Historically, an apparent absence of transitional forms has made it difficult to reconstruct the evolutionary affinities of the different modern avian ‘orders’. As you’ll know if you’ve been keeping up with the results of the various big molecular and morphological analyses – and who hasn’t – the avian cladogram is gradually coming together, though areas of debate and disagreement remain. One of the most vexing areas has been the origin of the parrots, or psittaciforms. An idea frequently mooted in the ornithological literature is that parrots are close kin of pigeons and doves – the columbiforms – and in fact some authors have even implied or stated that parrots are part of Columbiformes. Say what? I think we need to look at this in more detail…
The idea that parrots might be columbiform derivatives is perhaps best known thanks to its promotion by Alan Feduccia, author of the only ‘mainstream’ volumes on Cenozoic bird evolution (the 1980 The Age of Birds and 1996 The Origin and Evolution of Birds). In the first of these books, he wrote ‘shorebird derivatives include the sandgrouse, doves, and pigeons (Columbiformes), and, through the tooth-billed pigeons, the parrots (Psittaciformes)’ (Feduccia 1980, p. 98). In his second book, Feduccia referred in passing to parrots as ‘the columbiforms’ presumed close allies’ (Feduccia 1996, p. 252). Though the inference from Feduccia (1980) is that tooth-billed pigeons are ancestral to parrots, the figure caption on p. 99 states ‘The tooth-billed pigeons are probably surviving relicts of a group that became specialized after diverging from the main stem of columbiform evolution, and they show many anatomical features that would be found in the ancestral stock of parrots’. This implies convergence between tooth-billed pigeons and parrots, not direct affinity. Incidentally, the Tooth-billed pigeon Didunculus strigirostris is from Samoa, though it’s known from fossils to have lived on Fiji, and an unnamed species of the genus is known from Tonga.
While writing these contradictory comments, Feduccia probably had in mind Philip Burton’s 1974 paper on parrot and pigeon jaw and tongue morphology. Burton (1974)
showed that Didunculus was more like a parrot than was any other kind of bird in many details of cranial musculature. Burton listed five features seen in Didunculus which ‘showed some approach to [the condition in] parrots’ and was also impressed by features such as the form of the palatines and quadrates, the fused lower jaw, the long pterygoids, the short, deep upper jaw, and the broad nasal bars (pp. 272-273) [D. strigirostris shown below].
In the conclusions of the paper, Burton proposed that Didunculus may exhibit shared derived states with parrots. Rather than proposing that Didunculus might be near the ancestry of parrots however, Burton chickened out and wrote ‘More likely Didunculus is a relict survivor of a specialized group of pigeons which became separated early from the main stock of Columbiformes. This may not be the actual group from which parrots evolved, but clearly shows many of the anatomical features which might have been expected in the stock ancestral to parrots’ (p. 274).
So while Didunculus was used by Burton as evidence supporting a possible link between pigeons and parrots, the similarities discovered between Didunculus and parrots were regarded by him as convergent: in other words, they were were dismissed as evidence for a close affinity. As it happens, Didunculus is almost certainly nothing to do with the ancestry of parrots, but Burton would have been justified in proposing a possible affinity at the time. So: why propose an affinity of parrots and pigeons when the ‘trump card’ (= the cranial characters of Didunculus) is deemed an example of convergence? Yes, why indeed.
Recent work on columbiforms supports the view of Goodwin (1967) and others that Didunculus is well nested within pigeons and is not a disparate relict or part of a group well separated from other pigeons. Janoo (1996) wrote that ‘despite the morphological skull divergence, Didunculus is a typical columbine’; molecular studies show that Didunculus is part of the same columbid clade as Goura (crowned pigeons), Caloenas (Nicobar pigeons) and dodos and solitaires (Shapiro et al. 2002); and morphological work supports similar affinities but finds Gallicolumba to be close to these pigeons too (e.g., Goodwin 1967, Worthy 2001, Mahler et al. 2003) [dodo skeleton and model shown below, from the Oxford University Museum].
The bottom line is that, while Didunculus is rather like a parrot in some aspects of cranial morphology, it represents the extreme among a group of robust-skulled frugivorous pigeons and is nothing to do with parrot ancestry at all. It should not, therefore, be inferred to be suggestive of a pigeon-parrot link.
And – what are the affinities of parrots and pigeons? They are sort of close in one analysis (Livezey & Zusi 2007), but belong to fundamentally different branches of the neornithine tree in others (Fain & Houde 2004, Ericson et al. 2006, Mayr 2007, Hackett et al. 2008). Notably, no recent study has recovered a parrot-pigeon clade, least of all the inclusion of parrots within Columbiformes.
next soon: the Birds Come First hypothesis!
Refs – –
Burton, P. J. K. 1974. Jaw and tongue features in Psittaciformes and other orders with special reference to the anatomy of the Tooth-billed pigeon (Didunculus
strigirostris). Journal of Zoology 174, 255-276.
Ericson, P. G. P., Anderson, C. L., Britton, T., Elzanowski, A., Johansson, U. S., Källersjö, M., Ohlson, J. I., Parsons, T. J., Zuccon, D. & Mayr, G. 2006. Diversification of Neoaves: integration of molecular sequence data and fossils. Biology Letters 2, 543-547.
Goodwin, D. 1967. Pigeons and Doves of the World. British Museum (Natural History)
Fain, M. G. & Houde, P. 2004. Parallel radiations in the primary clades of birds. Evolution 58, 2558-2573.
Feduccia, A. 1980. The Age of Birds. Harvard University Press.
– . 1996. The Origin and Evolution of Birds. Yale University Press.
Hackett, S. J., Kimball, R. T., Reddy, S., Bowie, R. C. K., Braun, E. L., Braun, M. J., Cjojnowski, J. L., Cox, W. A., Han, K.-L., Harshman, J., Huddleston, C. J., Marks, B., Miglia, K. J., Moore, W. S., Sheldon, F. H., Steadman, D. W., Witt, C. C. & Yuri, T. 2008. A phylogenomic study of birds reveals their evolutionary history. Science 320, 1763-1768.
Janoo, A. 1996. On a hitherto undescribed dodo cranium, Raphus cucullatus L. (Aves, Columbiformes), with a brief taxonomical overview of this extinct flightless Mascarene Island bird. Bulletin du Muséum national d’Histoire naturelle, Paris 4e série, 18, Section C, no 1, 57-77.
Livezey, B. & Zusi, R. L. 2007. Higher-order phylogeny of modern birds (Theropoda, Aves: Neornithes) based on comparative anatomy. II. Analysis and discussion. Zoological Journal of the Linnen Society 149, 1-95.
Mahler, B., Araujo, L. S. & Tubaro, P. L. 2003. Dietary and sexual correlates of carotenoid pigment expression in dove plumage. The Condor 105, 258-267.
Mayr, G. 2007. Avian higher- level phylogeny: well-supported clades and what we can learn from a phylogenetic analysis of 2954 morphological characters. Journal of Zoological Systematics and Evolutionary Research 46, 63-72.
Shapiro, B., Sibthorpe, D., Rambaut, A., Austin, J., Wragg, G. M., Bininda-Emonds, O. R. P., Lee, P. L. M. & Cooper, A. 2002. Flight of the dodo. Science 295, 1683.
Worthy, T. 2001. A giant flightless pigeon gen. et sp. nov. and a new species of Ducula (Aves: Columbidae), from Quaternary deposits in Fiji. Journal of The Royal Society of New Zealand 31, 763-794.