Food Habits

Main Foods Taken

Mostly prosobranch freshwater snails of genus Pomacea (Howell 1932, Haverschmidt 1962, Beissinger 1983, Sykes 1987a; also see Fig. 4). Feeds primarily on apple snail (Cottam and Knappen 1939, Stieglitz and Thompson 1967, Snyder and Snyder 1969, Sykes and Kale 1974, Sykes 1987a) in Florida and on other species of Pomacea snails elsewhere in its range (Haverschmidt 1962, Beissinger 1983, Snyder and Kale 1983, Bourne 1985b), with several exceptions (Mader 1981, Snyder and Kale 1983, Beissinger 1990b).

Microhabitat For Foraging

Forages over open-water patches dispersed among emergent marsh vegetation, shallow lake edge, ponds, ephemeral wetlands, or along shallow banks of rivers, borrow pits, canals, etc. These may include the following habitats (Cowardin et al. 1979): (1) palustrine, emergent, narrow-leaved persistent, typically dominated by spike rush and maidencane; (2) palustrine, aquatic bed, submergent algal, dominated by periphyton; (3) palustrine, aquatic bed, submergent vascular, dominated by bladderworts (Utricularia spp.), etc.; (4) palustrine, aquatic bed, floating-leaved, dominated by white water-lily, etc.; (5) palustrine, aquatic bed, floating dominated by duckweed, etc.; and (6) palustrine, emergent, broad-leaved persistent, typically dominated by pickerel weed, arrowhead, etc., which may be intermixed or on edges of any of the above habitats. In South America also forages over flooded rice fields (Beissinger 1983).

Food Capture And Consumption

Uses 2 visual foraging methods: course-hunting, flying at height of 1.5–10 m above water surface; and still-hunting from perch (Snyder and Snyder 1969, Haverschmidt 1970, Beissinger 1983, Cary 1985, Sykes 1987a). In Belize known to steal food from Limpkins (Aramus guarauna; Miller and Tilson 1985). Course-hunting consists of slow wing-beats alternated with gliding, usually facing into wind, on straight course or making sharp turns, criss-crossing a foraging patch with head pointed downward searching for prey in water below. Once snail is detected, Snail Kite hovers just above water surface and extends feet to grasp prey with talons. Captures snails at or below water surface to depth of 16 cm (approximate maximum reach potential) while prey is moving about, grazing, at surface ventilating pseudolung, or resting on vegetation. Snail Kite never takes snail with its bill, nor does it plunge in the manner of fish-eating raptors; adults seldom get belly feathers wet (Snyder and Snyder 1969, Sykes 1987a). Forages throughout day in Florida with periods of inactivity, particularly during midday (Snyder and Snyder 1969, Sykes 1987a). Snail captures during course- and still-hunting dropped or rejected were 1 and 9%, respectively (Cary 1985). Spends 85% of time perched, 82% of flight time in search of prey, and 94% of time foraging by course-hunting, but all vary greatly (Cary 1985). Individuals may concentrate hunting efforts on a particular foraging patch and return over a series of days, as long as foraging conditions are favorable (Cary 1985). In Guyana rice fields, Snail Kites spent 62% of daylight period perched, 19% foraging, 6% flying, and 13% in maintenance activities (Beissinger 1983). Time foraging by adults nesting in Florida ranged from 25 to 50% and from 31 to 68% (pre- and postnest desertion periods, n = 4 nests; Beissinger and Snyder 1987).

Among adults, duration of foraging bouts (initiation of hunt until capture or hunt was discontinued) averaged 1.2 min for males (n = 18) and 2.5 min for females (n = 34), with a range of 0.5–12 min (n = 15+ individuals). Intervals between captures of snails in Florida ranged from 2 to 90 min (mean = 22.3) and capture rates ranged from 1.7 to 3.4 snails/hour (mean = 2.5, n = 109) (Sykes 1987a). Capture rates were higher in summer than winter (3.54 vs. 1.82/h in Water Conservation Area [WCA] 3A of Everglades; Cary 1985). Still-hunting perches ranged in height from 0.15 to 4.6 m, and distance from perch to capture point ranged from 0.9 to 12.2 m (Sykes 1987a).

Ambient air and water temperatures affect number of foraging bouts initiated and capture rates (Cary 1985). When temperatures < 10°C, no captures observed; 11–20°C, capture rate 0.72/h; 21–30°C, 2.68/h; and > 30°C, 5.28/h. Low capture rates or failure on cold days are explained by snails being inactive and remaining at greater depths (Hanning 1978). Based on availability of prey during colder temperatures, Snail Kite range in Florida may be limited to southern portion of peninsula in some winters when low temperatures persist longer than several days (Cary 1985). Foraging success of nesting birds declines in cold, windy weather following cold fronts on Lake Okeechobee—a cause of nesting failure and abandonment during winter months (Beissinger 1988, Snyder et al. 1989a). Distance regularly flown from nests in search of food (“foraging range”) at Lake Okeechobee: > 1 to > 6 km; varies from year to year depending on water levels and, presumably, snail densities (Beissinger and Snyder 1987), but generally < 2 km (PWS). Extreme distance flown from roost to foraging site 18.5 km (Palm Beach Co.; followed at distance by helicopter but not harassed; Rumbold and Mihalik 1994).

See Behavior: spacing, for defense of foraging patches.

Birds transport snails to a feeding perch in talons or bill. Transfer of prey between feet and bill is common in flight. Feeding perches include woody-stem plants (dead or alive), blades of sawgrass and cattails, fence posts, etc. Snail Kite’s slender, deeply hooked, sharp-tipped upper mandible permits it to cut the columellar muscle of Pomacea snails (Snyder and Snyder 1969) and remove soft tissues from the shells. Tomium arc of upper mandible approximates inner spiral of the snail’s shell where columellar muscle (the only muscle attaching snail to its calcareous shell) is attached. Extraction of snail from shell commences upon reaching feeding perch. This process is described in detail by Snyder and Snyder (1969), Voous and van Dijk (1973), and Snyder and Kale (1983).

Snail Kite holds snail with 1 or both feet and shell positioned with spire downward and aperture facing away from perch. Inserts tip of upper mandible between thin, horny operculum and shell. Removes operculum by twisting with bill and cutting or tearing muscle attachment with tip of upper mandible. Discards detached operculum and repositions shell with aperture upward and spire position to bird’s left (see Fig. 4). With upper mandible, cuts columellar muscle on inside curve of aperture with 1 or more strokes, rendering entire soft tissue of snail free of its attachment to shell. Using bill, bird pulls fleshy snail free of shell. May drop empty shell any time after soft tissue has been extracted. Eats snail whole or tears in pieces.

Entire handling procedure of snail at perch for adult bird (n = 16 males, 53 females) is 1–7 min (mean = 2.7), and number of attempts to remove operculum range from 3 to 14 (Sykes 1987a). Later study found mean handling time of Pomacea snails to be 95.7 ± 37.3 (SD) s in Florida and 93.0 ± 48.0 (SD) s in Venezuela (Beissinger 1990b). Handling time positively related to snail size (Beissinger et al. 1994). Adult females are more efficient at extracting snails than adult males (mean = 2.4 vs. 4.7 attempts). Extracted soft tissues of snails eaten in 1–16 pieces. Snail Kites do not eat albumen glands (orange in color; noxious and distasteful) of female apple snails; attempts to remove the gland ranged from 5 to 20 per snail (Snyder and Snyder 1969, Sykes 1987a). Snail Kites may eat all of snail viscera, or discard all or most of it. At completion of each feeding, adults and larger nestlings wipe sides of bill one or more times against the perch or nest material. Such behavior cleans the bill and may keep the tomium sharp. Discarded empty shells may accumulate beneath a frequently used feeding perch; one such site had 1,174 discarded shells (Sykes 1987a).

Apple-snail shells from 13 Snail Kite feeding stations (n = 697 shells) ranged from 25.2 to 71.3 mm in length (mean = 42.8; measured from outer edge of lip of aperture across shell at 90° to axis of spire) and from 27.4 to 82.4 mm in diameter (mean = 45.8; measured along axis of spire). Size class selection in Florida was primarily in the 30- to 60-mm range (98.5%) for length and diameter. Intact apple snails wet weight 12.7–38.1 g; mean extracted soft parts 48% of intact animal, and mean soft tissues with viscera (including albumen gland of females) removed 15%. Dry mass of apple snail soft parts 14.5% of wet mass (Sykes 1987a).

Birds take alternative food on rare occasions in Florida, particularly during or following drought conditions and during and after passage of cold fronts in winter when usual prey is scarce (Sykes and Kale 1974, Woodin and Woodin 1981, Takekawa and Beissinger 1983, Sykes 1987a, Beissinger 1990, Bennetts et al. 1994a). Alternate foods often resemble Pomacea snails (Beissinger 1990) and include mystery snail (Pomacea bridgesi), small aquatic snail (Viviparus georgianus), crayfish (Procambarus spp.), speckled perch (Promoxis nigromaculatus), musk turtle (Sternotherus odoratus), mud turtle (Kinosternon bauri), Florida cooter (Chrysemys floridana), peninsula cooter (Chrysemys nelsoni), soft shelled turtle (Trionyx ferox), small unidentified turtle, small unidentified mammal, ring-necked snake (Diadophis punctatus), unidentified snake, and dead American Coot (Fulica americana). Snail Kites consumed small turtles (carapace length 30–90 mm) using techniques similar to snail-eating behavior. Birds held turtle on back with head near perch and posterior facing away from them, then entered turtle body cavity by piercing right rear leg or caudal region, which strongly resembles aperture of apple snail (Sykes and Kale 1974, Beissinger 1988, 1990b). Handling times for turtles > 60 min, much longer than for apple snails; Snail Kites rarely consume all of turtle soft tissues (Beissinger 1990).

Food items taken by Snail Kites outside Florida are primarily snails of genus Pomacea, particularly P. doliodes (Haverschmidt 1962, Snyder and Snyder 1969, Collett 1977, Snyder and Kale 1983), but birds regularly take freshwater crabs (Dilocarcinus dentatus) in Venezuela (Beissinger 1990) and Marisa snails (M. cornuarietis) in Colombia (Snyder and Kale 1983).

Quantitative Analysis

Little quantitative information from Florida based on crop and gizzard contents. Of 4 crops and gizzards examined (2 taken in May and 1 each in Mar and Dec), 3 gizzards contained only fleshy remains of apple snails, and fourth had crop and gizzard gorged, containing 97% apple snails (8 whole and 12 fragments), 2 mites (Galumnidae), a midge larva (Chironomidae), and 3% plant debris; latter 3 believed incidental ingestion (Cottam and Knappen 1939).

Food selection discussed under Feeding and Diet. Caching not reported.

Nutritional content of pooled sample of apple snails (n = 875) in Florida by dry weight: 36.3% protein, 3.4% fat, 0.9% fiber, 32.1% ash (includes 10.9% calcium, 0.4% phosphorous, 0.2% sodium, 0.4% potassium, 20.2% other unidentified minerals), and 27.4% miscellaneous (carbohydrates, nucleic acids, etc.) (Sykes 1987a). Nutritional values based on separate analysis for 6 snails by Beissinger (1984) are similar for fat (lipid) but differ for other nutrients. These apparent differences may be due to sample size, time of year samples were collected, locality, differences between labs, or may be within normal range of variability for apple snails.

Mean available gross energy value from apple-snail soft tissue in Florida is 4.52–4.60 kcal/g dry mass (Beissinger 1984, Sykes 1987a), mean with viscera removed is 4.57 kcal/g, and mean of only female snails with albumen glands removed is 4.78 kcal/g (Sykes 1987a). Mean gross energy value for Pomacea doliodes in Guyana is 4.04 kcal/g (Beissinger 1983). This may be evidence of regional differences in caloric content of snail tissues. Beissinger (1983) estimated energy intake from P. doliodes by Snail Kites in Guyana rice fields to be 104.2 kcal/g/d with an expenditure of 95.7 kcal/g/d (based on dry weights). Total energy intake during egg-laying (134.8 kcal/d) in Florida far exceeded daily needs of females (98.2 kcal/d). Female expenditure in activities (76.3 kcal/d) was significantly less than that for males (134 kcal/d) (Beissinger 1987a). Energy expenditures during incubation, brooding, and postbrooding for deserting and tending parents are in Beissinger 1987b and Beissinger and Snyder 1987 .

No information on metabolism, and very little known about temperature regulation. Cary (1985) reported what he interpreted as thermoregulatory behavior. On cold (air temperature < 15°C) and windy days, Snail Kites switched perches more frequently, positioned themselves out of wind in sunny locations, perched with backs oriented toward sun more frequently, partially spread wings on 2 occasions, and perched significantly lower than on warm days (air temperature > 20°C).

Not observed drinking, but apple snails comprise approximately 86% water (Sykes 1987a); prey probably provides all water that birds require.

Little information on pellet-casting. A pellet found in a nest on St. Johns River was elliptical-ovate in shape, measured 22.3 x 11.0 mm, and weighed 0.5 g (dry weight); contained radula, jaw elements, and 1 columellar retractor muscle of P. paludosa, but bulk consisted of plant materials, latter believed to have been ingested inadvertently; not known if this pellet was from adult or nestling. At least 3 other pellets found at Snail Kite nests in Florida and 2 from recently captured nestlings in Argentina (Sykes 1987a).

Snail Kites usually defecate following each feeding (Sykes 1987a). To defecate, birds lower head and raise body and tail to a horizontal plane or slightly higher and direct excrement away in long stream. Nestlings, once strong enough, use same posture as adults and direct their tails toward outer rim of nest to evacuate; material may travel ≥ 0.2 m. Fecal material liquid in consistency; color a mixture of whites, browns, and grays (PWS). No fecal samples have been analyzed.