NONLINEAR POPULATION DYNAMICS OF SIX SPECIES OF FLORIDA CICONIIFORMES ASSESSED BY CHRISTMAS BIRD COUNTS

Abstract

We analyzed 30 years of Christmas Bird Count (CBC) data for Florida populations of six species of wading birds: Great, Cattle, and Snowy Egrets; Little Blue and Tricolored Herons; and the Wood Stork, a federally listed endangered species. These species are conspicuous components of wetland ecosystems, and the target of numerous management efforts in the state and region. The nonlinear dynamics of these populations were assessed through time series of birds seen during volunteer-based surveys and the first differences of successive annual counts, autocorrelations, and two new metrics: cor- relation time and a momentum oscillator. CBCs have the advantage of being conducted regularly over a large geographic scale. When properly analyzed, CBCs are among the few available sources of reliable population information for small, dark-plumaged species such as Little Blue and Tricolored Herons. Population trends assessed for all species by CBCs paralleled the known or suspected trends determined by breeding-season aerial and ground surveys, conducted by professional biologists. Cattle Egret populations have declined fol- lowing rapid expansion through the 1960s and 1970s; Snowy Egret and, to a lesser extent, Tricolored Heron populations have declined throughout the study period; Little Blue Heron and Great Egret numbers have been relatively stable. Wood Storks, following a period of decline that lasted into the mid-1970s, slowly recovered through the 1980s. Only Cattle Egrets displayed strongly deterministic population dynamics, indicating that predictions of population trends for the other species will require stochastic models with broad confidence bands.