2.2 Environmental variables and study area

We used 19 bioclimatic variables with a resolution of 2.5 minutes obtained from WORLDCLIM as predictors (www.worldclim.org). Although there was low spatial autocorrelation and dependence of points in environmental space (Olivier Broennimann et al., 2012; Guisan & Zimmermann, 2000), to compare the relationship between environmental factors and species, we used consistent variables used in two different space. Pearson correlation analysis was used to remove the data of 19 environmental layers by correlation value | r | > 0.7, considering Asian openbill habits, 10 environmental variables were retained bio1, bio3, bio8, bio9, bio10, bio12, bio15, bio17, bio18, and bio19. These nonlinear data were then converted into niche quantification analysis through principal component analysis. We used the first three principal components, which summarized 84.8% of the overall variation in the environmental data, to represent environmental variability across the landscape for model analysis. PC1 was mainly composed of precipitation factors related to annual average temperatures and low temperatures, PC2 mainly represented changes in low temperatures and temperature differences, and PC3 was related to annual precipitation and temperature in the rainy season (see Table S1).
The extent of the model calibration area had key impacts on the ENM results (Barve et al., 2011). Considering the distribution status, dispersal trends, dispersal ability, and ecological habits of the Asian openbill, we decided to use four different study areas. 1) The native range consisted of 9 administrative borders of India, Nepal, Sri Lanka, Bangladesh, Myanmar, Vietnam, Laos, Thailand, and Cambodia. 2) For the new distribution areas, we delimited the administrative boundaries of Yunnan, Guangxi and Guizhou provinces of China as the new northern range, while we chose the geographical division of the Malay Peninsula as the new southern range. 3) The calibration range, based on the native range, added accessible areas of the species including southwest China, the Malay Peninsula, Sumatra Island, and Pakistan. 4) The modeling prediction extent was comprised of the boundaries of east Asia, southeast Asia, South Asia, and Australia (Fig.1). The native range and new distribution range were used for comparative ecological niche analysis in environmental space, while the calibration range and project range were used for modeling and prediction in geographical space.