DOI: 10.5061/dryad.z08kprrdc
title: Data for: Interspecific competition and facilitation coexist in mixed-species bird flocks of montane coniferous forests in Taiwan

description: 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.
description: Fieldwork was carried out in
 the Tataka (N23.487546, E120.891723), Hehuanshan (N24.162243,
 E121.287129), and Dasyueshan (N24.255348, E121.008445) areas, Taiwan,
 during both the non-breeding (September-February) and the breeding
 (March-August) seasons in 2015 and 2016. Mixed-species flocks were
 regularly formed from September to February when cold weather prevails. We
 focused on flocking members of mixed flocks and recorded their foraging
 behavior and microhabitat use. Extensive trails within each study site
 were used for searching birds. A two-person team worked together and
 followed targeted bird species closely for as long as possible to collect
 data on each individual’s foraging behavior. Different social contexts
 (mixed-species flock, monospecific flock, solitary, or in pairs) were
 noted whenever a flocking species was encountered. All the species in a
 mixed flock were identified, and the number of individuals of each
 participant species was determined with the aid of binoculars. After the
 flock disappeared, both observers double-checked and compared the numbers
 of all species identified in the flock. If the recorded numbers for the
 individuals of a particular species were not the same between the two
 observers, we used the larger one, the reason being that one of the
 observers might have had a better view of the flock or was situated at the
 right spot to count the birds when they crossed a trail (Chen and Hsieh
 2002, Kotagama and Goodale 2004). The proportion of all flocks taken
 together occupied by each flocking species was then calculated as the
 flocking frequency of each species; in other words, we calculated a mean
 proportion across all the data from the 81 flocks (Hutto 1994). We also
 collected foraging data of flocking species both inside and outside of the
 mixed flocks. The foraging behavior and categories for microhabitat use of
 a foraging bird as defined by Remsen and Robinson (1990) were used. The
 main foraging behaviors included gleaning, hanging, probing,
 sally-striking, and sally-hovering. Reaching and pecking occurred very
 rarely; thus, we lumped them with gleaning and probing. We made only one
 observation from each individual in a mixed flock to maintain the
 independence of our behavioral data. To achieve this, we always shifted to
 another species whenever observations of one individual had been
 completed. In some cases, we observed more than one individual of a
 particular species if we were able to assure that both individuals were
 not the same one. For example, we occasionally moved on to another
 individual of the same species which was located somewhere away from the
 previously targeted bird, or we switched to an individual of the opposite
 sex which we could distinguish by its distinctive sex-specific plumage.
 (For more details, please read the paper, which will be published in the
 Journal of Avian Biology)