Bird sampling
Bird species were sampled along the elevational gradient from 2,350 m
and 4,950 m a.s.l. using the line transect method. Field sampling could
not be done lower or higher because of the inaccessible topographies
such as cliffs and glaciers. The entire 2,600 m elevational gradient was
divided into 26 consecutive 100-m elevational bands. One transect (the
length of each transect was 2.5 km) was placed in each elevational band
(for a total of 26 transects, Figure 1). Bird censuses were carried out
between 20 min after dawn and 10:00 hours and between 16:00 hours and 20
min before sunset (local time) by the same proficient observers. Birds
within 50-m of the observers were recorded. The transects were sampled
in a random order to reduce the temporal autocorrelation. Replicated
bird censuses were made four times during the breeding season, from June
to August 2018 and June to July 2019. For each transect, we made two
replicated surveys in the morning and two at dusk, respectively. The
taxonomic system of birds was followed Zheng (2011) (Zheng, 2011). We
used breeding birds (including residents and summer migrants) for
statistical analyses because seasonal and long-distance migratory birds
could cause potential bias.
We interpolated the presence of species to 26 elevational bands
according to their lowest and highest observed presences as the
relatively small study area and strong movement ability of birds. This
interpolation method is often used in elevational studies, and it can
reduce the bias of underestimation of species richness due to
insufficient sampling. We used species accumulation curves to evaluate
whether species diversity was sampled sufficiently. If the species
accumulation curve becomes an asymptote after a sharp rise and the rise
is slow, the sampling is sufficient (Ugland, Gray, & Ellingsen, 2003).
Estimator (Chao2) was used to compute estimated species richness. The
Chao2 statistics of each 100-m elevational band were calculated in
EstimateS 9.10 (http://purl.oclc.org/estimates) (R K Colwell,
2013). Pearson correlation analysis was used to assess the relationship
between observed species richness and Chao2 to assess how well the
species richness was sampled within each 100-m elevational band.