Factors affecting the distribution of landbirds on the Falkland Islands

A high proportion of island birds are threatened with extinction as a result of their vulnerability to introduced predators, habitat destruction, and fragmentation/isolation effects. In order to conserve island species effectively, it is necessary to disentangle these effects on distribution and abundance. We attempt to do this for the nine native passerines in the Falkland (Malvinas) Islands, using a database of presence/absence on 59 offshore islands in the archipelago, linked to data for each island on mammal presence, habitat modification, and isolation. Falklands native passerines are of considerable conservation importance, comprising one endemic globally threatened species, several endemic subspecies, and several restricted range species. Presence of rats on islands was by far the most important predictor of passerine presence, overriding the effect of habitat modifications. The globally threatened endemic Cobb's Wren Troglodytes cobbi was absent from all islands containing rats. Some species were more likely, and others less likely to occur on islands where tussac Poa flabellata grassland had been destroyed by grazing. The former species were primarily those adapted to dwarf-heath vegetation, and/or that thrive around human settlements. Island size and isolation were important predictors of occurrence for several bird species. The analyses show that, if vegetation restoration in the Falklands is to meet conservation aims, then it should be accompanied by introduced mammal control. Secondly, they indicate that biogeographical effects on bird distribution among islands in the Falklands are important, and need to be considered when assessing the conservation status of species, and when considering conservation action.