Data and code for: Behavioral plasticity shapes participation in a mixed-species flocking community of birds

Behavioral plasticity can modulate the costs and benefits of sociality, and thus may play a prominent role in mediating competition and facilitation during social interactions in mixed-species groups. However, investigations of assembly patterns of mixed-species groups typically treat species’ behavioral attributes as static rather than dynamic features that can change in social contexts. We investigate four axes of behavioral plasticity that may modulate interaction within mixed-species groups: 1) species’ selective preference for joining certain groups, 2) species’ ability to flexibly change their behavior in response to groupmates’ behavior, and 3) shifts and/or 4) expansions of species’ niche occupancy when foraging with conspecifics versus when foraging with heterospecifics. We assess variation in these axes of behavioral plasticity in an Australian mixed-species avian community. All species had selective preferences for flocks of certain strata, and some flexibly matched their flockmates’ foraging strata. Three species exhibited patterns of niche shift, and one species showed niche expansion. These findings suggest that species converge in strata in mixed-species flocks despite the potential for increased competition and emphasize that species can plastically react to changes in their social environment in numerous ways. Acknowledgment of such plasticity is likely integral to understanding the nuances of heterospecific interactions.

Methods
We collected this data in an approximately 1000 ha study site within Brookfield Conservation Park in the lower Murray-Darling River Basin in South Australia. Flock observations occurred from August 2019 to December 2019, just before the beginning of the austral spring and bird breeding season, continuing through the end of the breeding season when most species had fledged young. Here we discuss only mixed-species foraging flocks, which we define as groups that contain two or more species and that actively move in a coordinated fashion while foraging. During the study period, we opportunistically observed foraging flocks throughout the day (beginning at dawn and ending before dusk) to determine species composition and foraging behavior of flock members. In total, we recorded 445 mixed-species foraging flocks, formed of a combination of 38 species. Only flocks that were observed that were not categorized as "foraging flocks" were excluded from this data. Any other necessary data parsing is present in the code provided. We also include data on foraging observations of species in and out of flocks. For every species in each flock, we recorded the highest and lowest foraging strata in which its members were observed. We designated foraging strata by separating the environment into five distinct, ecologically relevant categories – ground, scrub, bush, low canopy, and high canopy (Supplementary Figure 1). The descriptions used to designate each category are as follows: 1) ‘ground’ was defined as open ground, free of vegetation, 2) ‘scrub’ consisted of plants taller than 5 cm but shorter than 20 cm—typically saltbush (Atriplex spp.), 3) ‘bush’ consisted of several species of bush that varied in height but the dominant species at our site were sheepbush (Geijera linearifolia), senna (Senna artemisioides ssp.), and bluebush (Maireana spp.), all of which are easily distinguishable from trees by their shape and structure, 4) ‘low canopy’ consisted of the lower half of branches and leaves on an individual tree (primarily Eucalyptus spp. and Myoporum platycarpum) (5) and ‘high canopy’ consisted of the upper half of branches and leaves on an individual tree.

To explore how joining mixed-species flocks influences the foraging niches of focal species, we opportunistically recorded foraging behavior both in and out of flocks for a select set of focal species. For each individual of a species observed, we recorded a singular attempt to capture prey, successful or not, during that given foraging bout. For each foraging event, we recorded whether the individual was participating in a mixed-species or a conspecific flock, the stratum in which the foraging event took place, and the location of the event via GPS point. The strata designations used for these observations are the same as described in flocking observations above. Included in the data here are only those species for which 15 or more foraging observations were observed.