Food Habits

Main Foods Taken

Small fishes make up >90% of diet in nearly all regions; insects, crustaceans, and frogs taken probably only when superabundant (Appendix 1).

Microhabitat For Foraging

Able to forage in wide variety of habitats by shifting foraging behaviors and taking advantage of either flock or solitary foraging strategies (Murdich 1978). Forages entirely in wetlands, even more so than other herons, usually in open or semiopen areas with low vegetation (Kushlan 1978). Uses brackish or salt water most frequently; in coastal N. Carolina, Custer and Osborn (1978) recorded no use of available freshwater habitat, but see Bent 1926 and Belser and Post 1987 . Observed foraging in slightly deeper water than Snowy Egret or Little Blue Heron, often in water touching belly-feathers. Powell (1987) suggested 18 cm as maximum foraging depth, as determined by leg length.

Foraging may be particularly productive in thick aquatic vegetation of freshwater lakes—e.g., torpedograss (Panicum repens, Hydrilla) sp. mats—which supports weight of stalking birds but allows access to fishing sites (Jenni 1969, Smith and Collopy 1995). On Lake Okeechobee, FL, open, wet prairies dominated by spike rushes (Eleocharis) selected in much greater proportion than availability would suggest (Smith et al. 1995). In salt marshes of New Jersey, uses edges of potholes and channels as sites from which to strike (Willard 1977). In w.-central Florida, foraged more often along shoreline than other herons (46.7% of observations), 20% of time in pools and open water, and 12% in canals (n = 98 observations; Kent 1986a).

Foraging habitat probably constrained to foraging radius (mean 5.6 km ± 6.0 SD, up to 25 km in Florida Everglades; n = 265; Bancroft et al. 1988) around breeding colony; postbreeding birds shift foraging habitat significantly farther from colony than during breeding (Bancroft et al. 1990).

Food Capture And Consumption

Wide variety of foraging behaviors, including Standing-and-Waiting, Walking Slowly, running pursuit followed by low crouch and quick, nearly horizontal slanting stabs into water (see descriptions of these behaviors in Kushlan 1978). Crouch often very deep, with neck touching breast-feathers, and belly nearly touching water (Willard 1977). Active pursuit common in 2 forms: loping chases, with flapping wings and lunging strikes, or tight pirouetting while turning in direction of open wing. May also feed by covering head and forward body with outstretched wings (Canopy-Feeding), or may use feet (Foot-Raking) to flush bottom-dwelling prey, particularly in cold weather (McIlhenney 1936, Meyerriecks 1971). May also strike at prey while hovering over water (Rodgers 1971, Hancock and Elliott 1978).

Foraging behaviors vary according to habitat. Active techniques found more often at coastal than inland sites (Meyerriecks 1962, Jenni 1969). On coastal flats, used Standing, Walking Slowly, Disturb-and-Chase (“Disturbing-and-Chasing” in Kushlan 1978), and Standing-and-Chasing 17.0, 36.7, 35.6, and 10.7% of the time, respectively (Kent 1986b). Rodgers (1983) reported that vast majority of foraging behavior in a coastal area in Florida was Walking Slowly and Walking Quickly.

Striking efficiencies are higher with Walking Slowly than with Disturb-and-Chase and higher along shorelines, canals, and in pools than in open water (Kent 1987). An entirely diurnal feeder; not noted feeding at night, even in mixed-species nocturnal flocks (L. Bryan unpubl. data, G. V. N. Powell unpubl. data). In multispecies foraging aggregations, usually found at edge of group; not reported to benefit greatly from piracy, beating, or other social interactions (Kushlan 1978).

Major Food Items

Mostly small fish; topminnows and killifishes (Poeciliidae and Cyprinodontidae) are the most common food items in U.S. (see Appendix 1). No information from outside of U.S.

Quantitative Analysis

All recent studies indicate that small fishes constitute >89% of diet consistently over range in se. U.S. (see Appendix 1). Much of the diet (often >50%) is composed primarily of killifishes (genus Fundulus), represented by both estuarine species such as mummichog (F. heteroclitus), and by freshwater species such as golden topminnow (F. chrysotus). Live-bearers (Poeciliidae) are another important group of prey animals, and cichlids (Cichlidae) probably become important in Central and South American range (Haverschmidt and Mees 1994).

Despite this overwhelming dominance of fishes in diet, 2 studies early in twentieth century reported that insects make up large part of diet (O. Baynard in Bent 1926, E. A. Chapin in Sprunt 1954). In freshwater areas of n. Florida, Baynard found >200 grasshoppers (Orthoptera) in a single stomach, and in 50 young birds, 2,876 grasshoppers, 8 frogs, 17 cutworms (Noctuidae), 6 lizards, 67 crayfish (Macrura), but no fishes. In other studies, invertebrate food items represented tiny proportion of diet, including clam worms (Annelida), spiders (Arachnida), weevils (Coleoptera), giant water bugs (Belostomatidae), dragonflies (Odonata), water beetles (Hemiptera), and ground beetles (Carabidae). Marine crabs (Uca spp.) are sometimes captured in coastal areas, but never eaten, suggesting aversion (Kent 1986b).

In Tampa Bay, FL, percentage of consumption of fish in classes 18–30 mm, 31–52 mm, 53–75 mm, and 76–100 mm was 43, 34, 15, and 8% of diet, respectively (Kent 1986b). Took relatively larger fishes than Snowy Egret, relatively smaller fishes than Little Blue Heron, and had a much higher predominance of fish in diet than the other 2 species. Mean weight of whole fishes recovered from regurgitated boluses of nestlings in a s. Florida colony was 0.4 g ± 0.50 SD and averaged total length of 30 mm ± 10 SD (n = 83 prey items; Bancroft and Jewell 1987). Mean volume of prey items in central Florida was 0.46 ml (n = 15 boluses; Jenni 1969). In coastal S. Carolina, 88% of fishes captured were 1–5 cm in total length (n = 90 regurgitated boluses; Post 1990). In coastal Everglades, fishes ranged from 13 to 44 mm in total length; vast majority were 19–35 mm long (n = 514 fish; Ogden 1977).

Basic energy requirements for nonbreeding adults estimated as 322 kJ/d; nesting attempt (adults and young) estimated to require 116,911 kJ of energy to complete; breeding adults expend 63,037 kJ per nesting attempt; young expend 26,937 kJ from hatch to fledging (Frederick and Powell 1994). In Tampa Bay, FL, feeding adults observed to consume 0.94 g/min (267 J/min); estimated foraging times were 51–91 min/d to satisfy minimum daily energetic requirements (n = 98 foraging observations; Kent 1986a).

Because of higher reflectance of lighter plumages, costs of thermoregulation are higher for dark-plumaged than for light-plumaged Egretta herons (Ellis 1980). Nesting in shaded habitats may be an evolutionary response to heat-loading in tropical and subtropical environments; however, light-colored Snowy Egrets also nest in shaded locations. Dark plumage may have other effects, such as making herons less conspicuous to predators, and less likely to be joined by other wading birds in foraging flocks (Kushlan 1978, Caldwell 1986).

An estimated 1.77 kg (dry weight) feces total are produced by parents and young during a nesting attempt (Frederick and Powell 1994). Pellet-casting not noted in this species, although other herons frequently cast boluses of shell, bone, and other indigestible ejecta.