1. Describing the patterns and revealing the underlying mechanisms responsible for variations in community structure remains a central focus in ecology. However, important gaps remain, including our understanding of species abundance. Most studies on abundance-based relationships are from either temperate ecosystems or tropical ecosystems, and few have explicitly tested abundance-based relationships across a temperate to tropical ecotone.

2. Here, we use a comprehensive dataset of breeding birds across elevation spanning a temperate to subtropical gradient in the Himalayas-Hengduan mountains of China to examine the relationship between species abundance and (1) elevational range size, (2) body size, (3) elevational range center and, (4) endemicity.

3. We tested a priori predictions for abundance-elevational range size relationship, abundance-body size relationship, and abundance-elevational range center relationship and explored how these relationships change along this temperate to subtropical mountain ecosystem.

4. We found that species abundance was significantly positively correlated with elevational range size across the study sites, demonstrating the key importance of elevational range size towards species abundance. Body size and elevational range center are weakly correlated with abundance. A novel finding of our study is that the abundance-elevational range size relationship gradually weakened from temperate to subtropical ecosystems, adding to a growing body of evidence suggesting that abundance-elevational range size tracks a temperate to tropical ecotone.

5. Our study demonstrates that abundance range-size relationship can transition across ecotones where faunas of different evolutionary origins converge. Furthermore, measuring abundance relationships across different environmental variables at the same spatial scale with comparable biogeography is a key strategy that can reveal the underlying mechanisms behind abundance patterns.

We made four replicated bird censuses during the breeding season (April to October) in each site along the elevational gradient. These bird censuses were from May-June 2012, August 2012, September-October 2012, and July-August 2013 in Gyirong; April and May in 2017 and 2018 in Gaoligong; August and September in 2018, and June and July in 2019 in Lebu. For each site, we divided the entire elevational range into 300-m elevational bands: twelve in Gyirong from 1,800 to 5,400 m, nine in Lebu from 2,300 to 5,000 m, and nine in Gaoligong from 700 to 3,400 m. We did not survey at lower or higher elevations because of inaccessible topography (e.g. turbulent rivers and cliffs) or harsh climate. We set two to four transects in each 300-m elevational band, all transects were in areas with the most undisturbed habitat available at each elevational band, and the habitat was contiguous between all elevational bands. We standardized sampling effort in each gradient by limiting the total length of transects in each 300-m elevational band around 7.5 km in Gyirong, and 7 km in Lebu and Gaoligong (length of individual transects were between 1.5 and 4 km). Details of the transects are provided in Tables S1-S3 of Appendix 1. We carried out field surveys between 30 minutes after dawn and 10:00 hours, and between 16:00 hours and 30 minutes before sunset (local time) but not conducted in extreme weather conditions (heavy rain or fog). For each transect, we made two replicated surveys in morning and two at dusk, respectively. We sampled the elevational bands in a random order to reduce the temporal autocorrelation. Field surveys in each elevational gradient were conducted by the same proficient observers. The observers recorded all bird species within 50 m of each transect. We used taxonomy of birds following Zheng (2011), and only used breeding birds (residents and birds breeding in the Himalayas-Hengduan mountains) for statistical analyses, because migratory birds could cause potential bias of the elevational range since we sampled the birds during breeding season.