Aim
Seasonal variation in community composition and species distributional ranges along elevational gradients remain poorly known but are essential to inform conservation. In this study, we aim to understand how species richness, community composition, and elevational ranges of montane birds change between the breeding and the non-breeding season.

Location
The east slope of the southern Gaoligong Mountains, Yunnan, southwestern China, elevational range: 700 - 3400 m a.s.l.; latitudinal range: 24°56´- 26°09´ N.

Methods
We compared bird species richness and community composition in nine 300-m elevational bands in the breeding (April - May) and non-breeding (December - January) seasons. We also calculated seasonal elevational shifts of 97 species with sufficient data recorded in both seasons and assessed how species’ traits influenced these shifts.

Results
Species richness declined in high and low elevations between the breeding and non-breeding season. The temporal beta diversity shift from the breeding to the non-breeding season was mainly caused by species losses rather than species gains in high- and low- elevation communities. Communities in middle elevations showed a contrasting pattern, with seasonal composition change resulting mainly from species gains. We also found that species’ seasonal distribution shifts were mainly associated with breeding elevation and diet. Notably, high- and middle-elevation breeders and insectivores significantly shifted their elevational ranges downslope in the non-breeding season. In addition, species that participate in mixed-species flocks and that rely on forests also showed significant downslope shifts in the non-breeding season.

Main Conclusions
These results show complex patterns of the interconnectedness of bird communities along the elevational gradient. Keeping forests at middle elevations intact appears especially important as they are used in winter by species that breed at both high and middle elevations. Furthermore, our results suggested conservation actions maintaining connectedness in low and middle elevations are urgently needed to conserve regional biodiversity and highlight the importance of seasonality in montane ecosystem research.

From April 2016 to December 2020, we conducted avian community surveys using standard line transects along the elevational gradient in breeding and non-breeding season. We divided nine 300-m elevational bands from 700 m a.s.l. (around the lowest elevation in Salween valley) to 3400 m a.s.l. (the mountain peak). For each elevational band, we located two to four transects varying in length, but the total lengths of transects in each elevational band were approximately seven km to ensure comparable efforts. For the breeding seasons, we conducted our surveys eight times in all transects in April and May (i.e., two replicates per year) when birds were actively breeding. We conducted non-breeding surveys along the same transects in two non-breeding seasons from December 2018 to January 2019, and in December 2020. 

We used N-mixture model structures for our occupancy models and estimated both occurrence and detection probability of 97 species in each elevational band in each season with temporal replicates.  We determined the upper-elevation limit, the lower-elevation limit, and the elevational range size of these 97 species using these criteria for true absences in elevational bands in each season. We then calculated the abundance-weighted mean elevation for each species. The seasonal shifts in these four measures for individual species were the differences in corresponding values from non-breeding to breeding seasons. Thus, positive values of seasonal shifts in the upper-elevation limit, the lower-elevation limit, and the abundance-weighted mean elevation represent upslope movements, while negative values represent downslope movements, and zero represents no change.

Finally, we assessed how species’ traits affected seasonal elevational shifts. Species’ traits included the abundance-weighted mean breeding elevation, body mass, diet, flocking status, and forest dependency. We classified the 97 species into low-elevation (700 - 1600 m a.s.l.), middle-elevation (1600 - 2800 m a.s.l.), and high-elevation breeders (2800 - 3400 m a.s.l.) based on their abundance-weighted mean elevations recorded in the breeding season. We classified the 97 species into low-elevation (700 - 1600 m a.s.l.), middle-elevation (1600 - 2800 m a.s.l.), and high-elevation breeders (2800 - 3400 m a.s.l.) based on their abundance-weighted mean elevations recorded in the breeding season. We extracted body mass and dietary groups (insectivore, omnivore, plant-seed eater, and frugivore/nectarivore) from the Elton traits 1.0 database (Wilman et al., 2014). We extracted flocking status (flocking species vs. non-flocking species) from Birds of the World (Billerman et al., 2020) and an empirical database of mixed-species bird flocks in southwestern China (Zhou et al., 2019). We classified two categories of forest dependency (forest birds: medium and high forest dependency; and non-forest birds: low forest dependency) provided by BirdLife International Data Zone (BirdLife International, 2020).

Below are descriptions of each column of the dataset: