Food Habits

Adult White Ibis foraging, St. Augustine, FL, December

Adult White Ibis foraging, NC, January

Feeding

Main Foods Taken

Aquatic crustaceans and insects.

Foraging Conditions

Prey availability depends on multiple environmental variables. In a controlled study that manipulated prey availability, more ibises foraged in ponds with high prey density and low to medium water depths than in ponds with lower fish density and deeper water (Gawlik 2002). In the Florida Everglades, foraging ibises were most likely to be found in low water (14.0 cm ± 5.0 SD, max depth 24 cm), high crayfish density, and wet prairie areas (Surdick 1998, Lantz 2008). Decoys placed in shallow water (10.5 ± 0.67 cm) with low vegetation (25.6 ± 5.14 cm) were more successful at attracting ibises to foraging areas than decoys placed in deeper water (17.1 ± 1.85 cm) with taller vegetation (49.0 ± 9.18 cm) (Heath and Frederick 2003).

In Spartina marsh, ibises forage most often in areas with < 30 cm vegetation (Petit and Bildstein 1987). In a study on the effect of periphyton density, water depth, water temperature, wind speed, vegetation density, Julian date, time after sunrise, fish density, prawn density and crayfish density on foraging success, only periphyton density negatively affected ibis foraging success; fish density was positively correlated with success (Surdick 1998). Emergent vegetation density and structural complexity were important factors determining foraging success (Adams and Frederick 2008a).

Food Capture And Consumption

Long, decurved bill, long legs, and neck facilitate foraging while wading in shallow water and probing for prey items beneath the surface (Kushlan 1977b). Eyes situated to focus below head on bill tip. Most foraging, especially when in standing water or soft mud, is tactile, without use of eyes. Bill tips appear to be well innervated, and contact with prey probably elicits a reflex closing action. Even so, captive individuals whose bills have been broken off and reattached or replaced with a prosthesis are capable of feeding on live crustacean prey. Can pick prey items from the surface of the ground or a plant; sweeps bill like spoonbill (Kushlan 1977b, 1978c, 1979b). In Spartina marshes, ibises alternate between tactile and visual foraging, both probing for fiddler crabs in burrows and chasing crabs encountered on the surface (Petit and Bildstein 1987).

Usually probe while walking slowly, stopping periodically to concentrate probing under a plant or down a crayfish or crab burrow. Bill is inserted into water or substrate closed or with tips held 1–2 cm agape. Bill has slight gap in center, which permits tips to function tweezer-like in grabbing and holding a prey item. Potential prey items seldom pursued, unless on land or in very shallow water.

Review by Kelly et al. (2003) refers to foraging behavior in Kushlan (1976d) to describe the following techniques used by ibis: stand and wait, walk slowly (≤1 step/s), walk quickly (≥1<2steps/s), running (>2 steps/s), foot raking, head swinging, groping, probing, pecking, stealing, and prey dropping. Prey dropping describes a focal bird repeatedly dropping a prey item rather than eating it. Foraging techniques vary according to prey type and availability (Henderson 1981).

Prey items swallowed with a quick forward thrust of head (Kushlan 1978c). Prey also worked toward gullet by closing bill tips, propelling prey backward into slight gap in bill. Hard prey items bitten in mandibles, thereby crushing snails and crustaceans, removing claws of crustaceans and heads of some insects prior to swallowing. Mud often washed from prey, even if bird must walk to a water source several meters from the capture site. Large prey items often stabbed and bitten into smaller pieces, but handling time increases exponentially with prey size, and such prey are susceptible to piracy (Kushlan 1979a). Proficiency in handling prey improves with age (Henderson 1981).

Piracy common in some areas, both among ibises and with foraging associates. Ibises are primary victims for other larger birds in aggregations (Kushlan 1978a, Frederick and Bildstein 1992). Intraspecific piracy occurs at colony sites with adults (mostly males) taking prey from adults (mostly females) regurgitating food to nestlings, as well as directly from nestlings (Frederick 1985). Ibises used by other species commensally, with visually foraging species following closely behind walking ibis, using it as a beater (Kushlan 1978b). In turn, ibises follow ungulates in flooded pastures in Venezuelan llanos.

Intra-specific predation rare, but reported from one colony in s. Florida where a fledgling attacked and probably ate a nestling (Herring et al. 2005).

Feeds in large aggregations and flies in cohesive flocks among feeding and roosting sites (Bildstein 1983, Petit and Bildstein 1986). Aggregations can exceed 5,000 birds in suitable habitat (Kushlan 1979a). White ibises recorded in 70% of wading-bird aggregations counted in s. Florida.

Juveniles often flock separately or feed on the periphery of adult flocks, being less successful in foraging than adults even when they are one-year-old (Bildstein 1983, 1984), and taking 2 months to develop flock flying (Petit and Bildstein 1986). Adults on the peripheries of large flocks in salt marshes forage visually more than do birds on the interiors of flocks (Petit and Bildstein 1987). Feeding aggregations form by local enhancement, the attraction of birds to already feeding white-plumaged birds (Kushlan 1976b, 1977e). Most (97%, n = 286) ibises leaving colony landed near or with conspecifics in foraging flocks (Smith 1995b). Three-dimensional decoys placed in flock-like arrangement can successfully lure individuals to specific areas (Crozier and Gawlik 2003a, Heath and Frederick 2003). Individually marked birds may return to same feeding site for at least several days. Feed primarily in morning and evening, except while nesting, spending other time resting near feeding site.

Males larger than females (see Table 1), thus have higher overall energy costs (Bildstein 1987). Although free-ranging males and females feeding side-by-side on fiddler crabs had similar capture rates, suggesting that males need to forage longer than do females (Bildstein 1987), captive males were more successful than females at extracting fiddler crabs from artificial burrows that approached the length of females’ bills (Bildstein et al. 1989).

Diet

Major Food Items

Small aquatic and semiaquatic organisms, especially crustaceans and aquatic insects. Also fish, especially when these occur in high densities. Specialize on crustaceans, especially freshwater crayfish (Cambaridae) and estuarine crabs (Ocypodidae; Nesbitt et al. 1975, Kushlan and Kushlan 1975, Kushlan 1979a, Bildstein 1983). Total diet is nevertheless exceptionally broad because of ability to pick up whatever items they touch with their bill.

Diet includes aquatic and terrestrial arthropods, polychaetes, snails, frogs, lizards, young snakes, and small fish. Outside the nesting season ibises are primarily coastal, most using mangrove swamps, Spartina marshes, and coastal lagoons, where they often specialize on fiddler crabs (Uca). However, in winter many forage in freshwater areas, particularly in s. Florida. In non-breeding season in Florida, earthworms, cockroaches and other insects in lawns are common prey, especially during the wet season when flooded wetlands are typically too deep for foraging.

Most information on prey from boluses collected from young birds. In s. Florida fish and crustaceans averaged about 2 cm long (fish 2.0 cm [SE = 0.4, n = 341]; crustaceans 2.2 cm [0.6, 62]). In another study crayfish averaged 18.7 mm (se = 0.4 mm) carapace length (equivalent to approximately 3.3 cm total length, Dorn et al. 2008). Compared with available crayfish, ibises selected larger size classes (> 20mm). In s. Florida fish commonly consumed when available include mosquitofish (Gambusia holbrooki), flagfish (Jordanella floridae) and occasionally sunfish (Lepomis spp., Dorn et al. 2008). In Louisiana crayfish (Procambarus) averaged 4.2 cm (2.2, 94) (Martin 1985). Capable of taking considerably larger prey. In Trinidad, for example, Scarlet Ibises take polychaetes (Nereis) up to 17 cm long; and in Venezuela white and red ibis forms take 4 cm aquatic insects, 5 cm blue crabs (Callinectes sapidus), and 4–6 cm fish. Captive individuals take crayfish in excess of 10 cm long.

Quantitative Analysis

Diet in inland s. Florida (Kushlan and Kushlan 1975): inland, crayfish (Procambarus alleni) 52% of biomass and 82% frequency, fish 19% biomass, insects 15%; in coastal habitats, crayfish 11% of biomass, fiddler crabs 20%, fish 31%, insects 10%. In same region Dorn et al. (2008) report high variability (11-fold difference) in proportions of fish and crayfish consumed among colonies. In n. Florida (Nesbitt et al. 1975): freshwater, crayfish 45% by volume and 70% frequency, insects 37% of volume; salt water, crayfish 46% of volume, 70% frequency, insects 24% of volume, crabs (Uca) 5%.

In Orange Lake, FL (Baynard 1912): 352 cutworms, 308 grasshoppers, 602 crayfish, and 42 small snakes in 50 regurgitations from young. In coastal Louisiana (Hammatt 1981): mud crabs (Panopeus herbstii) 89% by volume and fish 11%, in 16 regurgitations from young.

In coastal S. Carolina near Charleston (Post et al. 1985): fish 4% by number, estuarine crabs 63%, crayfish 20%, insects 10% in 22 regurgitations from young. In coastal S. Carolina near Georgetown (Bildstein et al. 1990): mainly crayfish early in the breeding season (39–85%), but less so later, depending upon rainfall. In a S. Carolina Spartina marsh, Uca pugnax and U. pugilator comprised over 95% of the prey taken (Henderson 1981, Bildstein 1983).

In llanos of Venezula (Aguilera et al. 1993): coleoptera 48%, other insects 34%, crustaceans 3%, fish 7% of prey items from 9 ibis stomachs.

Nutrition

Basal metabolic rate of adult, 81 to 88 kcal/d (Benedict and Fox 1927). Existence metabolism, 113.8 kcal/d and 0.12 kcal/g/d; aviary existence metabolism 138.7 kcal/d (Kushlan 1977c). Daily energy expenditure, 164.6 kcal/d, or 21% of body mass.

Young require 8,620 cal of food during nestling period and the s. Florida population of 62,000 birds in the mid-1970s required 9.3 x 108kcal/yr (Kushlan 1977c, 1977d). Assimilation efficiency (AE) of nestlings is 68% to 92% on a diet of shrimp and fish, AE of adult is 80% (55%–92%). Caloric content of feces averages 2.48 kcal/g dry weight (0.22, 66; 1.89–2.95). Daily energy use of young peaks at mass of 600 g, decreases to fledging. Energy content of diet, 2.7 to 4.5 kcal/g.

For inland Florida nesters, crayfish made up 60% of energy consumed in s. Florida and 72% in central Florida (Kushlan 1979a). In coastal S. Carolina, adults feeding on fiddler crabs need to do so for 2.5 (females) to 3.5 (males) hr/d (Bildstein 1987). Because of salt stress, nestlings do not develop normally on brackish-water prey such as fiddler crabs. Even at estuarine colony sites, the bulk of the nestling diet consists of freshwater prey, especially crayfish (Bildstein et al. 1990, Johnston and Bildstein 1990).

Drinking, Pellet-Casting, And Defecation

Drink by inserting lower bill horizontally into water and lifting head to allow water to flow into gullet. Non-digestible hard parts, such as fish bones, arthropod exoskeletons, and crayfish gastroliths are cast in pellets. Pellets also may be cast in flight (Below 1979).

Food Selection And Storage

Food selection exhibited through tactile and visual foraging (Kushlan 1978c, 1979a, Bildstein et al. 1989). During nesting, selectively consume crayfish, crabs, insects, and frogs in higher proportion than available at feeding sites. When feeding on large fiddler crabs, select females over males, apparently avoiding major claw of latter (Bildstein et al. 1989). Able to catch small fish in situations when they are trapped in shallow water, although some kinds are selected among those available, such as Cyprinodon variegatus, Fundulus confluentus, Jordanella floridae. Do not select relatively high energy prey, but when raising nestlings select low-salt prey (crustaceans and fish) captured at freshwater sites (Bildstein et al. 1990, De Santo et al. 1997). Will reject prey requiring prolonged handling time.