Male Birds-of-Paradise, in repose (top) and on display (bottom)
Left to right: "Le Sifilet" [the Western or Arfak Parotia] - Parotia sefilata, Superb Bird-of-Paradise - Lophorina superba , “Le Nébuleux” [The Nebulous Bird-of-Paradise] - ??
Despite their incredibly different outward appearances, the Birds-of-Paradise are all very closely related. We know this because of their skeletal similarities, and, these days, their genetic similarities.
However, their close affiliation to one another genetically has also caused problems in pinning down the exact number of species in the family Paradisaeidae, as it turns out that within each of the fourteen genus, the species are able to (and occasionally naturally do) cross-breed with one another. This wild cross-breeding is believed to be the source of many of the specimens and illustrated birds that have never again, or very rarely, been seen in the wild.
An example of this cross-breeding is shown above, at right. The "Nebulous Bird-of-Paradise" is thought to either be a misrepresented Twelve-Wired Bird-of-Paradise (unlikely, given Jacques Barraband's reputation as an ornithological illustrator), or a cross between two species living in nearby ranges of New Guinea. As the ranges of many different species and genus overlapped at the time of the specimens being gathered, it’s unknown which two would have created such a bird, or whether it would have been fertile (most wild cross-breeds of birds-of-paradise are). Genetic tests may give us the answers to these questions in the future.
Histoire naturelle des oiseaux de paradis et rolliers. Francois Levaillant, illustrated by Jacques Barraband, 1806.