Interspecific competition and facilitation coexist in mixed-species bird flocks of montane coniferous forests in Taiwan

Besides competition, positive interactions also play an important role in shaping the social structure of mixed-species bird flocks. This study aimed to illuminate the interspecific interactions of competition and facilitation in mixed-species bird flocks. We recorded the foraging behavior and microhabitat use of flocking species in montane coniferous forests of Taiwan under different social contexts. Foraging niche breadth and niche-overlap with other flocking species were compared between individuals inside and outside of mixed flocks. For the three microhabitat variables (foraging locations, vertical strata and horizontal strata), relationships between niche-overlaps of heterospecific pairs of these flocking species and their corresponding interspecific associations were determined using a simple linear regression. While in mixed flocks, two understory species, Taiwan fulvetta Fulvetta formosana and yellowish-bellied bush warbler Horornis acanthizoides, shifted their foraging from shrubs upwards into coniferous trees. Meanwhile, flamecrests Regulus goodfellowi moved downwards vertically within the canopy, and black-throated tits Aegithalos concinnus spread out horizontally along branches. In addition, flamecrests applied many more sally-hovers inside of mixed flocks than outside of flocks. All four species are insectivores which might find it more difficult to obtain sufficient food during the colder winters when food resources become scarcer. Therefore, they may be using the increased vigilance afforded by the flock to expand their foraging niches and thus to increase their foraging opportunities inside mixed flocks. Furthermore, niche-overlaps of heterospecific pairs of the 11 common flocking species were positively correlated with their corresponding interspecific associations on all three microhabitat variables. These results indicate that a greater foraging niche-overlap between two flocking species would result in higher coexistence of the two species in mixed flocks. Consequently, facilitative interactions occurred in these mixed-species flocks in addition to competitive interactions.