Create a new order, Phaethontiformes, for the Phaethontidae
NOTE: The following proposal was submitted to and passed NACC, and is here submitted with the authors’ permission.
Background: The tropicbirds (Phaethontidae), as currently recognized by the AOU, form part of the order Pelecaniformes. The traditional members of the Pelecaniformes, including the tropicbirds, share a number of morphological characters, including totipalmate feet, which occur only in the traditional Pelecaniformes (del Hoyo et al. 1992, Hedges and Sibley 1994, Kennedy and Spencer 2004). The traditional Pelecaniformes were also united by the placement of the salt gland and the lack of any brood patch, among other synapomorphies (Hedges and Sibley 1994). The monophyly of this group was questioned by Sibley and Ahlquist (1990) and again by Hedges and Sibley (1994) based on DNA-DNA hybridization work. However, even in the DNA-DNA hybridization-based reconstructions, the tropicbirds formed a monophyletic group with three other traditional families of Pelecaniformes: the cormorants (Phalacrocoracidae), anhingas (Anhingidae), and gannets and boobies (Sulidae) (Hedges and Sibley 1994).
Sequence-based molecular phylogenies provide a different perspective on relationships of the tropicbirds. Kennedy and Spencer (2004), in a species-level phylogeny of the frigatebirds (Fregatidae) and tropicbirds, found that the tropicbirds are not closely related to the other Pelecaniformes, although their ability to fully explore this issue was compromised by limited sampling outside their focal groups. Molecular phylogenies with greater taxon sampling include Ericson et al. (2006), who, using five nuclear loci (~5000 base pairs of data), found that the tropicbirds occupy a position in a basal clade (“Metaves”) of the Neoaves where they are very distantly related to the other Pelecaniformes. A similarly distant relationship between the Phaethontidae and the other Pelecaniformes was recovered by Hackett et al. (2008), in a study using 19 nuclear loci (nearly 32,000 base pairs). Both of these studies placed the tropicbirds near the doves (Columbidae), mesites (Mesitornithidae), sandgrouse (Pteroclidae), and grebes and flamingos (Podicipedidae and Phoenicopteridae) (Ericson et al. 2006, Hackett et al. 2008). Although support for the separation of the Phaethontidae from the other Pelecaniformes was very strong in both studies, support for placement near the groups listed above was strong only in the Ericson et al study, and in that study the result may be driven by a single atypical gene.
Recommendation: It seems clear that the tropicbirds do not belong in or near the traditional Pelecaniformes. We propose recognizing a new order, the Phaethontiformes, already adopted by Christidis and Boles (2008) and Gill et al. (2009), and placing this new order before the Ciconiiformes (see Proposal 2009-C-02) in the linear classification.
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Ericson, P.G.P., Anderson, C.L., Britton, T., Elzanowski, A., Johansson, U.S., Källersjö, M., Ohlson, J.I., Parsons, T.J., Zuccon, D., and Mayr, G. 2006. Diversification of Neoaves: integration of molecular sequence data and fossils. Biol. Lett. 2 543-547
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Hackett, S.J, Kimball, R.T., Reddy, S., Bowie, R.C.K., Braun, E.L., Braun, M.J., Chojnowski, J.L., Cox, W.A., Han, K., Harshman, J., Huddleston, C.J., Marks, B.D., Miglia, K.J., Moore, W.S., Sheldon, F.H., Steadman, D.W., Witt, C.C., and Yuri, T. 2008. A phylogenomic study of birds reveals their evolutionary history. Science. 320 1760
Hedges, S.B. and Sibley, C.G. 1994. Molecules vs. morphology in avian evolution: The case of the “pelecaniform” birds. Proc. Natl. Acad. Sci. USA. 91 9861-9865
Kennedy, M. and Spencer, H.G. 2004. Phylogenies of the frigatebirds (Fregatidae) and tropicbirds (Phaethontidae), two divergent groups of the traditional order Pelecaniformes, inferred from mitochondrial DNA sequences. Molecular Phylogenetics and Evolution. 31 31-38
Sibley, C.G. and Ahlquist, J.E. 1990. Phylogeny and classification of birds. Yale. Univ. Press, New Haven, Connecticut
Shawn Billerman, Irby Lovette, Terry Chesser, August 2010
Comments from Robbins: “YES. The genetic data clearly demonstrate that the tropicbirds are not closely related to the Pelecaniformes.”