Altitudinal migration behavior patterns of birds on the eastern slope of Mt. Gongga, China

Many bird species in montane regions exhibit altitudinal migration behavior; however, altitudinal migration of birds is still understudied, especially in Asia. Mt. Gongga (7556 ​m) is the highest peak of the Hengduan Mountains in Southwest China. The steep elevation gradient and the high bird diversity make the eastern slope of Mt. Gongga (Hailuo Valley) an ideal place for studying the altitudinal migration behavior patterns of birds. We synchronously recorded the local bird diversity, temperature and humidity at two sites (1800 and 3000 ​m a.s.l.) during three migration seasons from September 2020 to June 2021 to identify birds exhibiting altitudinal migration behavior. During our surveys, we recorded 146 bird species in total and 20 passerine bird species were altitudinal migrants according to our altitudinal migration formula. Among those 20 altitudinal migrant species, eight bird species displayed a typical altitudinal migration pattern (upward migration during the breeding season and downward migration during the non-breeding season). Moreover, temperature was correlated with the daily number of individuals (DNI) at each study site. Therefore, increasing temperatures possibly caused upward migration of birds (DNI decreased at 1800 ​m and increased at 3000 ​m) and vice versa. To further elaborate, the Rufous-gorgeted Flycatcher (Ficedula strophiata), the species with the most prominent altitudinal migration behavior, initiated upward migration at a temperature of 11–12 ​℃ at 1800 ​m and initiated downward migration at a temperature of 12–13 ​℃ at 3000 ​m. Besides, humidity was positively correlated with the DNI. Therefore, increasing humidity (equals to increasing rainfall or snow) possibly caused downward migration of birds (DNI increased) and vice versa. Furthermore, bird species exhibiting a typical altitudinal migration behavior pattern are feeding on insects. Thus, the spatial and temporal changes of the invertebrate biomass might be an important ecological driver for the altitudinal migration of invertivorous (birds eating invertebrates) birds. This study provides fundamental data for the altitudinal migration of birds in the Hengduan Mountains and shows how altitudinal migration is seasonally dynamic across an elevational gradient in a subtropical mountain region.