Divergence of morphological characters in two white-eye species (Passeriformes: Zosteropidae) in sympatry

Abstract

When two ecologically similar, sympatric species compete in their zone of overlap, their traits are expected to diverge in the area of sympatry, while remaining more similar in the areas of allopatry. This reduces competition. In this study, we document divergence of some morphological characters in allopatry and sympatry in two species of white-eye occurring in Sri Lanka, viz. the Sri Lanka white-eye (Zosterops ceylonensis) and Oriental white-eye (Zosterops palpebrosus). The extent of the overlap zone may be as large as 20–30 km in some areas. The study was carried out from February 2013 to July 2015. Birds were captured using mist nets in sympatric and allopatric zones of the two species, and 580 individuals belonging to both species were caught. We found that the bill length, bill width, and mass of the Sri Lanka white-eye in the zone of overlap were significantly different from those in the allopatric zone. In general, the bill length, bill width, and mass showed an increase in the zone of overlap for the Sri Lanka white-eye. The sympatric population of the Sri Lanka white-eye has evolved sufficient differences to be ecologically isolated from the smaller Oriental white-eye, thereby facilitating coexistence. The larger bill of the Sri Lanka white-eye in sympatry enables it to be more successful in obtaining food than the other species. The trends of these changes showed that there is gradual replacement of the Oriental white-eye in the upper elevations.