SYMPATRIC FLIGHTLESS RAILS GALLIRALLUS-DIEFFENBACHII AND GALLIRALLUS-MODESTUS ON THE CHATHAM ISLANDS, NEW-ZEALAND - MORPHOMETRICS AND ALTERNATIVE EVOLUTIONARY SCENARIOS

The extinct rails Gallirallus dieffenbachii and G. modestus were sympa tric on at least three islands of the Chatham group. Morphological and genetic evidence indicates that they evolved from the same volant anc estor, putatively the banded rail, G. philippensis. Morphometric analy ses were used to compare these three species and the New Zealand fligh tless weka, G. australis. It is evident that both the Chatham rails we re flightless, and each had undergone significant changes in body shap e relative to G. philippensis. G. dieffenbachii was similar in overall form to the weka and most other flightless Gallirallus spp., being la rger 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 e vidence of the evolution of specialised feeding behaviour. General sim ilarity between G. dieffenbachii, G. australis and most flightless mem bers of the group results from the parallel evolution of these species in allopatry, whereby a more or less enlarged generalist feeder almos t invariably results. The ancestral stock of G. modestus is hypothesis ed to be the more recent arrival on account of specialisation resultin g from competition between the coloniser and the generalist resident, G. dieffenbachii. A general principle is proposed by which the product s of later colonisations will be the more specialised through the addi tional selective effect of the earlier occupant.