Hybrid birds of paradise may occur when individuals of different species, that look similar and have overlapping ranges, confuse each other for their own species and crossbreed. Erwin Stresemann hypothesised that hybridisation among birds-of-paradise might explain why so many of the described species were so rare. Stresemann examinined many controversial specimens and, during the 1920s and 1930s, he published several papers on his hypothesis. Many of the species described in the late 19th and early 20th centuries are now generally considered to be hybrids, though some are still subject to dispute; their status is unlikely to be settled definitely without genetic examination of museum specimens.