The Least Bittern breeds from southeast Canada through the U.S. and Mexico. Populations west of the Mississippi River are discontinuous and concentrated in low-lying areas of the Central Valley and Modoc Plateau of California, Klamath and Malheur Basins of Oregon, and along the Colorado River of southwest Arizona and southeast California (Poole et al. 2009). In Montana, the Least Bittern is an accidental species (Montana Bird Distribution Committee 2012).
For a comprehensive review of the conservation status, habitat use, and ecology of this and other Montana bird species, please see Marks et al. 2016, Birds of Montana.