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.

Altitudinal bird migration involves seasonal shifts up and down the altitude gradient annually. Asia as the place with the largest number of altitudinal migrants, has quite few related studies, especially for montane and temperate avifaunas. To explore the potential drivers of seasonal altitudinal migration for birds in the middle of Hengduan Mountains, we conducted a three-year investigation on breeding and non-breeding season bird communities at eight elevational bands (1200–4200 ​m) in the Gongga Mountains. We examined the altitudinal migration patterns and relationships between seasonal distribution shifts and species' traits of 50 species with sufficient data recorded in both seasons. We found that a large proportion of breeding birds underwent altitudinal migration and showed three migration patterns (downslope shift, upslope shift, no shift). Seasonal distribution shifts were mainly correlated with certain ecological traits. Species breeding at high and mid-elevations, nesting in scrub and being omnivorous are more likely to show downslope movements during the non-breeding season. In addition, territorially weaker species exhibited more diverse migration patterns. Notably, we found the hand-wing index (HWI) was actually more convincing than body mass in explaining altitudinal migration. These results consolidate the studies of seasonal altitudinal migration in montane birds. Our study could be used to bridge existing knowledge gaps that currently impeding effective conservation for montane avifaunas in the Hengduan Mountains.

Gut microbiota and host interactions co-evolve and develop into stably adapted microbial communities and play vital roles in maintaining the health of organisms. Diet is supposed to be an important driver of differences in gut microbiota, but previous studies would commonly use literature depictions, which are essential but inaccurate, to explain the effects of diet on the gut microbiota of wild birds. In this study, we collected intestinal samples from five sympatric flycatchers to compare the gut microbial differences using bacterial 16S rRNA genes from Illumina MiSeq platform. Over 1,642,482 quality-filtered sequences from 18 16S rRNA libraries were obtained and distinct compositions and diversities of gut microbiota were found in five flycatchers. Their gut microbiota is mainly from the four bacterial phyla of Proteobacteria, Firmicutes, Actinomycetes, and Bacteroidetes, but at the genus level showed a significant difference. Functional predictions revealed that the metabolic capacity of the gut microbiota of five flycatchers is greatly distinguished at KEGG level 3. And multiple food fragments showed a significant correlation with gut microbiota. Besides, the significant differences in the specific composition of the diets of the five insectivorous flycatchers indicated the differentiation of dietary niches. The study of the gut microbiota and feeding habits of sympatric flycatchers would increase the understanding of the gut microbial diversity of wild birds, and also improve our cognition of the co-evolution and co-adaptation within the host gut microbiota relations.