Describing the pattern and variations in spatial pattern of biodiversity and revealing its underlying mechanisms remain a central focus in ecology. However, less attention was paid to the species range size, and few studies have explored the drivers of species range size and the relationship between species range size and species richness (rescue effect). Here, we use a comprehensive dataset of breeding birds collected from 2018 to 2019 along the elevational gradient in Lebu Valley, Eastern Himalayas of China to explore the a) species mean elevational range size pattern, b) drivers influencing species mean elevational range size, and c) rescue effect. We found that species mean elevational range size of birds in Lebu Valley was a hump-shaped pattern (species mean elevational range size was largest at middle elevations), and the annual temperature range and normalized vegetation index were the most important explanatory variables for the species mean elevational range size pattern. However, species mean elevational range size was negatively correlated with the annual temperature range and positively correlated with the normalized vegetation index, respectively. These results were contrary to the predictions of the climate variability hypothesis and the ambient energy hypothesis. In addition, the correlation between species mean elevational range size and habitat heterogeneity was weak, which indicated that the habitat heterogeneity hypothesis also failed to predict the breeding bird mean elevational range size pattern in Lebu Valley. Moreover, we found the hump-shaped species richness pattern, which could also be resulted from non-directional rescue effect. Given the uncertainty in mean elevational range size pattern and the fact that much of the previous research has rarely tested the relationship between species range size and richness patterns, hypotheses explaining the elevational range size and the underlying mechanisms should be tested in more studies of different taxa and regions.