1 IntroductionSpecies dispersal has a profound impact on species populations from ecological and evolutionary perspectives (Johnson & Gaines, 1990; Paradis et al., 1998), and represents the first stage of the invasion process (Davis, 2009). The causes of species dispersal are complex: it may be influenced by life-history, kin selection, avoiding of inbreeding depression, habitat loss, climate change, and species introductions (Clobert, 2012; Kokko, 2006; Travis & Dytham, 2002). Researching on species dispersal and distribution from the ecological niche perspective can help better understand biological responses to environmental changes (Ingenloff et al., 2017; McCauley et al., 2014). Otherwise, dispersal behavior into novel areas may pose challenges to the integrity of local ecosystems and biodiversity conservation (Tingley et al., 2014; Vilà et al., 2010). Therefore, it is necessary to study changing ecological niches and distribution trends during species dispersal.Over the past two decades, Ecological niche models (ENM) have been widely implemented in biological invasion research. Extensive research has revealed ecological niche dynamics, potential geographical distributions, driving factors behind invasion, and assessments of invasion risk using various modeling approaches, such as measuring niche characteristics and comparing differences between native and novel niches, analyzing habitat suitability, and predicting future spatial distribution trends (Beukema et al., 2018; Olivier Broennimann & Guisan, 2008; Mandle et al., 2010; Ørsted & Ørsted, 2019; Parravicini et al., 2015; Tingley et al., 2014; Villaverde et al., 2017). Studies of niche dynamics under climate change, which have received more attention, show the biological invasion often accompany with niche shift (O. Broennimann et al., 2007; Gallagher et al., 2010; Stiels et al., 2014). However, such conclusions can be suspicious because of insufficient evidence or inappropriate modeling frameworks, such as lack of analyses in environmental space, and excluding factors like partial niche filling, sampling bias, or the unequal availability of environmental conditions (Guisan et al., 2014; Peterson et al., 2011; Petitpierre et al., 2012; Qiao et al., 2017). Hence, an appropriate modeling method should be chosen rather than blind trust (Joppa et al., 2013; Qiao et al., 2015).On the other hand, the recent use of niche models for biological invasion-related studies on plants or animals have mainly focused on colonized species. For example, ring-necked parakeet (Psittacula krameri ) (Strubbe et al., 2015), brown marmorated stink bug (Halyomorpha halys ) (Zhu et al., 2017), and eastern gray squirrel (Sciurus carolinensis ) (Creley et al., 2019) are such studies. They emphasize the fundamental niche of the species. With the development of niche theory, the relevant niche concepts have been constantly improved (Guisan et al., 2014; Jackson & Overpeck, 2000; Peterson et al., 2011; Sax et al., 2013). In particular, Sax (2013) proposed the existence of a marginal zone outside of the fundamental niche, a ”tolerance niche” area in which individuals of a species can survive even if they do not currently have self-sustaining populations. Tolerance niches are prevalent among many species, especially those with dispersal and migration behaviors, such as the Amur falcon that breeds in east Asia and winters in Africa, so the winter niche is the tolerance niche. Combined with the stages of biological invasion (Davis, 2009), individual dispersal behavior before the establishment of a population in a new distribution area should also be included in the tolerance niche discussion. However, in contrast to colonized species, there is much less information about the niche dynamics mechanisms for species that have not yet successfully established in an invaded area (Bush et al., 2018; Feng & Papeş, 2017; Rosenblad et al., 2019). Here, we select a representative species with natural dispersal to investigate its niche dynamics and the potential effects of dispersal on the population.The Asian openbill (Anastomus oscitans ) is a large wading bird that specializes in forage mollusks. It belongs to the stork family Ciconiidae and is mainly distributed in South Asia and Southeast Asia, including India, Sri Lanka, Bangladesh, Myanmar, Thailand, Vietnam, and other countries (Elliott et al., 2020). Observed data in recent years describe a large-scale phenomenon of Asian openbill dispersal: the first was documented in Dali, China in 2006 (Wang, 2007), it was recorded by Perlis State in Chuping, Malay peninsula in 2008 (Lim et al., 2008), and it was first discovered in Singapore in 2013 (Low et al., 2013). The dispersal behavior can be divided into two directions: south to southern Thailand, Malaysia, Singapore and north to northern Vietnam and China, and the number of dispersed individuals has increased from a few to thousands each year (Han et al., 2016; Jiang, 2010; Liu et al., 2015; Low et al., 2013). Data showed that no breeding behavior was found in any population of Asian openbill in the new distribution areas (Han et al., 2016; Low et al., 2013; Zainul-Abidin et al., 2017), and presented that majority population are subadult birds and the population is more in summer and less in winter in China (Han et al., 2016; Lei et al., 2017). In comparison to temperate species, tropical species are generally adapted to a narrow and constant range of abiotic conditions (Gaston & Chown, 1999; Janzen, 1967), It is interesting when tropical birds naturally spread to both low and high latitudes, hence the following questions about abiotic conditions during Asia openbill dispersal: 1) Did the niche change during the dispersal process? 2) Is it possible to establish populations successfully in new areas? 3) Based on the present niche characteristics, how might the Asian openbill spread in the future?