.