Hypotaenidia is a genus of birds in the family Rallidae. The genus is considered separate by the IOC and IUCN, while The Clements Checklist of Birds of the World / eBird consider the species to be part of Gallirallus. Since the mid-19th century, when these rather terrestrial birds were recognized to form a genus distinct from the decidedly aquatic Rallus, the validity of Hypotaenidia has been disputed.
Only in the 2010s has this question be resolved to some degree of satisfaction. While the present genus does indeed belong to the Gallirallus group in tribeRallini of subfamilyRallinae, it stands apart from the numerous more basal lineages therein, and constitutes a well-distinct clade of smallish to mid-sized species distributed from Sahul far into the Pacific – across a range of at least 9,000 by 8,000 km (almost 6,000 by 5,000 miles). Like their larger more ancestral relatives, they readily evolve into flightless island endemics in the absence of terrestrial predators; however, their smaller bulk makes it easier for them to get airborne, and more prone to be swept away by strong winds. Consequently, they are (together with the Zapornia crakes) suspected to make up much of the prehistoric diversity of flightless rails of the oceanic islands of the Pacific.[2][3]
Hypotaenidia rails are typically a brownish hue on the upperside; this can range from almost beige to blackish in lightness and generally tends toward reddish hues, and many species have light spots or stripes on the remiges or the entire wings; often, conspicuous light stripes run above and/or below the eyes. The underside is barred black-and-white in almost all members of this genus; unlike in the Zapornia crakes (which are also smaller on average, but with some overlap, and generally similar in coloration) this barring is not restricted to the area between legs and tail, but extends across the entire belly up to the neck. A few species are so cryptic that they appear almost uniformly medium to blackish-brown at a distance. The eyes are red to chestnut, and the bill and legs are either more or less bright red to yellowish, or blackish, usually in line with the overall darkness of the plumage.[4]
Species
Genus Hypotaenidia, as circumscribed after the split-up of Gallirallus, contains the following species:[5]
"Sharpe's rail" (traditionally Gallirallus sharpei) is the name given to a singular and enigmatic specimen, Naturalis Biodiversity Center RMNH 87485. Probably collected around 1860, the specimen seems to have arrived at the museum without any accompanying data. It has been suggested to be an aberrant individual of H. philippensis which it certainly resembles much in size, shape, and some plumage details; unpublished DNA data also indicates it belongs to this species. Consequently, it would belong in Hypotaenidia if Gallirallus is split.
Extinct
Several rail species that have gone extinct in fairly recent times and which were traditionally placed in Gallirallus are better assigned to Hypotaenidia:
The aberrant Chatham rail may also belong in Hypotaenidia
The Chatham rail (Cabalus modestus) is another species that was usually included in Gallirallus in the genus' loose circumscription, and often considered a close relative of H. dieffenbachii with which it shared its island home. However, these two species are far less closely related than presumed, leading to reestablishment of the genus Cabalus when Gallirallus was split up. Even so, the Chatham rail is very closely related to the Hypotaenidiaclade, and analyses are hampered by the lack of data from the extinct and little-known species. Some datasets do resolve the Chatham rail as member of Hypotaenidia, and it may eventually be moved here.[2][3][6][7]
Also, many Pacific rails died out before they could be studied alive by scientists, and are not even attested by skin specimens, but only from fragmentary evidence such as subfossil bones or brief reports in travellers' accounts. They have been traditionally placed in Gallirallus, but most if not all would seem to belong elsewhere, and at least in the case of Polynesian species almost certainly in Hypotaenidia:[8]
In addition, the Small Oʻahu "crake" ("Porzana" ziegleri) is anatomically similar to Hypotaenidia and could have been be a Zapornis-sized species of the present genus, while the Snipe rail (Capellirallus karamu) of New Zealand might be closely enough related to the Chatham rail to be included in its genus, i.e. Hypotaenidia if Cabalus is considered invalid.[6]
References
^"Rallidae". aviansystematics.org. The Trust for Avian Systematics. Retrieved 2023-07-27.
^Taylor, P. Barry & van Perlo, Ber (1998): Rails: a guide to the rails, crakes, gallinules, and coots of the world (Helm Identification Guides). Yale University Press, New Haven. ISBN0-300-07758-0.
^Kirchman, Jeremy J. et al. (2021): Phylogeny based on ultra-conserved elements clarifies the evolution of rails and allies (Ralloidea) and is the basis for a revised classification. Ornithology138(4): 1–21. doi:10.1093/ornithology/ukab042PDF fulltext
^A similar bird was found to live on nearby Vava‘u in 1793. Given that G. vekamatolu was flightless, this may just as well represent a related species.
^Kirchman, J.J.; Steadman, D. W. (2005). "Rails (Aves: Rallidae: Gallirallus) from prehistoric sites in the Kingdom of Tonga, including description of a new species". Proceedings of the Biological Society of Washington. 118 (2): 465–477. doi:10.2988/0006-324x(2005)118[465:rargfp]2.0.co;2. S2CID86755841.
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