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, southwest China, elevational range: 700–3,400 m a.s.l.; latitudinal range: 24°56′–26°09′N.Methods: We compared species richness and community composition of birds 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 the 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.