Phenotypic Clumping Decreases With Flock Richness in Mixed-Species Bird Flocks

Kartik Shanker,Hari Sridhar,Priti Bangal

doi:10.3389/fevo.2020.537816

Abstract

Animals that live in groups may experience positive interactions such as cooperative behavior or negative interactions such as competition from group members depending on group size and similarity between individuals. The effect of group size and phenotypic and ecological similarity on group assembly has not been well-studied. Mixed-species flocks are important subsets of bird communities worldwide. We examined associations within these in relation to flock size, to understand rules of flock assembly, in the Western Ghats of India. We examined the relationship between phenotypic clumping and flock richness using four variables—body size, foraging behavior, foraging height and taxonomic relatedness. Using a null model approach, we found that small flocks were more phenotypically clumped for body size than expected by chance; however, phenotypic clumping decreased as flocks increased in size and approached expected phenotypic variation in large flocks. This pattern was not as clear for foraging height and foraging behavior. We then examined a dataset of 55 flock matrices from 24 sites across the world. We found that sites with smaller flocks had higher values of phenotypic clumping for body size and sites with larger flocks were less phenotypically clumped. This relationship was weakly negative for foraging behavior and not statistically significant for taxonomic relatedness. Unlike most single-species groups, participants in mixed-species flocks appear to be able to separate on different axes of trait similarity. They can gain benefits from similarity on one axis while mitigating competition by dissimilarity on others. Consistent with our results, we speculate that flock assembly was deterministic up to a certain point with participants being similar in body size, but larger flocks tended to approach random phenotypic assemblages of species.

Highlights

Group living is widespread in different animal taxa and contexts across the world (Krause and Ruxton, 2002)
We examined variation in phenotypic clumping relative to flock richness using the dataset reported in Sridhar et al (2012b) from sites across the world
Phenotypic Clumping: Flock Heterogeneity as a Function of Flock Richness For 2010, we found that phenotypic clumping in body size was highest and statistically significant for small flocks (Table 1 and Figure 2)

Summary

Introduction

Group living is widespread in different animal taxa and contexts across the world (Krause and Ruxton, 2002). The drivers of group assembly and group size likely vary. There is both theoretical and empirical work on the drivers of group sizes in single-species groups, our understanding of group sizes in a mixed-species context is limited. Increase in group size in single-species groups is likely to lead to competition for resources. Mixed-species groups may allow species to separate on different axes of ecological and morphological similarity (Sridhar and Guttal, 2018). In effect, mixed-species groups may be affected by the same factors as single-species groups but species can differ in the degree of similarity across traits, potentially mitigating the effects of competition. Understanding group assembly is critical to deciphering the processes that drive mixedspecies grouping

Methods

Results

Discussion

Conclusion