4.1 Tolerance niche expansion during dispersal

The occurrence data used in this study were composed of breeding and non-breeding populations, so the niches of several populations were collectively referred to as the observed niche in our analysis. Compared with the original native observed niche, the two new distribution sites had different ecological niche characteristics. The Malay Peninsula had niche conservatism, but southwest China had niche expansion. As opposed to common niche expansion, however, the Asian openbill distributed in China and the Malay Peninsula did not exhibit documented breeding behavior and it lacked self-sustaining capacity (Han et al., 2016; Zainul-Abidin et al., 2017). Therefore, the measured results should be defined as the expansion of the tolerance niche.
Niche breadth has a close relationship with range size (Kambach et al., 2019), so understanding niche breadth and physiological tolerances can result in better estimates of range size (Austin et al., 2009; Bush et al., 2018). The results of our measurements indicated that Asian openbill in China had a stronger tolerance to low temperatures. Indeed, Field surveys have found that some individuals in China can stand the temperature below zero (Han et al., 2016; Lei et al., 2017). Furthermore, native species ranges underestimate climate tolerance (Bocsi et al., 2016), especially that endotherms able to endure a wide range of environmental conditions (Strubbe et al., 2015). Hence abiotic factors were not the direct cause of the dispersal of Asian openbill.
Despite the obvious niche expansion, the observed niche of the whole population was still over a relatively short period, and niche conservatism cannot be rejected. However, this does not mean that the distribution of Asian openbill was in an equilibrium situation. At least in the tolerance niche, it is not known whether the bird has completely occupied the non-breeding space and presumably will continue to expand.