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Snail Kite
Rostrhamus sociabilis
Order
FALCONIFORMES
– Family
ACCIPITRIDAE
Authors: Sykes, Jr., P. W., J. A. Rodgers, Jr., and R. E. Bennetts

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Habitat

Figure 3. Typical Snail Kite habitat of flooded freshwater marsh

Breeding Range

Figure. 3.  In Florida, large inland freshwater marshes, edges of shallow lakes, and other flat water courses with marsh edge where apple snails can be found. These habitats are in humid temperate and humid tropical ecoregions (Bailey 1976, 1978) of peninsular Florida and are palustrine emergent wetlands semipermanently flooded (Cowardin et al. 1979) often on organic substrate of peat overlying oolitic limestone or sand or directly on limestone or marl (Davis 1946). Marsh vegetation is usually of low profile (≤ 3 m), often dense, dominated by sawgrass (Cladium jamaicense), with an interdigitated matrix of shallow, open-water areas (depth 0.2–1.3 m), with or without sparse emergent vegetation, and with relatively clear and calm water. About 60–70% of Everglades is sawgrass. Open-water areas consist mainly of white water-lily (Nymphaea odorata) sloughs, flats of spike rush (Eleocharis cellulosa), maidencane (Panicum hemitomon), etc., shallow lake edge, or canal edge. Other emergent or floating plants associated with open-water areas in pure stands or mixed with aforementioned species are arrowhead (Sagittaria lancifolia), pickerel weed (Pontederia lanceolata), floating heart (Nymphoides aquatica), water fern (Salvinia rotundifolia), duckweed (Lemna spp.), water bonnets (Pistia stratiotes), and introduced water hyacinth (Eichhornia crassipes) from tropical America. Low trees and shrubs scattered through these marshes include coastal-plain willow (Salix caroliniana), dahoon holly (Ilex cassine), pond apple (Annona glabra), bald cypress (Taxodium distichum), pond cypress (T. ascendens), introduced widespread punk-tree (Melaleuca quinquenervia) from Australia, cocoplum (Chrysobalanus icaco), wax myrtle (Myrica cerifera), and buttonbush (Cephalanthus occidentalis). Tree islands of woody plants are found in some marshes, mainly Everglades and headwaters of St. Johns River. Willow thickets are found in most marshes. Cattails (Typha domincensis and T. angustifolia) and reed grass (Phragmites australis) are present, often as a result of disturbances or nutrient overload from adjacent agricultural or suburban development. Giant bulrush (Scirpus validus) is present on deep-water edge of marshes at Lakes Kissimmee, Okeechobee, E. Tohopekaliga, and Tohopekaliga (Loveless 1959, Sykes 1978, 1979, 1983b, 1987b, Rodgers 1995).

Nearly continuous flooding of wetlands for ≥ 1 yr is needed to sustain large apple-snail populations and to enable snails to remain active and available as food for Snail Kites (Sykes 1979, Beissinger 1988). If wetlands are dry, snail mortality is high, and the few remaining mollusks burrow into soft sediments to estivate and are no longer available to Snail Kites (Kushlan 1975, Sykes 1979). During dry conditions, Snail Kites disperse widely throughout peninsula to smaller wetlands that remain flooded (Beissinger and Takekawa 1983, Takekawa and Beissinger 1989).

About one-quarter of peninsular Florida was originally flooded much of each year except during drought (Tebeau 1971). Drainage projects were initiated in 1881 and continued intermittently into 1970s (Parker et al. 1955, Tebeau 1971, Johnson 1974). Widespread drainage permanently lowered the water table 1.5 m in areas of s. Florida (Parker 1951, Klein et al. 1974) and as much as 2.1 m on headwaters of St. Johns River. Lowering Lake Okeechobee > 0.5 m resulted in lower minimum lake levels, shorter duration of wet conditions, less time between dry-downs, and less water storage (Beissinger 1986). Large tracts of marsh have been eliminated throughout the peninsula, the largest being Everglades Agricultural Area (312,354 ha) on s. shore of Lake Okeechobee. Little effort was made to control drainage of Everglades (originally 802,900 ha) through canals into tidal estuaries from 1910 to 1950. Prior to construction of the 3 water conservation areas (350,945 ha), 1950–1962, Everglades was dry much of time, with fires frequent and widespread (Parker et al. 1955, Hofstetter 1974). With creation of these 3 areas by diking, flow of water to the sea was controlled and provided flooded conditions over much of these large impounded marshes affording habitat for Snail Kites. Water levels and water flow through the Kissimmee Valley—Lake Okeechobee—Everglades system are now completely controlled (except in Everglades NP at the downstream end) by structures, canals, and pumps for flood control, agricultural and municipal uses, recreation, and to supply water on a set schedule to Everglades NP. The original sheet flow of water under which this freshwater marsh system evolved about 5,000 yr ago has not existed for approximately 75 yr (Parker 1955, Leach et al. 1972, Klein et al. 1974).

Foraging patches at some localities are rendered unusable because of rapid growth of floating plants such as water hyacinths and water bonnets. These plants form solid mats on the water surface, and because Snail Kites hunt visually, they cannot see snails below such vegetative blankets. This problem is enhanced by excess nutrients in the water. Source of nutrients is agricultural and suburban developments adjacent to or upstream of wetlands. These plants are a localized problem throughout the Snail Kite’s range in Florida, and control is mainly by periodic spraying with herbicides (Holm et al. 1969, Sykes 1979, 1983b).

Roosting Sites

Almost always over water. In Florida, 91.6% are located in stands of coastal-plain willow, 5.6% in punk-tree, and 2.8% in pond cypress. Roost sites tend to be in taller vegetation units in low-profile marshes. Willow stands grow in thickets, clumps, or elongated strands, on berms of old American alligator (Alligator mississipiensis) holes, burned-out peat pockets, former tree-island sites, borders of tree islands, or disturbed sites. Heights of willows at roosts are 1.8–6.1 m and stand size 0.02–5 ha. Birds tend to roost around small openings in willow stands. In large stands, they use < 10% of stand for roosting. Punk-tree and pond cypress roosts are no larger than 0.02 ha and tree heights 4–12 m (Sykes 1985a).

Winter Range

Generally same as breeding range, but in some years birds may move southward from northern part of range.