Habitat Use by Wading Birds in a Subtropical Estuary: Implications of Hydrography

-The dynamics of foraging habitat use by long-legged wading birds was analyzed with respect to water-level fluctuation patterns in Florida Bay. Wading-bird presence at four sites situated to sample the heterogeneity of the bay was quantified by repeated surveys collected throughout the day and year. Models for habitat availability were generated using water-level data collected from continuous recorders, staff gauges, and habitat profile maps. These models were tested against the survey data. Roseate Spoonbills (Ajaia ajaja) foraged on the study areas primarily at night. Great Blue Herons (Ardea herodias) fed both day and night, but primarily at night where the tidal range was small. Great Egrets (Casmerodius albus), Snowy (Egretta thula) and Reddish (E. rufescens) egrets, Little Blue (E. caerulea) and Tricolored (E. tricolor) herons, and White Ibis (Eudocimus albus) fed during daylight. Where tidal range was small (< 5 cm) diurnal species fed throughout the day. Florida Bay has a pronounced annual water-level cycle that causes monthly mean water levels to vary by as much as 30 cm between October (high) and May (low). Models derived from hydrology data predicted that this seasonal variation in water level would have a major impact on habitat availability, particularly where tidal flux was small. The predictions were supported by survey data. At sites with minor tides, most wading-bird species had a cycle in seasonal abundance that correlated with seasonal changes in water level; only the tallest species, Ardea herodias, was uniformly present throughout the year. The large daily range in tide (xf = 80 cm) afforded year-round access to foraging habitat, and these abundance patterns did not exist. The seasonal variability in habitat availability has major management implications because the maintenance of stable wading-bird populations depends on the availability of alternative foraging sites when water levels are high. Historically, these sites have tended to be targeted for human development. Received 20 October 1986, accepted 11 June 1987. THE dynamics of foraging habitat use by wading birds have been well studied for shortlegged waders (Charadriiformes; Goss-Custard 1970, Holmes 1970, Burger et al. 1977, GossCustard et al. 1977, Silliman et al. 1977, Myers 1980, Connors et al. 1981, Fleischer 1983) but are poorly known for long-legged species (Ciconiiformes). This is especially true for marine systems, where studies have focused on major environmental cycles (see Kushlan 1981 for review). Ciconiiformes (hereafter "wading birds") are known to alter their feeding-site selection in relation to major seasonal climatic changes (Ryder 1967, 1978; Henny 1972) and the lunar tidal cycle (Krebs 1974, Brandman 1976, Bayer 1978, Custer and Osborn 1978a, Pratt 1980). Beyond the patterns imposed by these major cycles, little is known about the dynamics of habitat use by wading birds. In marine and estuarine ecosystems of southern Florida, where seasonal variations in the annual temperature cycle are relatively minor and the lunar tidal range tends to be small (NOAA tide tables 1986), habitat use by wadingbird populations is potentially independent of these factors. Therefore, an analysis of habitatuse patterns may identify other factors that function as determinants of habitat use, By combining data on habitat use by wading birds at a series of locations in Florida Bay in conjunction with an analysis of habitat topography and hydrography, I tested two null hypotheses. First, in the absence of restrictions imposed by a tidal cycle, foraging habitat use would be independent of time. And second, in the absence of a strong annual climatic cycle (temperature), habitat use by wading birds would be constant throughout the year.