Using a large citizen science dataset to uncover diverse patterns of elevational migration in Himalayan birds

Among montane birds, elevational migration is a well-described phenomenon. Yet, apart from mountain ranges in the Americas, there is little information on the large-scale patterns and extent of elevational migration. Using a large citizen science dataset (eBird), we determine the elevational ranges of 377 Himalayan bird species in their breeding and non-breeding periods. Based on the position of species' seasonal elevation ranges, we describe five elevational migration patterns that broadly include post-breeding upslope and downslope migration. Most high-elevation breeders (65-75%) were downslope migrants, which were further subdivided into four distinct patterns: 'displace' (complete downslope), 'shift' (partial downslope), 'expand' (lower limit expansion) and 'contract' (upper limit contraction). We find significant intraspecific variation in migration patterns across the Himalayas, possibly determined by local biotic and abiotic conditions. Specialized dietary guilds like invertivores were more likely to show shift or displace migration, potentially tracking seasonally fluctuating food resources, while generalists like omnivores and human commensals were more likely to be resident. Territorial birds were largely non-migratory, most likely to retain high-quality breeding territories. As mountains are a bounded domain with limited combinations of species' seasonal elevation ranges, the patterns we describe here are useful for understanding elevational migration globally.