Frontiers of Taxonomy: Parsing the Purple Swamphens
The Purple Swamphen (Porphyrio porphyrio), as traditionally classified, is a group of related forms scattered across the Old World and Pacific Islands from Portugal to Samoa. This group is visibly diverse and has long been suspected of including multiple species. Three subspecies in particular—those of the western Mediterranean (Western, porphyrio), Africa and Madagascar (African, madagascariensis), and the Philippines (Philippine, pulverulentus)—are readily identifiable by sight, and seem natural candidates for recognition as separate species. The remaining nine subspecies—those of Asia, Indonesia, Australasia, and Oceania—are also diverse but not as clearly distinguishable, as their features appear to blend from one group into the next.
Odd Branches on the Family Tree. A genetic analysis published in 2015 (García-R. and Trewick) validated the three readily identifiable forms and also produced an unexpected result that points to a three-way split of the remaining subspecies: in Asia (Gray-headed, poliocephalus); in Indonesia (Black-backed, indicus); and eastward (Australasian, melanotus).
The surprise was that the geographically central and apparently transitional form, the Black-backed Swamphen of Indonesia, is not the closest relative of any of its neighbors. Instead, all of its neighbors are more closely related (by ancestry, as measured by mitochondrial RNA markers) to one another and to a very different species, the huge, flightless, tailless South Island Takahe (P. hochstetteri) of New Zealand, which naturally occurs alongside the Australasian Swamphen.
The logical inference seems straightforward: if Black-backed is a distant branch of the family tree, more distant than the obviously separate Takahe, then Black-backed must also be separate from the others, and therefore apparently serves as a wedge rather than a bridge between the dissimilar Asian and Australasian populations.
Australasian Swamphen, P. melanotus. (Westgate Park, Melbourne, Victoria, Australia; July 14, 2018.) © Dan Forster
South Island Takahe, flightless and tailless. (Zealandia Wildlife Sanctuary, Wellington, North Island, New Zealand; September 22, 2019.) © André Zambolli
South Island Takahe, showing its small frontal shield and purple, turquoise, green, and bronze coloration. (Orokonui Ecosanctuary, Dunedin, South Island, New Zealand; July 19, 2020.) © Oscar Thomas
García-R. and Trewick’s research indicates a subdivision of the former Purple Swamphen into a total of six species, plus three more based on their genetic proximity—although two of the additional three are extinct. Starting from the oldest divergent branch in the family tree, they are:
Western Swamphen (Porphyrio porphyrio)
Black-backed Swamphen (P. indicus)
African Swamphen (P. madagascariensis)
North Island Takahe (P. mantelli) †
South Island Takahe (P. hochstetteri)
Australasian Swamphen (P. melanotus)
Gray-headed Swamphen (P. poliocephalus)
Philippine Swamphen (P. pulverulentus)
White Swamphen (P. albus) †
Some authorities have adopted this proposal and some have questioned it. The main source of skepticism has been the apparent difficulty of drawing lines between Gray-headed and Black-backed due to apparent intergradation. Less clear is the possibility that Black-backed and Australasian might not be distinguishable in the field.
Philippine Swamphen, slaty-gray overall with a brown back. (Agusan Marsh Wildlife Sanctuary, Loreto, Mindanao, Philippines; November 26, 2015.) © Erickson Tabayag
Western Swamphen, showing all-coral-pink legs and violet upperparts. (S’Albufera, Mallorca, Spain; September 22, 2018.) © Guido Bennen
African Swamphen, showing all-coral-pink legs and bronze-green upperparts. (Marievale Bird Sanctuary, Gauteng, South Africa; August 4, 2018.) © Richard Gray
Australasian Swamphen, P. m. melanotos, showing dark knees and black upperparts. (Western Springs Park, Auckland, New Zealand; July 28, 2018.) © Oscar Thomas
An Easy Problem in Southeast Asia. Attention has been misplaced on the variable populations native to Southeast Asia, from northern Thailand and southern China south to Sumatra and Singapore, collectively classified as the subspecies viridis. Confusion about the significance of this variation apparently began from a presumption (of uncertain origin) that viridis was part of the Black-backed group. Field observers immediately knew that this couldn’t be right, for the simple reason that most viridis have unambiguously gray heads. A related, but less significant cognitive problem is that many viridis also have black backs, which seems to imply that Gray-headed and Black-backed cannot be separate species (or at the very least that these names are hopelessly misleading).
Gray-headed Swamphen, a dark individual with a blue-and-purple head. (Khateshwar Tank, Yavatmal, Maharashtra, India; April 29, 2013.) © Jayant Atrey
Taking a broader view, however, the range of variation observed in viridis occurs to some degree throughout the Asian populations traditionally classified as poliocephalus (mainly those of the Indian subcontinent). In other words, when considered as a species widespread from Turkey to Sumatra and Singapore, Gray-headed is highly variable in many conspicuous aspects of its coloration—including but not limited to the head and back—so while viridis certainly varies widely, its variation appears to be consistent with the variation observed elsewhere in Asia.
The obvious solution—which quickly became consensus—is to regard viridis as conspecific with poliocephalus. This revision seems uncontroversial, but it still leaves the fundamental ambiguities unresolved. Is there a line be drawn between Gray-headed viridis and Black-backed indicus? On the other side of indicus, is there a line between Black-backed and the various Australasian populations? Do we classify them as three, two, or one species?
Gray-headed Swamphen, a pale individual with a whitish-gray head. (Nong Bong Kai Wildlife Refuge, Chiang Rai, Thailand; February 20, 2019.) © Elizabeth Skakoon
Black-backed Swamphen, P. indicus, showing its typical plumage, which is approximately intermediate between Gray-headed and Australasian. (Muara Angke, Jakarta, Java, Indonesia; April 22, 2019.) © Lars Petersson
A Harder Problem in Indonesia. The central problem in swamphen reclassification is what to make of indicus, the Black-backed Swamphen. Visually, it appears to be a transitional form that blends into the neighboring Asian and Australasian populations, but genetic markers indicate that it has maintained a wholly separate identity for millions of years despite its inevitable interaction with those similar-looking populations.
If there are lines to be drawn on either side of indicus, they are difficult to ascertain—in part because information about populations that occupy potentially transitional areas is sparse, and in part because the available information seems to provide at least superficial evidence of intergradation.
To the west, although viridis is clearly allied with the Gray-headed, there is evidence of possible hybridization (or perhaps merely cohabitation) between viridis and indicus on the southern Malay Peninsula and Sumatra. So the reassignment of viridis to Gray-headed merely pushes the gray area southward.
Australasian Swamphen, P. m. pelewensis, of a population with coloration intermediate between Black-backed and melanotos. (Ngatpang Aquaculture Ponds, Ngatpang, Palau; December 31, 2018.) © Robert Tizard
To the east, certain populations assigned to the Australasian group—pelewensis of Palau, melanopterus of the Moluccas and Lesser Sundas, and bellus of southwestern Australia—appear visibly transitional between Black-backed indicus and the bulk of the Australasian populations farther east. It seems conceivable that some portion of these populations might belong with the Black-backed instead of the Australasian, or that their visible similarity is due to a history of hybridization—or that it is due to coincidence, or has another, unrelated explanation.
(To the extent that García-R. and Trewick were able to obtain and test tissue samples from these subspecies, the RNA markers placed them on the Australasian branch. So for now at least, that is how they are classified.)
Assuming that the genetic analysis is sound, the position of Black-backed in the Porphyrio family tree appears facially inconsistent with the relationships implied by its resemblances to neighboring populations. Giving full credence to objective evidence from the laboratory seems to dictate that it qualifies as a separate species, but this conclusion and the delineation of which populations are included must remain provisional while we continue to receive additional evidence from the field.
References
eBird. 2020. eBird: An online database of bird distribution and abundance. Cornell Lab of Ornithology, Ithaca, N.Y. http://www.ebird.org. (Accessed September 23, 2020.)
García-R., J.C., and S.A. Trewick. 2015. Dispersal and speciation in purple swamphens (Rallidae: Porphyrio). Auk 132:140-155.
Taylor, B., and B. van Perlo. 1998. Rails: A Guide to the Rails, Crakes, Gallinules, and Coots of the World. Yale University Press.
Text © Russell Fraker / September 23, 2020