Taxonomy
Its taxonomy is confusing and remains unsettled. It has sometimes included the Torresian, Yellowish and White Imperial-pigeons as subspecies. The widespread nominate subspecies of the Pied Imperial-pigeon differs from all these by its plain white thighs and undertail coverts (though often with a dark spot at the very tip), and its narrowly dark-tipped bluish bill.
For comparison, the other species' have black-spotted undertail coverts and thighs (spotting mainly near thighs in White Imperial-pigeon), the bill of the Torresian Imperial-pigeon is greenish-yellow, and the bills of the Yellowish and White Imperial-pigeons are bluish at the base and yellowish at the tip. Furthermore, the Yellowish Imperial-pigeon has a distinctive yellowish tinge to its plumage (some Pied Imperial-pigeons may also appear yellowish, but infrequently to the same extend), and the White Imperial-pigeon has silvery-grey remiges. However, the taxon melanura of the Moluccas, which usually is considered a subspecies of the Pied Imperial-pigeon, resembles the Torresian Imperial-pigeon in bill, thighs and undertail coverts, but has a significantly broader black tail-tip. Consequently, some have suggested it should be placed under the Torresian Imperial-pigeon, while others have suggested it should be considered an entirely separate species, D. melanura (for which the name Black Imperial-pigeon has been used – an unfortunate choice, as only the tail has significantly more black than the other members of this group, and the name Black Imperial-pigeon usually has been used for D. melanochroa). Yet others have considered melanura to be invalid, instead believing it only is a morph of D. b. bicolor, as both types can be found on some islands.
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