Evidence for the erstwhile reality of the double-banded argus pheasant Argusianus bipunctatus, one of the world's most mysterious crypto-birds, is best described as feather-light - in every sense.
Male great argus pheasant (© Francesco Veronesi/Wikipedia CC BY-SA 2.0)
Truly a giant among pheasants, an adult male specimen of the great argus pheasant Argusianus argus (=giganteus) can measure up to 6.5 ft long. Native to the Malay Peninsula, Sumatra, and Borneo, it is also instantly recognised by virtue of its extremely long slender tail, and its huge fan-like wings, whose broad feathers are ornately embellished with rows of spectacular ocelli (eye-like markings) with which to capture the attention of female argus pheasants during courtship (and which earned these birds their name too - Argus being the many-eyed watchman from Greek mythology). There is also a lesser-known related species, the crested argus pheasant Rheinardia ocellata, once again native to southeast Asia but housed in a separate genus.
On 8 April 1871, moreover, T.W. Wood published a short report in The Field documenting a singular partial feather of unknown provenance and very unusual appearance. A portion of a male bird's primary from the right wing, on first sight it resembled those of the great argus. Closer observation, however, revealed that this strange plume bore two bands of speckled, chocolate-brown colouration - one on its broad web, and one on its narrow web. In contrast, corresponding plumes from the great argus bear only one such band, on their broad web.
1870s engraving showing a close-up of the unique double-banded argus pheasant feather (top) alongside that of a great argus pheasant feather (bottom) (public domain)
Consequently, Wood deemed that the aberrant feather must have originated from a second, hitherto-unknown species of argus, and he duly dubbed this unseen species Argus [now Argusianus] bipunctatus, the double-banded argus pheasant.
The first three feathers in this illustration are from the crested argus pheasant, the fourth is the single, unique feather from the double-banded argus pheasant (public domain)
The lone feather from Wood's newly-created species was presented by Edward Bartlett in 1891 to the British Museum (Natural History)'s ornithological collection at Tring, where it still resides today as the only tangible evidence for the double-banded argus's reality. No sightings of this most mysterious bird have ever been reported either - which can be explained at least in part by the simple fact that no-one really knows where to begin looking for it.
Pheasant expert Dr Jean Delacour suspected that Java would prove to be the homeland of this cryptic bird, but his several searches for it here all proved unsuccessful. In 1983, ornithologist G.W.H. Davison nominated Tioman, an offshore island of eastern Malaysia, as a more plausible provenance, though not a very promising one. This is because Tioman had been well-explored scientifically during the 20th Century, thereby rendering it unlikely that a bird as sizeable as a species of argus pheasant could still exist there undetected.
In other words, if this speculative species was indeed native to Tioman, it must surely now be extinct. Yet if so, this would be exceptionally tragic, because the aerodynamic properties of its unique feather as determined from its precise physical structure are so poor that this ostensibly lost species might conceivably have been flightless - and, if so, would have constituted the only species of flightless modern-day pheasant known to science.
Having said that, in a Journal of Field Ornithology paper from 1992 K.S. Parkes dismissed this perplexing plume as nothing more than a freak feather from a great argus, in which the normal single band of speckled brown colouration had been duplicated via the expression of a mutant gene. Moreover, following the International Ornithological Congress's removal of the double-banded argus from its list of valid taxa in 2011, a year later the IUCN followed suit by removing it from its list of extinct bird species. Hence the double-banded argus is currently the avian equivalent of a persona non gratis as far as ornithological taxonomy is concerned, and unless any new discovery is made in the future it is likely to remain so.
This ShukerNature blog article is expanded from my book Mysteries of Planet Earth.