Foraging Decisions, Patch Use, and Seasonality in Egrets (Aves: Ciconiiformes)

Abstract

Feeding Snowy (Egretta thula) and Great (Casmerodius a/bus) egrets were observed during two breeding seasons in coastal New Jersey and two brief winter periods in northeast Florida. A number of tests based on assumptions of foraging models, predictions from foraging theory, and earlier empirical tests concerning time allocation and movement in foraging patches was made.Few of the expectations based on foraging theory and/or assumptions were supported by the empirical evidence. Snowy Egrets fed with greater intensity and efficiency during the breeding season (when young were being fed) than during winter. They also showed some tendency to leave patches when their capture rate declined, and they spent more time foraging in patches when other birds were present nearby. Great Egrets, in contrast, showed few of these tendencies, although they did leave patches when their intercapture intervals increased. Satiation differences had some influence on feeding rates in Snowy Egrets, but only at the end of feeding bouts.Some individuals of both species revisited areas in patches that had recently been exploited, and success rates were usually higher after the second visit. Apparently, for predators of active prey, shortterm changes in resource availability ("resource depression") may be more important than resource depletion, a common assumption in most OFT models.

Quantitative analysis of foraging habitat use by ciconiiformes in the upper Paraná river Floodplain, Brazil

“…The species with tactile (Wood Stork and Roseate Spoonbill) or tactile-visual (Jabiru) foraging techniques were more dependent on the patches with low depth and high prey density than were species that forage visually (Kushlan et al, 1985;Gaines et al, 1998). Snowy Egret is often the first species to locate high-quality patches, acting as a catalyst for larger aggregations (Caldwell, 1981), on which it depends in the periods of high energetic demand (Erwin, 1985;Master et al, 1993). These four species have white or white-pink (Roseate Spoonbill) plumage and forage socially, two characteristics intimately related (Beauchamp and Heeb, 2001).…”

“…Consequently, the studies on Ciconiiformes habitat use must employ large spatio-temporal scales (McCrimmon et al, 1997;Strong et al, 1997). In addition, most Ciconiiformes prey primarily on the fishes (Del Hoyo et al, 1992), whose distribution is strongly influenced by water levels (Erwin, 1985;Gawlik, 2002). This opportunistic way of habitat selection is commonly described in the wading bird literature (Fasola, 1986;.…”

An extension of the floodpulse concept (FPC) for lakes

“…High trophic-level organisms such as birds and mammals have received much less consideration in recent WLF-related studies (see review paper by Leira & Cantonati, this issue); however, there is a large database on wading bird species that depend on the floodpulse for foraging in shallow waters and exposed sediments, and for breeding on the dessicated ground (e.g., Erwin, 1985;David, 1994).…”

Influence of lagoons size and prey availability on the wading birds (Ciconiiformes) in the upper Paraná river floodplain, Brazil

“…According to Gawlik (2002), when referring to the prey availability is necessary not only the quantification of these, but also know to that point are its vulnerable to the predation. When the preys are fishes, very mobiles animals that evolved effective mechanisms to reduce the probability of predation, the vulnerability of these fishes in a certain habitat can be more important for the predator than the amount of individuals preys presents (Erwin, 1985). The weak and no significant correlation values of the Ciconiiformes diversity, number of species and relative abundance with the fish density during February were probably associated to the hydrometric level of the floodplain.…”

“…Although there are no studies about the Ciconiiformes breeding in the region, possibly the most of the species breed in the drought period, as happens at the Pantanal Matogrossense (Willis, 1995). Adult birds raising nestlings face intense demands for food acquisition, needing to optimize its foraging efficiency and energy intake (Erwin, 1985). This can be facilitated foraging in the lagoons with more fish availability in the study area.…”