Bill Dimensions of the Three Species of Zosterops on Norfolk Island

Abstract

Species pairs of birds on islands, originating from double invasions of single stocks from other islands or mainland, differ greatly in bill size where they are sympatric. Differences in bill length have been found to range from 15.6 to 50% (Grant 1968). This has been interpreted to indicate that substantial differences in diet, presumed to be associated with the differences in bill length, are necessary for the sustained coexistence of two species derived in the above manner. One of the species pairs used in the analysis was Zosterops albogularis and Z. tenuirostris on Norfolk Island. Mean measurements given by Lack (1944) were found to yield a difference of 16.7%. Mees (1970) has recently pointed out that these measurements are not correctly representative of the populations. They are from single individuals of each species, and were made much earlier by G. Matthews. Mees's own more extensive, and hence more representative, measurements yield a difference in mean bill length of no more than 4.5%. Such a small difference renders these two Zosterops species an exception to the empirical rule that sympatric, double-invasion, species pairs of birds differ in bill length by at least 15%.1 Further, it raises a doubt that appreciable differences in diet are a necessary condition for coexistence. However, dimensions other than length are as important or more so in contributing to the efficiency of the bill in dealing with foods (Grant 1968). Differences in other dimensions may reflect differences in diet more accurately for some species than differences in length. Only length was used