Width of Range Overlap in Congeneric Bird Species Pairs Along An Elevational Gradient In The Eastern Himalayas

Abstract

We investigated the distribution of range overlap between contiguous pairs of congeneric bird species along a 200-2800 m elevational gradient in the eastern Himalayas. Over a 3-year field survey between from 2012 to 2014 we recorded 15867 birds comprising 245 species in elevational intervals of 50 m. From this data set, we selected 107 congeneric pairs and fit a logistic curve to the abundance ratio versus elevation data to estimate the width of transition zone over which the species ratio changes from 90% to 10%. We found that the distribution of the transition widths range from < 50 m (about a quarter of the pairs) to more than 500 m. This lack of physical exclusion suggests that there is considerable niche separation between closely related species. For a smaller subset of species pairs with better quality data and model fit we investigated the correlation between the degree of range overlap (transition width) on the one hand, and phylogenetic distance and morphological trait difference on the other. We found a significant correlation with mass, beak width and wing size, but not with tarsus length, beak length and phylogenetic distance. This suggests that mass beak width and wing size may play an important role in niche differentiation in the area.