APPARENT AVIAN EXTINCTIONS FROM ISLANDS IN A MAN-MADE LAKE, SOUTH-AFRICA

We surveyed avian diversity and abundance on islands in a 16 year old man-made lake in southern Africa. The vegetation was semi-arid shrubla nd and grassland. We compared species lists on islands with matched si tes on the surrounding mainland. We recorded a total of only 19 specie s on islands, 25 on islands connected by landbridges to the mainland a nd 32 species on the mainland. Islands had significantly fewer species than their mainland pairs but the difference was unrelated to island size. Three species (Fairy Flycatcher Stenostira scita, Redeyed Bulbul Pycnonotus nigricans, and Namaqua Dove Oena capensis) were common in mainland censuses but absent on islands. These species are insectivoro us, frugivorous and granivorous respectively. Two species, Cape Sparro w Passer melanurus and Mountain Chat Oenanthe monticola, were more com mon on islands than mainland sites. Larger species and those with patc hy mainland distributions were most prone to extinction on islands. Th is study shows that habitat fragmentation can cause rapid species loss , even in vagile groups such as birds. It points to the importance of recolonisation ability for predicting minimum viable areas. The persis tence of species in habitat fragments may depend critically on corrido rs over which birds are willing to fly.