Cory’s Bittern, Ixobrychus “neoxenus” exilis: morph, mutant, or mixed bag?

Résumé

Cory’s Bittern (Ixobrychus “neoxenus” exilis) is an enigmatic form, usually referred to as a rare dark colour phase or morph of Least Bittern (Ixobrychus exilis [Gmelin, 1789]). A review of the literature indicates that birds identified as Cory’s Bittern show considerable plumage variation and commonly exhibit multiple plumage anomalies. Temporal and spatial clumping of records shows that they are of irregular occurrence; thus, they do not meet the criteria for recognition as a colour morph. Cory’s Bittern should thus be viewed as a colloquial name that refers to any one of a number of abnormally dark-plumaged, genetically undefined and phenotypically heterogeneous Least Bitterns.