A large proportion of bird species in mountain regions engages in altitudinal migration, a behavior involves annual movements between breeding and nonbreeding ranges at different elevations. Compared to latitudinal migrations, altitudinal migrations, which usually involve short-distance movements, are understudied and their patterns and causes are poorly understood. While most altitudinal bird migrants breed at higher elevations and spend their winter times at lower elevations, other patterns do occur. To better understand the different patterns and their potential causes, we characterized altitudinal patterns of ~100 resident bird species in Taiwan, a sub-tropical island with an almost 4000m elevational gradient, and investigated the differences in birds’ functional traits (including morphological, ecological and life history traits) among pattern types. We characterized the migration patterns of individual species based on the seasonal changes in upper and lower elevational limits of their distributional ranges using the occurrence records in the eBird database. We then built multinomial logistic regression models to examine the associations between functional traits and different migration patterns.
Our results showed that the niche width of a species, defined as the air temperature range across its distributional range, was the most important determinant of whether the species moved along an elevation gradient seasonally. Species with wide niches tended to stay within the same elevational ranges during the whole year (without altitudinal migration), whereas species with narrow niches shifted their ranges along elevation between breeding and nonbreeding seasons. Interestingly, niche width could not explain why some migrants moved up and the others moved down in nonbreeding seasons although the latter, not the former, were expected to be associated with narrow niches under a thermal tolerance hypothesis. Besides niche width, body size and habitat diversity also had significant, but weaker, influences on the altitudinal migration patterns. While down-migrants tended to be smaller than non-migrants, up-migrants tended to be habitat specialists. These results suggested that post-breeding down-migrations of birds may be driven by cold temperatures in winter, and up-migrations may be related to specific habitat requirements. Using long-term and large-scale data collected by citizen scientists, this study demonstrates an alternative approach to provide new and general insights into understanding of altitudinal migrations of birds.