Patterns of bird species richness and composition on islands off Arnhem Land, Northern Territory, Australia

The bird faunas of the adjacent Wessel and English Company island chains we re sampled at two scales (0.25 ha quadrats and entire islands). Ninety-six species were recorded from 226 quadrats, with the most frequently recorded species being mistletoebird Dicaeum hirundinaceum, brown honeyeater Lichmer a indistincta, silver-crowned friarbird Philemon argenticeps, bar-shouldere d dove Geopelia humeralis, northern fantail Rhipidura rufiventris and yello w white-eye Zosterops lutea. At the quadrat scale, vegetation type was a ma jor determinant of the abundance of individual species (and hence species c omposition), species richness and total bird abundance. Bird species compos ition and richness at the quadrat scale was also significantly affected by island isolation (particularly the amount of land within 20 km of the islan d perimeter). Island size had no effect on quadrat-scale richness or total abundance. However, the abundance of 10 of the 38 most frequently recorded individual species was significantly related to island size, in most cases even when the comparison was restricted to similar habitats. The most strik ing cases were rufous fantail Rhipidura rufifrons, mangrove golden whistler Pachycephala melanura, brown honeyeater and yellow white-eye, which were a ll significantly more abundant on smaller islands. One hundred and seventy- one species were recorded from the 62 islands sampled. There was a very tig ht relationship between island size and the number of terrestrial species ( 73% of deviance explained) and of all species (84% of deviance explained). This relationship was improved (marginally) when isolation was included in the model. Ordination of islands by their terrestrial bird species composit ion was related to island size and isolation, and suggested an erratic spec ies composition on small islands.