Habitat selection and population trends in terrestrial bird species of Robinson Crusoe Island: habitat generalists vs. forest specialists

Hahn I, Vergara P, Römer U

Abstract

Habitat loss and degradation on oceanic islands are key processes leading topopulation decline of endemic birds and facilitating the establishment of invasive birdspecies. In this study, carried out in the Robinson Crusoe Island, we assessed density andhabitat selection of terrestrial bird species, including juan fernandez firecrown and juanfernandez tit-tyrant, two endemics, as well as green-backed firecrown and austral thrush,which apparently originate from the mainland. Results show that perturbed habitats containeda low density of the endemic species whereas the mainland species were significantlymore abundant in perturbed scrub habitats. Bird species show different habitatselection patterns, with endemics selecting for native forest and mainland species selectingfor perturbed habitats, or using them at random. Bird species experienced temporal trendsin their overall population sizes, with the endemic tit-tyrant suffering a significant declinein its population size of about 63% between 1994 and 2009. Only mainland speciesexhibited temporal changes in habitat use, significantly reducing their densities in thepreferred scrub habitats, possibly as a response to decreased habitat quality. Thrushesapparently were able to compensate the population decrease in one non native habitat typeby using native forests, a habitat giving them the opportunity of preying on nests ofendemic species. We conclude that endemic bird species behave as specialists whereas themainland species must be treated as invasive generalists on Robinson Crusoe Island.