New insights into the patterns and drivers of avian altitudinal migration from a growing crowdsourcing data source

Altitudinal migration is a common and important but understudied behavior
in birds. Difficulty in characterizing avian altitudinal migration has
prevented a comprehensive understanding of this behavior. To address this,
we investigated the altitudinal migration patterns and explored potential
drivers for a major proportion (~70%) of the entire resident bird
community along an almost 4,000 m elevational gradient on the main island
of Taiwan. Based on the occurrence records collected by citizen
scientists, we examined the seasonal shifts in the center and the upper
and lower boundaries of elevational distributions for 104 individual
species. We then built phylogeny-controlled regression models to
investigate the associations between the birds’ seasonal distribution
shifts and seven of their traits, and examined whether the observed shifts
can be explained by three main hypotheses on potential drivers. Results
showed that at least 60 species (58%) seasonally changed their
distributions along elevations. While most of them (42 species) tended to
move downhill in winter, a considerable number of species (14) tended to
move uphill. While the species breeding at high or low elevations tended
to move downhill in winter, those breeding at medium-low elevations tended
to move or extend their distributions to higher elevations. Our regression
models suggested that seasonal variations in climates and food
availability could be major drivers of the behavior. However, the three
hypotheses can only partially explain the observed downhill migration
patterns and none of them can well explain the uphill patterns, indicating
an important knowledge gap. This study investigated avian altitudinal
migration from a new perspective with a novel and generalizable approach,
and revealed interesting patterns that could be difficult to identify with
conventional approaches. It demonstrated the power of citizen science data
to provide new insights into this behavior by characterizing the general
patterns and mechanisms across a large number of species.