<a name="RTFToC7">Trewick--Sympatric Chatham rails </a>

Journal of the Royal Society of New Zealand abstracts

**Sympatric flightless rails Gallirallus dieffenbachii and G. modestus on the Chatham Islands, New Zealand; morphometrics and alternative evolutionary scenarios**

Steven A. Trewick*

The extinct rails Gallirallus dieffenbachii and G. modestus were sympatric on at least three islands of the Chatham group. Morphological and genetic evidence indicates that they evolved from the same volant ancestor, putatively the banded rail, G. philippensis. Morphometric analyses were used to compare these three species and the New Zealand flightless weka, G. australis. It is evident that both the Chatham rails were flightless, and each had undergone significant changes in body shape relative to G. philippensis. G. dieffenbachii was similar in overall form to the weka and most other flightless Gallirallus spp., being larger than G. philippensis in all except the wings. It is possible that G. dieffenbachii was a generalist feeder, as is the weka. G. modestus was a considerably smaller bird with a long beak that is presumably evidence of the evolution of specialised feeding behaviour.

General similarity between G. dieffenbachii, G. australis and most flightless members of the group results from the parallel evolution of these species in allopatry, whereby a more or less enlarged generalist feeder almost invariably results. The ancestral stock of G. modestus is hypothesised to be the more recent arrival on account of specialisation resulting from competition between the coloniser and the generalist resident, G. dieffenbachii. A general principle is proposed by which the products of later colonisations will be the more specialised through the additional selective effect of the earlier occupant.

Keywords: Rallidae, Gallirallus, Capellirallus, flightless rails, Chatham Islands, sympatry, morphometrics, speciation.

(c) Journal of The Royal Society of New Zealand,

Volume 27, Number 4, December 1997, pp 451-464

PDF file of entire paper: medium quality (1403K); (scanned from paper original: notes about this process)

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