Food Habits

Main Foods Taken

Wide range of prey items including earthworms, annelid worms, aquatic and terrestrial insects, crabs, shrimp, prawns, crayfish, other crustaceans, snails, freshwater and marine fish, frogs/toads, and snakes/lizards (Kushlan 1978a, 1978b).

Microhabitat For Foraging

Great variety including salt-marsh pools, tidal channels/flats, freshwater marshes/swamps, ocean inlets and lake margins. Generally prefers brackish/marine habitats with relatively shallow water (Willard 1977, Custer and Osborn 1978a). Has taken advantage of prey concentrations caused by declines in water level and areas where brackish and salt water mix (e.g., ocean inlets; Kushlan 1976b, Parnell and Soots 1978, Kushlan et al. 1985, Powell 1987, TLM). High water increased foraging flight distances for individuals at Lake Okeechobee, FL (Smith 1995). Water depth determined number of foraging individuals in Florida and N. Carolina wetlands (Kushlan 1976b, Custer and Osborn 1978b, Edelson and Collopy 1990). Mean water depth used for feeding (including gleanings off vegetation) in San Francisco Bay, CA, was 3 cm, with strikes taking place in shallow water near dense vegetation (Hom 1983). Water depth in salt-marsh pools used in s. New Jersey: 2–15 cm (Willard 1977). Experimental flocks of model Snowy Egrets attracted significantly more individuals to shallow (<20 cm) as opposed to deep (>20 cm) salt-marsh pools in s. New Jersey (TLM). In Florida, foraged along shorelines (47% of microhabitats) using walk slow behavior, and in pools (34%), where they foot-stirred (Kent 1986a, 1987). Preferred foraging along the edge (<1 m from shoreline) of salt marsh pools in s. New Jersey where foot-stirring, striking, and captures were most prevalent; larger pools (>100 m²) visited more frequently than small ones (Master 1989).

Food Capture And Consumption

Broadest behavioral repertoire (21 of 34 described behaviors) of all North American herons (Willard 1977, Kushlan 1978a). Behaviors include standing, bill-vibrating (tongue-flicking), head-swaying, pecking, walking slowly, walking quickly, running, hopping, leapfrog feeding, wing-flicking, openwing-feeding, underwing-feeding, foot-stirring, foot-raking, foot probing, foot paddling, hovering, hover-stirring, dipping, disturb and chase, and foot-dragging. Effectiveness may be enhanced owing to greater visual acuity than most other wading birds (Caldwell 1981).

Many behaviors make use of distinctively colored feet (Kushlan 1978a). Only heron that uses 4 of the 5 foot movement behaviors (Hancock and Kushlan 1984). Foot-stirring is particularly suited for small prey consumed by this heron (Willard 1977, Hom 1983; see Diet, below). Foot-stirring associated with 66% (n = 6043) of strikes in San Francisco Bay (Hom 1983). Most frequently used foraging behaviors in s. New Jersey and Tampa Bay, FL, were walking slowly, foot-stirring, and disturb-and-chase (Willard 1977, Kent 1986a). In Tampa Bay, strike efficiency much lower using walk slow, foot stirring, and disturb-and-chase behaviors than standing, but did not differ between microhabitats (Kent 1987).

In central Florida, breeders fed using hovering foraging techniques (including hovering over water, and sometimes raking water with feet) in artificial wetlands with fish concentrated at water surface (Edelson and Collopy 1990). In Louisiana, fed by dipping (i.e., putting head down while flying to catch prey) on dead fish discarded by shrimp trawlers (Rodgers 1974).

Variety of behaviors used not reflective of broad diet. A general survey indicated 4 most common prey items make up almost 75% of diet, suggesting species can be a specialist (Kushlan 1978a; see Diet, below). Individuals faced with relatively wider prey choice in Venezuela used fewer behaviors than those in s. Florida Everglades where prey choice was narrower (Kushlan et al. 1985). Behavioral flexibility suggests adaptability to a wide range of environmental and social foraging conditions. Used a variety of animated behaviors, especially disturb-and-chase, when foraging within confines of isolated salt-marsh pools in s. New Jersey (TLM). Switched to bill vibrating during spring tides when marsh is flooded and fish no longer confined to discrete pools (Master 1991). Luring fish may be more efficient than chasing under such conditions (Buckley and Buckley 1968, Master 1991). Bill-vibrating specifically attracted mosquito fish (Gambusia spp.) in Florida (Kushlan 1973).

Breeding individuals foraged primarily during morning and evening hours in central Florida (Edelson and Collopy 1990), central and s. New Jersey (Erwin 1985, Master 1989), and California (Hom 1983). Nonbreeders did not feed after dark in Florida Bay (Powell 1987). In s. New Jersey, 97.4% of observed mixed-species foraging aggregations formed at or just before sunrise (Master 1992).

Foraging activity, including strike and capture rates, was greater during breeding season (central New Jersey) than during winter (n. Florida; Erwin 1985). Capture efficiency in Florida during the nonbreeding season was 42.8% (Kent 1986b). Immatures struck more frequently than adults in Tampa Bay (3.1 vs 1.6 strikes/min) but success rates were not different (Rodgers 1983). Capture rate was greater under cloudy than sunny conditions (Rodgers 1983). Commonly revisits feeding patches, 7.5 min (range 1–15 min) after first visit, experiencing higher foraging (Erwin 1985, 1989).

Group flights from breeding colonies to feeding sites made up 7% of foraging flights in New York (Maccarone and Parsons 1988), 4–5% of flights in N. Carolina (Custer and Osborn 1978a, Erwin 1983), and 29% of flights in Florida (Ogden 1978). Individuals flew an average of 2.8 km from colonies to foraging areas in vicinity of Lake Okeechobee, FL (Smith 1995). Such group flights may facilitate the formation of foraging aggregations.

Species functions as focal species attracting other species to foraging aggregations, a process known as local enhancement (Rodgers 1974, Caldwell 1981). At Stone Harbor, NJ, model Snowy Egret flocks, as compared to Great Egret (Ardea alba) and mixed-species model flocks, attracted the most Snowy Egrets and individuals of all other species combined (Master 1992). In Panama, attractiveness of foraging flocks was also a function of Snowy Egret abundance (Caldwell 1981).

In New Jersey, feeding flock formation and size during the breeding season was greatest following spring tides (142.8 ± 10.4 SE, n = 78), which replenish salt marsh–pool fish populations; mean size was 130 individuals (range = 5–530), of which about 75% were Snowy Egrets (Master 1992). Overall, membership in an aggregation prolongs time spent in a patch relative to solitary egrets (Itzkowitz 1984, Erwin 1985). Solitary foragers in s. New Jersey spent <30 min at foraging sites; aggregations typically remained as a group in the same area >4 h (n = 108; Master 1992).

Foraging in large aggregations, often with other wading species, improves foraging success (Kushlan 1977, Erwin 1983, Master et al. 1993; Fig. 2). Greater foraging rates (captures/min) and efficiency (captures/strike) in aggregations attributed to greater prey availability owing to disturbance (prey beating) by neighboring birds (Willard 1977, Master et al. 1993). Feeding in aggregations versus solitarily in s. New Jersey improved capture rate (1.9 vs. 1.3 captures/min) and efficiency (64% vs. 42%) although strike rate (3.4 vs. 4.1 strikes/min) and prey length (2.7 vs. 2.9 cm) were not significantly different (Master et al. 1993). Stepping rates were significantly lower in aggregations, reducing energy expenditure and thus improving net energy intake. Foraging was concentrated near center of aggregations (82% of observations of individuals within 10 m of center of aggregation; n = 170) where disturbance was most intense and need for stepping greatly reduced. In nonbreeding aggregations in Costa Rica, strike rate increased, apparently the result of disturbance, but capture efficiency dropped compared to feeding solitarily (Russell 1978). In Panama, by contrast, as proximity to other foraging wading birds increased, foraging success of associate species also increased; foraging efficiency and flock size decreased, and inter-individual distances increased after attacks by avian predator (Caldwell 1981, 1986).

Fed opportunistically and more efficiently in association with other nonwader species including Pied-billed Grebe (Podilymbus podiceps; Leck 1971), Common Tern (Sterna hirundo; Bertin 1977), Red-breasted Merganser (Mergus serrator; Parks and Bressler 1963, Emlen and Ambrose 1970, Lamm 1975), Hooded Merganser (Lophodytes cucullatus; Parks and Bressler 1963), and Cattle Egret (Wiese and Crawford 1974). Fed on insects stirred up by cattle in Florida (Rice 1954).

Few interactions among species foraging in aggregations and little interspecific diet overlap recorded in San Francisco Bay during the breeding season (Hom 1983) and in Florida during the nonbreeding season (Kent 1986a). Few aggressive interactions observed in s. New Jersey and those that occurred had no effect on foraging success (TLM). Herons in s. New Jersey aggregations also tended to separate by position within aggregations with white species near center and dark species on the periphery (Master et al. 1993).

Major Food Items

In 4 major estuaries along the Atlantic coast, regurgitations to nestlings revealed a narrow choice of prey—approximately 75% fish and 25% crustaceans; other studies have shown similar results (see Feeding, above). In Netherlands Antilles, mostly consumed small fish (Voous 1983). In a provisioning experiment, large fish selected over small fish, suggesting that experience affects choice (Itzkowitz and Makie 1986). In s. New Jersey, preferred prey included sheepshead minnows (Cyprinodon variegatus), mummichogs (Fundulus heteroclitus), and prawns (Palaemonetes pugio) with polychaetes, elvers, mud fiddlers (Uca pugnax), blue crabs (Callinectes sapdidus), and green-headed flies (Tabanus spp.) included occasionally (Willard 1977, Itzkowitz 1984, Master 1989). In central New Jersey, Atlantic silversides (Menidia menidia), killifish (Fundulus spp.), and bay anchovies (Anchoa mitchilli) were major prey items. Mummichogs were major prey in coastal N. Carolina (Kneib 1982). Pipefish (Syngnathus spp.), needlefish (Strongylura spp.), and mosquito fish (Gambusia spp.) consumed in Florida (Kushlan 1973, Kent 1986a). Mean total length of fishes caught in s. New Jersey was 2.81 cm (Master et al. 1993). Fish size ranged from 1.8 to 3.0 cm at Tampa Bay, FL (Kent 1986a). Mean prey size, including annelids, crustaceans, insects, and topsmelt (Atherinops affinis), of breeders in San Francisco Bay was <1 cm (Hom 1983).

Quantitative Analysis

In San Francisco Bay, CA, 94% of all prey (145 individuals; 3,100 captures) were too small to identify or estimate size (Hom 1983). Nestling regurgitations in Florida were 87% fish (primarily mosquito fish and flagfish [Jordanella floridae]), 7% invertebrates, and 5% amphibians by volume (Jenni 1969). In S. Carolina, regurgitations were 60% salt-water fish, 35% salt-water crustaceans (Palaemonetes spp.), and 4% freshwater fish by number (Post 1990). In inland Texas, regurgitations consisted of 4 species comprising 60% fish and 40% crayfish (Cambarus spp.; Telfair 1981).

The role of foraging behavior in prey selection has been little studied in any heron species (Kushlan 1978a). The degree of dispersion of food resources determines the tactics and behaviors used in acquiring prey. Snowy Egrets are selective foragers by virtue of their pursuit or chase style of foraging (Kushlan 1978a). Approximately 78% of their diet is composed of only 4 of the 29 species of prey generally consumed by this species across its range (Kushlan 1978a).

Estimated daily energy intake of 112.9 Kcal was derived from grams of prey and caloric intake/min figures of 0.6 and 563.2, respectively, during the nonbreeding season in Florida (Kent 1986b). These individuals used energetically expensive foraging behaviors (disturb-and-chase) to capture small fish and crustaceans, necessitating near constant foraging during low tides (Kent 1986b). In contrast, individuals conserved energy by using tongue-flicking in situations of reduced prey density (Master 1991). Consumed 277.4 Kcal/d in aggregations in s. New Jersey (Master et al. 1993). During spawning runs of silversides, ate 21% of body mass/d (Takita et al. 1984).

Mass specific basal metabolic rate = 0.784 cm³O₂/g h; feather reflectance 82% (300–2,700 nm wavelength range); thermal conductance 0.0515 cm³O₂/g h C° (Ellis 1980).

Nestlings defecate and regurgitate stomach contents when disturbed. Adults and juveniles defecate outside the nest; nestlings defecate in the nest. Not known to cast pellets or drink water.