Vegetation patterns in relation to bird nesting preferences on West Island, South Australia.
West Island near Victor Harbour, SA is an important near-shore nesting site for several bird species, but nesting behaviour in shorebirds is often affected by changes in rookery vegetation. Because the island is under increasing threat from invasion and expansion of weeds, its vegetation patterns were determined and preference analysis undertaken for nesting by Silver Gulls, (Larus novaehollandiae) and Little Penguins (Eudyptula minor), as studies in WA suggest they are affected by vegetation changes. The results indicated that there were four broad types of vegetation on the island: grass-dominated, shrub-dominated, mallow/weedy forb-dominated and native forb-dominated. These were correlated with soil and exposure features, the grasses and shrubs being most abundant on more sheltered alkaline and less saline soils, while shrubs dominated in rocky, leeward areas. The native forbs and low shrubs occurred in more exposed saline and alkaline areas, weedy forbs in more sheltered, less saline places. Nesting preference analysis showed that both the Silver Gulls and Little Penguins significantly preferred open, grass-dominated areas. Penguins were significantly less abundant in quadrats with high levels of exposed surface rock and shrubs or weedy annual forbs (although they did still nest under rocks in these areas), whereas the gulls significantly avoided native, perennial forb-dominated areas, although this may also be a reflection of the degree of exposure of the latter. As the weedy forbs are known to invade areas of increasing fertility resulting from nesting, this highlights the need for successful weed control in offshore rookeries if successful nesting is to continue in the long term.