The response of mixed-species bird flocks to anthropogenic disturbance and elevational variation in southwest China

Avian mixed-species flocks (MSFs) are an important example of species interactions threatened by the biodiversity crisis. They are found throughout the world in forested habitats but are generally reduced in size or frequency by human disturbance. In southern China, a unique MSF system is led by several species of closely- related fulvettas (Alcippe morrisonia, A. hueti, and A. davidi). Our objective was to understand how this system is distributed across elevational gradients, especially moving west into the Hengduan Mountains, and how it responds to human disturbance. We sampled leadership and composition of 375 MSFs over 2 yr in and surrounding 5 nature reserves in Guangxi Zhuang Autonomous Region and Yunnan Province, with transects placed in different land use types - protected forest, buffer areas, or agriculture - and ranging from 400 to 3,200 m. We also sampled birds outside of MSFs. We found MSFs led by fulvettas (A. fratercula and A. davidi) in forests across the region up to 2,900 m. Elevation was not a significant influence on MSF size or prevalence in models that also included land use. We found that MSFs were encountered at only one-third of the frequency in agriculture as in forest, and had strongly different composition and leadership. Although MSFs in buffer areas were more similar to those in forest, birds in buffer areas had lower flocking propensity, different flock leaders, and less complex social networks. In particular, buffer transects that were seeded pine had low numbers of fulvetta-led MSFs and forest specialists in MSFs. In the future, it is important to understand which vegetational characteristics allow MSFs, particularly fulvetta-led MSFs, to persist in buffer habitats.