Description
A stocky wading bird with a straight pointed bill, relatively short neck and legs, and somewhat pointed wings; darker flight feathers; bill dull yellow with a dusky tip on the upper mandible; legs and feet are greenish yellow; breeding feathering includes generally inconspicuous white ruffs on the shoulders and two small green patches on the back; wing span 107 cm (NGS 1983). Terrestrial locomotion is slow and stealthy; flight is rapid and usually low.
MORPHOLOGY AND PLUMAGES: A brown, medium-sized heron, 60-85 cm long, with a stout body and neck and relatively short legs (Palmer 1962, Cramp 1977, Hancock and Kushlan 1984). Adult plumage is all brown above (finely flecked with black) and heavily streaked with brown and white below. The crown is rusty- brown. An elongated, black patch extends from below the eye down the side of the neck, a characteristic unique among herons (Hancock and Kushlan 1984). The throat is white. Sexes are similar, except that the male is slightly larger (Palmer 1962). Juveniles differ only in lacking black neck patches, which are obtained in the first winter. Plumage does not change seasonally. In flight, seem hurried, ungraceful, and stiff. When disturbed, they often freeze in an upright, concealing posture, with head and bill upturned.
VOCALIZATIONS: During the breeding season, males repeat from two to 10 times a distinctive, far-carrying call, rendered as "pump-er-lunk" by Palmer (1962), and which is often preceded by a series of clicking and gulping sounds. A detailed, anatomical description of how they vocalize, through inflation of the esophagus, was provided by Chapin (1922). They rely on their resounding calls to communicate among the dense, visually restricting emergent vegetation that dominates nesting habitats. Low frequency sounds, such as their "booming," attenuate less rapidly and are audible at greater distances in dense marsh vegetation than high frequency sounds (Cosens and Falls 1984). When flushed from a marsh, they often emit a hoarse "kok-kok-kok" or nasal "haink" (Palmer 1962).
EGGS: Eggs measure 49 by 37 mm and are elliptical, buff-brown to deep olive-brown, smooth, and slightly glossy (Harrison 1978).
Diagnostic Characteristics
Differs from night-herons in the following ways: wings are pointed rather than rounded, flight feathers are much darker than back (vs. no contrast), upperparts lack white spotting, and bill is more slender. Much larger than the least bittern (average length 71 cm vs. 33 cm). Differs from similar juvenile green heron in being larger (length 71 cm vs. 46 cm) and in having flight feathers of wings obviously darker than the middle of the back.
Habitat
BREEDING: Primarily large freshwater and (less often) brackish marshes, including lake and pond edges where cattails, sedges, or bulrushes are plentiful and marshes where there are patches of open water and aquatic-bed vegetation. Occurs also in other areas with dense herbaceous cover, such as shrubby marshes, bogs, wet meadows, and, rarely, hayfields (Brewer et al. 1991). Readily uses wetlands created by impoundments. Wetlands of 2.5 ha or more may support nesting; smaller wetlands may serve as alternate foraging sites (Gibbs and Melvin 1992). See Hanowski and Niemi (1990) for a quantitative study of habitat in Minnesota.
Nests primarily in inland freshwater wetlands, sometimes in tidal marshes or in sparsely vegetated wetlands or dry grassy uplands. Breeding occurs primarily in wetlands with tall emergent vegetation. Sparsely vegetated wetlands and dry grassy uplands are sometimes used, as are tidal marshes in some areas (Stewart and Robbins 1958, Swift 1987). In comparison to the sympatric least bittern (IXOBRYCHUS EXILIS), uses a wider variety of wetland cover types, less densely vegetated sites, shallower water depths, and primarily freshwater habitats.
Wetlands used in Maine were dominated by emergent and aquatic-bed (floating-leaved and submergent) vegetation, had a high diversity of vegetative life forms, and a high degree of cover/water interspersion (Gibbs et al. in press; Gibbs and Melvin 1990). Portions of wetlands used were dominated by sedges (CAREX spp.), broad-leaved cattail (TYPHA LATIFOLIA), and ericaceous shrubs. In a study of Quebec lakes, lakes with patches of floating-leaved plants, emergent growth along shorelines, and abundant amphibian populations were preferred (DesGranges and Houde 1989).
At Horicon Marsh in Wisconsin, responded to tape-recorded calls only from shallow water cattail and dry cattail habitats and seemed to avoid deepwater cattails (Manci and Rusch 1988). At moist soil impoundments in Missouri, associated with water depths of less than 10 cm and vegetative cover characterized as "rank, dense, or sparse." Habitat use was not associated with "open" or "short" vegetative cover or water of depths of greater than 10 cm (Fredrickson and Reid 1986). In Minnesota, seven breeding territories had a mean water depth of 10 cm, vegetation height of 1.3 m, and density of sedge and grass stems of 117 stems/m squared (Hanowski and Niemi 1986).
NON-BREEDING: Migrant bitterns were flushed at 25 sites during spring in Missouri with mean water depth of 26 cm, vegetation height of 63 cm, and stem density of 157 stems/m squared. Characteristics of 35 flush sites in fall were similar, except that vegetation was taller (118 cm) (Reid 1989). In areas where temperatures stay above freezing and waters remain open, especially in coastal regions where the ocean moderates climate (Root 1988). Wintering habitat is much like breeding habitat, and overwintering populations are heavily dependent on managed wetland areas, such as those occurring at wildlife refuges (Root 1988). Occasionally occurs in habitats that are more open than the usual ones. Overwintering occasionally takes place in brackish coastal marshes (Hancock and Kushlan 1984).
Ecology
Basically solitary. Low population density. Too little is known of rates of predation on nests and adults to infer the importance of predation as a factor limiting populations. Similarly, only minimal information is available on the effects of parasites and disease. Wading birds are known to be susceptible Type C botulism (occasionally), avian cholera (infrequently), chlamydial infections (frequently), sarcocystis (rarely), and aspergillosis (infrequently) (Friend 1987). Known to harbor lice and flies (Peters 1936) and a number of species of nematodes (Boyd 1966).
Reproduction
Nesting occurs solitarily (non-colonially) on all-purpose territories that provide both feeding and nesting sites, but occasionally up to a few pairs nest in a small area (Harrison 1979). Polygyny is suspected to occur (Palmer 1962). Mating rituals are elaborate, and involve display of a tuft of white nuptial feathers located on the sides of the neck (see Baker 1980, Johnsgard 1980). BOTAURUS bitterns may undergo a change in iris color from yellow to orange during courtship (Wood 1986).
The nest consists of a platform of reeds, sedges, cattail, or other available emergent vegetation, and is lined with fine grasses. Nests are usually placed on the ground, in a tussock, a few inches above water, or are floating; are surrounded by water, and have dense, overhead cover (Bent 1926, Vesall 1940, Cogswell 1977, Harrison 1978, Terres 1980). Nests may also be built in grassy uplands (Vesall 1940, Duebbert and Lokemoen 1977). Nests are usually accessed by two, well-beaten pathways (Gabrielson 1914).
Egg-laying begins in late April or early May, about a month after the arrival at nesting areas, and continues until mid-June (Bent 1926). In the north-central U.S., nests may contain eggs from about early May to early July (Brewer et al. 1991). Clutch size ranges from two to seven eggs, usually three to five (Duebbert and Lokemoen 1977, Graber et al. 1978). Incubation, by the female only, begins with the first egg (Palmer 1962). Hatching occurs after 24-28 days (Burns 1915, Bent 1926, Vesall 1940), and chicks remain at or near the nest for two weeks (Gabrielson 1914, Vesall 1940, Harrison 1979, Terres 1980). Chicks are fed only by the female, and are given fish, frogs, snakes, crayfish, and mice (Gabrielson 1914, Byers 1951). Nests often become fouled with food debris (Vesall 1940). Post-fledging behavior of young is unreported.
Apparently single-brooded (Palmer 1962). In upland habitats in North and South Dakota, 57% of 72 nests hatched at least one egg (Duebbert and Lokemoen 1977). Little is known about sources of egg, chick or post-juvenile mortality, age at fledging or first breeding, or territory size. Age at fledging for the closely-related Eurasian bittern is 50-55 days (Cramp 1977). One banded American bittern lived at least eight years (Clapp et al. 1982). Probably undergoes extensive post-breeding dispersal, which has resulted in numerous sightings, mostly between September and December, at locations as distant as Iceland, Norway, and Great Britain (Cramp 1977). Information on molt is incomplete. Adults undergo a complete post-breeding molt from August to November, and possibly a pre-breeding renewal of body plumage (Palmer 1962, but see Bent 1926).