2.2.1 Plant–frugivore bird network
Based on the distribution characteristics of fruit plants, we set up
four transects (2-3km for each transect) in the survey area to observe
the feeding behavior of birds from September 2020 to August 2021. Once
birds were found to forage for plant fruits, we recorded which bird
species fed on which plant, the number of fruits that were removed by
each bird species either by swallowing or carrying away from the parent
tree, the number of birds per visit, and the total foraging time (from
arrival to departure). Observations were recorded during two foraging
periods of 7:00-10:00 a.m. and 14:00-17:00 p.m. with Safari l0×26 zoom
binoculars until no more fruits remained on the mother trees. The
observation frequency at least eight per month. If frugivorous birds
visited trees in conspecific flocks, foraging behavior was recorded for
a single, randomly chosen individual and was assumed to be
representative of the entire feeding flock (Breitbach et al., 2010).
During the field observations, we found that some birds were foraging
the fruits by pecking, and made it difficult to record realistic feeding
quantity. Thus, to avoid the influence of bird feeding patterns on the
network structure (Jordano et al., 2016; Zhang et al., 2022), we took
the frequency of bird visits to plant species to build the interaction
networks between birds and fruits plants.
2.2.2 Fruit and frugivoretraits
We measured the traits of all plant and bird species relevant to their
roles in interaction networks. Plant traits included fruit diameter (d,
mm), length (l, mm), mass (m, g), volume (v, mm3), and
color (co). Fruit length, fruit diameter, and fruit mass were measured
from 20 fruits obtained from one to three trees per species for each
fruiting plant species using Vernier calipers and an electronic balance.
The fruit volume was calculated using ellipsoid volume calculation:
v=4/3 (π×l/2×d/2×d/2) (Zhang et al., 2022). Bird traits included bill
width (bw, mm), body length (bl, mm), body mass (bm, g), wing length
(wl, mm), and wing loading (wlo)
as a proxy of bird movement capacity: Wlo=bm/2×wl. With increasing wing
loading value, the movement capacity of a bird species decreases
(Camargo et al., 2020). These bird traits were obtained from a field
guide to the birds of China (Mackinnon and Phillipps, 2000) and A
Handbook of the Birds of China (Zhao, 2001).