The Rallidae are undeniably a very successful group of birds. Rallids have spread to almost every corner of the globe, including a few island corners that were never reached by other terrestrial birds. The subject of today's post is one of the widespread genera of rallids - but it's one of the trickier ones.
Stable classification of rallids has eluded ornithologists for years, for two main reasons. One is that rallids are a prime example of what may be called the smudge effect - clearly distinct subgroups, but without clear boundaries. Take a rail and a moorhen, and the differences are easy to spot. But then someone comes along with a third species, that looks a bit like a rail and a bit like a moorhen, and it's back to the bench for the frustrated ornithologist. The other is the unusual nature of rallid evolution and dispersal. Rallids are, normally, surprisingly good fliers - that's why they are able to reach so many remote islands. On the other hand, they tend to be very reluctant fliers, only invoking those flying abilities if they absolutely have to. As a result, rallids have tended to disperse to remote localities, and then promptly lost the flying abilities that got them there at the first available opportunity. With those losses of flying ability come other related changes - larger body size and such - resulting in the flightless rails looking not very much like their flying descendants, and an awful lot like unrelated flightless species from completely different islands.
The genus Amaurornis belongs to the subgroup of rallids known as the crakes. Crakes are quite generalised rallids, distinguished from the other major generalised rallid group, the rails, by their shorter beaks. Most crakes are notoriously shy and retiring birds - as an example of their reserve, one crake species, Amaurornis magnirostris of the Talaud Islands in Indonesia, was only described in 1998 (Lambert, 1998). The crake species assigned to Amaurornis, also known as bush hens or water hens, tend to be larger than other crake species in the genus Porzana and its satellite genera. Other than this, however, there seems to have been little or no definition on what actually distinguishes one genus from the other, and species have been shuttled back and forth between the two for years.
Indeed, Olson (1973) regarded Amaurornis as the generalised ancestral group for a number of other rallid genera, including Porzana, Porphyrio (swamphens), Gallinula (moorhens) and Fulica (coots) - effectively paraphyletic, except that he didn't suggest any specific relationships between descendant genera and particular species within Amaurornis. As well as the Asian and Australasian species previously included in the genus, Olson (1973) also assigned three African species previously regarded as the genus Limnocorax to Amaurornis.
The first explicitly phylogenetic analysis of the Rallidae (and still the only morphological analysis of the group) was the gigantic production of Livezey (1998). Livezey confirmed Olsen's suggestion of a close relationship between Amaurornis, Porzana, Gallinula and Fulica (but not Porphyrio), but Porzana was massively paraphyletic, with the other three genera belonging to a clade that was nested within Porzana - specifically within the subgenus Limnocorax (which Livezey had removed from Amaurornis and returned to Porzana so that at least one of the genera could be monophyletic). Livezey refrained from carving up Porzana into bite-sized monophyletic chunks because support for the recovered relationships within the genus was negligible, and while he did support monophyly for the core group of Amaurornis, it apparently didn't take much fiddling with the analysis to make the whole thing topple over like a badly-cooked soufflé (certainly, Livezey's taxonomic separation of Amaurornis from the other crakes as a separate subtribe Amaurornithina fits right into the "tits on a bull" category). There are also hints that convergence may have befuddled a number of results of the Livezey analysis - elsewhere in the tree, for instance, was a highly suspect clustering of flightless taxa from New Zealand and Mauritius.
Subsequent molecular analyses have tackled sections of the Rallidae, but unfortunately none have had the coverage of the Livezey (1998) analysis. The most significant for the crakes has been that of Slikas et al. (2002), which answered the hanging question of "is Porzana or Amaurornis the polyphyletic genus?" with "As it happens, they both are". Their analysis divided Porzana and Amaurornis between three clades - one containing species of the former, one containing species of the latter, and one containing species of both. Christidis & Boles (2008) proposed that the names Porzana and Amaurornis be each restricted to the appropriate one of the first two clades, with the third clade recognised as a third genus. Oh, and the correct name for that third genus just happens to be Limnocorax. That's right - after years of being the subject of a game of taxonomic kickball, Limnocorax breaks free to become its own genus - and one considerably larger than either of the other two. Unfortunately, Slikas et al. didn't include any representatives of Gallinula or Fulica to test where they sat relative to the three clades.
If we accept the results of Slikas et al. (2002), Amaurornis includes five species from southern and south-east Asia and northern Australasia - A. olivacea, A. isabellina, A. phoenicurus, A. moluccana and A. magnirostris*. Most of these are some variation on chestnut-brown, but the striking white-breasted A. phoenicurus is a notable exception. An unfortunate omission from Slikas et al.'s (2002) analysis was the large New Guinean flightless rail Megacrex inepta, which was placed in Amaurornis by Livezey (1998). A molecular analysis by Trewick (1997) that included Megacrex placed it in association with Rallus and its relatives (the rails), and far away from Porzana, but (a) none of the other "Amaurornis" species were included in this analysis, and (b) it was a neighbour-joining analysis. Yuck.
*Brief taxonomic note - Amaurornis is one of those genera that has been subject to argument about whether it is masculine or feminine. It's feminine, so the species names have to be formed accordingly (phoenicurus retains the masculine ending because it's a noun, not an adjective).
Christidis, L., & W. Boles. 2008. Systematics and Taxonomy of Australian Birds. CSIRO Publishing.
Lambert, F. R. 1998. A new species of Amaurornis from the Talaud Islands,
Indonesia, and a review of taxonomy of bush hens occurring from the
Philippines to Australasia. Bulletin of the British Ornithologist’s Club 118 (2): 67 – 82.
Livezey, B. C. 1998. A phylogenetic analysis of the Gruiformes (Aves) based on morphological characters, with an emphasis on the rails (Rallidae). Philosophical Transactions of the Royal Society of London Series B – Biological Sciences 353: 2077-2151.
Olson, S. L. 1973. A classification of the Rallidae. Wilson Bulletin 85 (4): 381-416.
Slikas, B., S. L. Olson & R. C. Fleischer. 2002. Rapid, independent evolution of flightlessness in four species of Pacific Island rails (Rallidae): an analysis based on mitochondrial sequence data. Journal of Avian Biology 33: 5-14.
Trewick, S. A. 1997. Flightlessness and phylogeny amongst endemic rails (Aves: Rallidae) of the New Zealand region. Philosophical Transactions of the Royal Society of London Series B – Biological Sciences 352: 429-446.