2.1 Study site
Field experiments were performed in the Guilin botanical garden
(107°17′E,25°01′N) in Guangxi Zhuang Autonomous Region, Southwest China
(Fig.1). The altitudes range from 180~300 m above sea
level. The climate in this region is dominated by the mid-subtropical
zonal monsoon and the average annual air temperature is approximately
19.2 °C, ranging from -4.2 °C in January to 36 °C in July. The annual
average precipitation is 1800 mm and the mean annual relative humidity
is more than 78% (Tang et al., 2009). The Guilin botanical garden is a
comprehensive research base integrating ex-situ protection of endangered
plants, domestication and planting of garden ornamental plants. The
local vegetation is a middle subtropical evergreen and deciduous
broad-leaved mixed forest, and the fruits of plant species, such asCinnamomum camphora , Machilus thunbergii , and Ficus
concinna can provide sufficient food resources for birds.