Least Bittern Life History

Habitat

Least Bitterns are relatively scarce breeders in coastal and inland wetlands, but they can be locally numerous where food is abundant. They nest in freshwater and brackish marshes with tall aquatic vegetation such as cattails and other reeds and rushes, preferentially in places interspersed with patches of open water and small stands of woody vegetation. Some birds forage during summer in saltmarshes and mangrove swamps, but they nest in those habitats less commonly. Least Bitterns winter in saltwater, brackish, and freshwater wetlands in the southernmost coastal areas of their U.S. range (especially southern Texas and Florida), as well as in Mexico, the Caribbean, and Central America. They also often use created wetlands, such as golf course ponds or sewage treatment areas with reedbeds, during winter.

Food

Least Bitterns eat mainly small fish such as minnows, sunfishes, and perches. They also eat small snakes, frogs, tadpoles, salamanders, slugs, crayfish, other crustaceans, shrews, mice, dragonflies, and leeches. They may occasionally prey on eggs or young of Yellow-headed Blackbirds. When hunting, the birds stand (or hang from reeds) motionless near the water’s edge, jabbing at prey with the bill. Sometimes they stalk their prey by walking slowly, and sometimes swaying the neck (not the head) side to side just before striking, as other herons do. They may flick their wings to startle hidden prey into the open. Least Bitterns shake or soften larger prey items in their bill before consuming them. Crustaceans such as crayfish are sometimes trimmed to remove the hardest parts of the exoskeleton.

Nesting

Nest Placement

Based on observations of related species, it’s likely that the male selects the nest site, which is usually about 6-30 inches above the water, over water that is 3-38 inches deep. Nests are rarely more than 10 yards from the edge of the reed bed but are seldom at the edge itself.

Nest Description

The male constructs the well-hidden nest, and the female sometimes watches him as he works. He builds the nest by pulling on the reeds and crimping them to form a platform and a canopy, which are then held together with stalks and sticks arranged in a spokelike fashion. When complete, the unlined nest measures about 6-8 inches across and 2-5 inches deep. The foundation is sometimes a nest from previous years, or even an old nest of another species, such as Marsh Wren. The male occasionally repairs the nest even as the female incubates. Where food is abundant, Least Bittern pairs might nest very near others, almost colonially.

Nesting Facts

Behavior

Least Bitterns hunt alone, quietly perching at the edge of a stand of reeds, watching for prey. Their movements are slow and deliberate when walking or climbing, and they frequently grasp reeds with their feet when moving about above the ground. They can run rapidly and, like rails, can compress their bodies to move quickly through dense vegetation. Flying birds sometimes appear delicate in the air, rising up above the reeds and quickly fluttering downward and disappearing into the vegetation. On longer flights, their heronlike shape (pointed at both ends) is evident, and they fly rapidly and directly. Foraging birds impale prey with lightning-fast reflexes. Resting birds often preen or scratch with bill or foot, occasionally stretching out their long necks, which can seem improbably long for their small bodies. Threat displays include hissing, opening the bill, and tilting the wings forward. When disturbed by people, Least Bitterns sometimes compress the body, distend the neck and bill upward (but with eyes fixed forward), and even sway with the reeds, apparently an attempt at camouflage. Even tiny chicks at the nest will attempt to strike this “bittern pose.” Pairs are probably monogamous, at least for a single nesting season. Males preen the female’s neck and back before copulation.

Conservation

Least Bittern populations were stable over the last half-century according to the North American Breeding Bird Survey, although the bird’s retiring habits mean population trends are difficult to assess from standardized surveys. Partners in Flight estimates the global breeding population is 310,000 and rates them 11 out of 20 on the Continental Concern Score, indicating a species of low conservation concern. Continued loss of wetland habitats and the spread of invasive species of marsh vegetation may further threaten the species, though in some locations, Least Bitterns appear to be able to use the nonnative, invasive Phragmites reed for nesting.

Credits

Gibbs, J. P., F. A. Reid, and S. M. Melvin. 1992. Least Bittern. In The Birds of North America, No. 17 (A. Poole, P. Stettenheim, and F. Gill, Eds.). Philadelphia: The Academy of Natural Sciences; Washington D.C.: The American Ornithologists' Union.

Kushlan, J. A., M. J. Steinkamp, K. C. Parsons, J. Capp, M. A. Cruz, M. Coulter, I. Davidson, L. Dickson, N. Edelson, R. Elliott, R. M. Erwin, S. Hatch, S. Kress, R. Milko, S. Miller, K. Mills, R. Paul, R. Phillips, J. E. Saliva, W. Sydeman, J. Trapp, J. Wheeler and K. Wohl (2002). Waterbird conservation for the Americas: The North American waterbird conservation plan, version 1. Washington, DC, USA.

Partners in Flight. (2020). Avian Conservation Assessment Database, version 2020.

Pittaway, Ron and Peter Burke. (1996). Cory's Least Bittern. Ontario Birds 14 (1):26-40.

Sauer, J. R., D. K. Niven, J. E. Hines, D. J. Ziolkowski Jr., K. L. Pardieck, J. E. Fallon, and W. A. Link (2019). The North American Breeding Bird Survey, Results and Analysis 1966–2019. Version 2.07.2019. USGS Patuxent Wildlife Research Center, Laurel, MD, USA.

Sibley, D. A. (2014). The Sibley Guide to Birds, second edition. Alfred A. Knopf, New York, NY, USA.

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