Fig. 9. (a) The shifts of habitat population centroid and(b) the percent contribution of the environment variables in 3 climate zones.
The constraint of STZ distribution is LUCC accounting for 34.3% importance. The distribution in STZ is also influenced by bio14 and bio15 accounting for 21.6% and 11.4%,respectively. Seasonal rainfall significantly affected the distribution in this region. NDVI is the most important variable in the distribution of MSZ and MTZ which the percent contribution is 51.6% and 55.5% in 2 regions. Alt also plays a great role accounting for 24.6% in MSZ and LUCC accounts for 19.7% in MTZ.
The result shows that the reduction of the habitat area and the movement of the population centroid will cause an increase in the distance and the consumption, which forces the birds change their migration time and route. Moreover, some birds may be not even able to achieve migration. The habitat of migratory birds in central China trends to move towards east in future, and the habitat of migratory birds in the southern China trends to move towards south.
The result shows that NDVI is an important variable in southern China. The variable shows a great percent contribution to the distribution of migratory birds in southern China, which means the food is the constraint of birds. In the middle China, LUCC plays a great role to the distribution. Especially in the Haihe River Basin and Yellow River Basin, the importance of LUCC accounts for over 30%, which means human activities influence the distribution greatly. In the Yangtze River Basin of the middle subtropical zone, we can see alt is an important variable to the distribution. Because of the wide area, the altitude varies greatly in the regions.

Implications of birds protection

4.2.1 The status of birds protection in China
Species with broad tolerance may persist or even expand in the changing environments, but migratory birds are sensitive to the environment (Huang et al., 2017; Runge et al., 2015). It is not effective to protect the biodiversity in China though the country has committed great contribution in the remission of biodiversity loss (Young et al., 2014). The main measure of protecting the species is establishing the protected area in China. The identification and delineation of habitat is considered as the cornerstone of the establishment of protected areas. Whether nature reserves works well under climate change has been more important to protect species. Although the number of nature reserves increases recent year, the protective effect is not obvious. For there is no complete system to estimate the effectiveness of nature reserves (Liang et al., 2018c; Ma et al., 2019; Sang et al., 2011). It is more difficult to ensure the effectiveness of protected areas since they move to different places each year.
Considering the climate change and the distribution of the migratory birds, we compared the differences between the nature reserves and the suitable areas(Fig.10). Endangered species are more environmentally sensitive and the population of these species is rare. The migration of these endangered species makes it more difficult for human to protect them effectively.