Nests solitarily or in loose colonies (Kaufman 1996) in fresh, brackish, or salt water habitats that provide dense vegetation, especially if scattered bushes or other woody growth, such as mangroves, is present (Ehrlich et al. 1988, Palmer 1962). May nest in association with Boat-tailed Grackles (Kaufman 1996).

Nest is placed near open water in dense vegetation 8 to 14 inches above shallow water (Palmer 1962), on the ground, or at a height of up to 8 feet in a low shrub (Ehrlich et al. 1988). Forages by climbing on reeds over shallow or deep water, rather than by wading (Kaufman 1996).

NATURE SERVE GLOBAL HABITAT COMMENTS:

BREEDING: Tall emergent vegetation in marshes, primarily freshwater, less commonly in coastal brackish marshes and mangrove swamps. Prefers marshes with scattered bushes or other woody growth. In the northeastern U.S., breeds mainly in wetlands along lakes, rivers, and estuaries on the coastal plain; generally absent from the Appalachian highlands and mountainous parts of New York and northern New England (Gibbs and Melvin 1992). Readily uses artificial wetlands.

Habitats vary throughout North America, but nesting usually occurs among dense, tall growths of emergent vegetation (particularly cattail (TYPHA spp.), sedge (CAREX spp.), bulrush (SCIRPUS spp.), or common reed (PHRAGMITES AUSTRALIS)), interspersed with some woody vegetation and open, fresh water (Weller 1961, Palmer 1962, Kushlan 1973, Swift 1987, Frederick et al. 1990). Both fresh and brackish marshes are used (Palmer 1962, Swift 1987, Andrle and Carroll 1988). Occurrences have been associated particularly with cattail, vegetated edges along deep, open waters (Weller 1961), and nutrient-rich microhabitats (Kushlan 1973). Nests typically 0.15-0.75 m above water near open water (Terres 1980, Harrison 1979, Brewer et al. 1991), in water typically 10-50 cm deep. Spend nearly all the diurnal period in dense, grass-like vegetation; open habitats such as mats of emergent vegetation are rarely used (Frederick et al. 1990).

Weller and Spatcher (1965) found more nests at two Iowa marshes during years when ratios of emergent vegetative cover to open water were equal (the 'hemi-marsh' condition). More densely or sparsely vegetated wetlands contained fewer nests, and interspersion of water and cover may thus be an important characteristic of breeding habitats. In Wisconsin, were restricted to deep-water and shallow-water cattail habitats, apparently avoiding areas of dry cattail, river bulrush (SCIRPUS FLUVIATILIS), and sedge (Manci and Rusch 1988). Variably-sized wetlands were used in Maine, with dense, tall stands of cattail, which were often associated with relatively stable water regimes at managed impoundments and coves on lakes (Gibbs and Melvin 1990).

On moist-soil impoundments in Missouri, least bitterns were associated with waters up to 50 cm deep and rank, dense vegetative cover bordering open water (Fredrickson and Reid 1986). Were not associated with open, sparse, or short vegetative cover or muddy openings (Fredrickson and Reid 1986).

Among six tidal marshes along the Hudson River in New York, Swift (1987) reported that presence was related to the extent of tall bulrush-cattail cover and site elevation (i.e., depth of tidal flooding). Favored sites had tall, dense vegetation and low-lying, 'wetter' sites (peak water depths averaged 70 cm), perhaps because nests are usually placed over water or near open water (Weller 1961, Palmer 1962, Kushlan 1973, Aniskowicz 1981). In tidewater areas of Chesapeake Bay, Stewart and Robbins (1958) reported abundance in narrow-leaved cattail (TYPHA ANGUSTIFOLIA) marshes, common in other coarse marsh types and weak-stemmed brackish marsh types, but scarcity in salt marshes.

In the Florida Everglades, observed from airboats at densities (birds/km) of 0.13 in canals, 0.04 in open grasslands, and 0.37 along airboat trails (Frederick et al. 1990). Most individuals were seen in mixed sawgrass (CLADIUM JAMAICENCIS)/cattail (29% of birds), homogeneous sawgrass (23%), and homogeneous cattail (9%), and, within these vegetative associations, were seen twice as frequently in dense than sparse stands (Frederick et al. 1990). Open sloughs, rush prairies, mats of emergent vegetation, and burn areas were used infrequently (Frederick et al. 1990, but see Kushlan 1973). The strong association with cattail in northern regions (e.g., Weller 1961, Swift 1987, Manci and Rusch 1988, Gibbs and Melvin 1990) may occur because cattail is the most common tall plant growing in dense stands above deep water in most northern areas (Frederick et al. 1990). In South Carolina, nests frequently were associated with boat- tailed grackle colonies (Condor 95:139-144).

Sites where least bitterns were flushed during spring migration in Missouri (n = 61) had a mean water depth of 23 cm, vegetation height of 64 cm, and stem density of 287 stems/meter squared (Reid 1989), and were dominated by burreed (SPARGANIUM EURYCARPUM), water smartweed (POLYGONUM COCCINEUM), and cattail. Reid (1989) observed a shift in habitat use to taller (121 cm), sparser (165 stems/meter squared) stands in fall (n = 15 flush sites). Feeding platforms (n = 52) occurred over water averaging 29 cm in depth and among stands of emergent vegetation averaging 199 stems/meter squared.