Predicting territory density of Dusky Orange-crowned Warblers Oreothlypis celata sordida breeding on Santa Catalina Island, California

Abstract

Capsule Territory density of the Dusky Orange-crowned Warbler Oreothlypis celata sordida, endemic to the California Channel Islands, USA, varied with vegetation greenness and oak coverage on Santa Catalina Island.Aims To model factors affecting territory density of warblers on Santa Catalina Island.Methods Breeding warblers were monitored in a dry and a more mesic habitat on the island in 2006–2008 and modelled using a GIS tool to predict territory density. Data of three-year territory boundaries with ecological covariates were used to develop a regression tree and project the distribution of territory density on an island-wide scale. The fitted model was indirectly evaluated using the island-wide point counts.Results The territory density of warblers differed between mesic and xeric habitats in response to oak coverage and food abundance in oaks. In the model, Normalized Difference Vegetation Index and oak coverage were the best predictors. Individual density estimates from distance sampling were comparable with the model prediction.Conclusion Plot-based surveying and statistical modelling provided an insight on how breeding warblers responded to resource abundance. This approach may facilitate the management of these insular birds that are highly dependent on oak habitats on the island where habitat loss has been occurring due to anthropogenic fire.