The eggs are pale blue or green. Both parents feed the young by regurgitating food. A second brood is often produced in a season. These birds migrate from the northern parts of their range in winter for the southernmost coasts of the United States and areas further south, travelling at night. They mainly eat fish, frogs, crustaceans and insects, which they capture with quick jabs of their bill while climbing through marsh plants. The numbers of these birds have declined in some areas due to loss of habitat. They are still fairly common but are more often heard than seen. They prefer to escape on foot and hide than to take flight. These birds make cooing and clucking sounds, usually in early morning or near dusk. The least bittern was originally described in 1789 by J. F. Gmelin based on specimens from Jamaica. The least bittern forms a superspecies with the little bittern and yellow bittern. There are five widely recognised subspecies. Birds from Ecuador are sometimes assigned to a sixth subspecies, "I. e. limoncochae" Norton, DW, 1965. North American birds were formerly divided into two subspecies, eastern ("I. e. exilis") and western ("I. e. hesperis"), but this is no longer believed to be a valid distinction. A dark rufous morph, "I. e. neoxenus", termed "Cory's bittern" or "Cory's least bittern" was originally described by Charles Cory as a separate species in 1885 from a specimen collected on or near the Caloosahatchee River, near Lake Okeechobee, in southwest Florida. Cory stated that the specimen was "without doubt perfectly distinct from any other known species". Further specimens followed over the next decades from Florida, Michigan, Illinois, Wisconsin, Ohio, and Ontario. Initially, Cory's least bittern was accepted as a valid species, with Elliott Coues and Richard Bowdler Sharpe both including it in published species lists. As early as 1892, however, doubts were raised about the validity of Cory's least bittern as a separate species. Nonetheless, in 1896 Frank Chapman wrote a detailed paper supporting its retention as a valid species. Outram Bangs later argued, in 1915, that this view was wrong and proposed that Cory's should become a junior synonym of least bittern. This view eventually prevailed, with the American Ornithologists' Union removing the species from their list of North American birds in 1923, although others held dissenting views until at least 1928.