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Least Bittern
Ixobrychus exilis
– Family
Authors: Gibbs, J. P., F. A. Reid, S. M. Melvin, Alan F. Poole, and Peter Lowther
Revisors: Poole, Alan F., and Peter Lowther

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Adult male Least Bittern
Figure 1. Breeding and non-breeding ranges of the Least Bittern, nominate race.

The Least Bittern, the smallest member of the heron family, is among the most inconspicuous of North American marsh birds. Within suitable habitats, however, it is often neither shy nor rare. This colorful heron is quite vocal and can be found at surprisingly high densities—up to 15 nests per hectare. Suitable habitats include fresh and brackish water marshes with tall, dense emergent vegetation and clumps of woody plants over deep water.

Least Bitterns forage by stalking along the open-water side of emergent vegetation, grasping clumps of plants with their long toes and curved claws. They also build small foraging platforms at rich feeding sites, catching fast-moving prey, mainly small fish and insects, with their small, thin bills. An extremely narrow body permits this bittern to pass easily through tangled vegetation.

Their nest, an elevated platform with an overhead canopy, is built of emergent aquatic vegetation and sticks. Males and females appear to help equally in rearing the young. When encountered, Least Bitterns typically burrow like rodents through dense vegetation, fly away weakly over marsh vegetation with legs dangling, or “freeze” with their bill pointed upward, feathers compressed, and eyes directed forward.

Five similar subspecies are distributed throughout much of North, Central, and South America. This biography is restricted to the North American and nominate subspecies exilis. Populations north of areas with prolonged frosts are migratory; others seem to be resident. Least Bitterns are often overlooked in surveys of bird populations because the species is so secretive. Reports from birdwatchers, however, have suggested that populations are reduced over much of the species’ range and extirpated in some areas. Basic research on the Least Bittern’s nesting biology, movements, population dynamics, and overwintering biology would help to clarify the status of the species and facilitate its conservation.