Systematic studies have three main objectives: firstly to determine the unique properties of taxa at and above the species level; secondly, to determine common properties of selected taxa and to suggest reasons for similarities and differences; and thirdly, to investigate variation within taxa (Mayr, 1969). Oystercatchers provide excellent material for such studies. Despite their distinctiveness and evolutionary conservatism, they have had a surprisingly unstable systematics. This has occurred mainly because Old and New World species are rarely sympatric, and thus the latter have commonly been regarded as only subspecies of the former. New Zealand is perhaps the only country where species of both Worlds overlap in their distributions, and thus afford opportunity for comparative study. Further impetus for a systematics study of oystercatchers is provided by their relative homogeneity. They comprise a monogeneric family of shorebirds which has not undergone any major adaptive radiations into a diversity of ecological niches, but rather has dispersed from original centres of distribution to occupy identical niches in new geographical localities. The uniformity of structure and habit of this family has been attributed by Larson (1957) to extreme ecological specialization coupled with strong stabilizing selection Although Lack (1961) has pointed out that ecological divergence of closely related forms is not known to initiate species-formation, the inherent potential of such divergence to function as an isolating mechanism is well established (Mayr, 1963; Cain, 1966). Conversely, specialization is likely to restrict speciation and result in evolutionary conservatism. This restriction is reflected in the taxonomy of oystercatchers, with a maximum of eight species being generally accepted at present.