A classification scheme for mixed-species bird flocks.

The literature on mixed-species flocks references a wide variety of bird associations. These studies, however, have used an array of unstructured characteristics to describe flocks, ranging from the temporal occurrence of flocking to the identity and behavioural features of constituent members, with little consensus on which key traits define and characterize a mixed-species flock. Moreover, although most studies report species-specific roles, there is no clear consensus about what these roles signify nor how to define them. This lack of consistency limits our ability to compare flocks from different habitats, regions and species pools. To unify this sizable body of literature, we reviewed and synthesized 538 studies on mixed-species flocks. We propose 13 categories to classify mixed-species flocks using behavioural and physical traits at the flock and participant level, as well as the habitat where the flock occurs. Lastly, we discuss the historical terminology for different species roles and propose definitions to clarify and distinguish among nuclear, leader, sentinel, and flock-following species. We envision that these guidelines will provide a universal language for mixed-species flock research, paving the way for future comparisons and new insight between different regions and systems. This article is part of the theme issue 'Mixed-species groups and aggregations: shaping ecological and behavioural patterns and processes'.

Revisiting the 'nuclear species' concept: do we really know what we think we know?

The idea of 'nuclear species' has received a lot of attention in mixed-species flock research. Our impression of this literature is that referenced statements tend to cite the same papers in support of a small set of ideas, and often there is a mismatch between what papers contain and what they're cited for. Motivated by these impressions, we built and quantitatively examined a database of referenced statements about nuclearity in flocks. This confirmed our impression quantitatively, but more strikingly, a single paper stood out in its influence on ideas around nuclearity in flocks. Moynihan's 1962 monograph on mixed-species flocks in Panama, 'The organization and probable evolution of some mixed-species flocks of neotropical birds' published in Smithsonian Miscellaneous Collections, was cited twice as much as the next most-cited paper and was the most-cited paper for 10 out of 15 most-discussed ideas related to nuclearity. Further, a number of other highly cited papers are strongly influenced by Moynihan's ideas, i.e. its influence is much greater than what a count of citations conveys. We also found that Moynihan was mis-cited frequently. We juxtapose what we found from the citation analysis with what the paper actually contains to better understand the nature of support that Moynihan provides, and discuss the implications of our findings for what we know about and how we research nuclearity in flocks. This article is part of the theme issue 'Mixed-species groups and aggregations: shaping ecological and behavioural patterns and processes'.

Malate synthase contributes to the survival of Salmonella Typhimurium against nutrient and oxidative stress conditions.

To survive and replicate in the host, S. Typhimurium have evolved several metabolic pathways. The glyoxylate shunt is one such pathway that can utilize acetate for the synthesis of glucose and other biomolecules. This pathway is a bypass of the TCA cycle in which CO2 generating steps are omitted. Two enzymes involved in the glyoxylate cycle are isocitrate lyase (ICL) and malate synthase (MS). We determined the contribution of MS in the survival of S. Typhimurium under carbon limiting and oxidative stress conditions. The ms gene deletion strain (∆ms strain) grew normally in LB media but failed to grow in M9 minimal media supplemented with acetate as a sole carbon source. However, the ∆ms strain showed hypersensitivity (p < 0.05) to hypochlorite. Further, ∆ms strain has been significantly more susceptible to neutrophils. Interestingly, several folds induction of ms gene was observed following incubation of S. Typhimurium with neutrophils. Further, ∆ms strain showed defective colonization in poultry spleen and liver. In short, our data demonstrate that the MS contributes to the virulence of S. Typhimurium by aiding its survival under carbon starvation and oxidative stress conditions.

Flock-species richness influences node importance and modularity in mixed-species flock networks.

Interdependencies in social groups of animals are a combination of multiple pairwise interactions. Heterospecific groups are often characterized by important species that contribute more to group initiation, maintenance or function than other species. However, in large heterospecific groups, many pairwise interactions are not realised, while others may not be biologically significant, confounding inferences about species importance. Hence, in this study, we examine context dependent changes in species importance and assortment in mixed-species bird flocks from a tropical field site in Southern India using social network analysis. Specifically, we ask how the structural importance of a species and the clustering patterns of species relationships depends on species richness in mixed-species flocks. We constructed both raw and filtered networks; while our results are largely correlated, we believe that filtered networks can provide insights into community-level importance of species in mixed-flocks while raw networks depict flock-level patterns. We find significant differences in flocks of different richness in that different species emerge as structurally important across flocks of varying richness. We also find that assortment is higher in two-species flocks and decreases with an increase in the number of species in the flock ('flock richness' hereafter). We argue that the link between structural importance of species in mixed-species flock networks and their functional significance in the community critically depends on the social context: namely, the species richness of the mixed-species flock. We propose that examining species structural importance at different flock-richness values provides insights into biologically meaningful functional roles of species. More generally, we suggest that it is important to consider context when interpreting species centrality and importance in network structure.

Mixed company: a framework for understanding the composition and organization of mixed-species animal groups.

Mixed-species animal groups (MSGs) are widely acknowledged to increase predator avoidance and foraging efficiency, among other benefits, and thereby increase participants' fitness. Diversity in MSG composition ranges from two to 70 species of very similar or completely different phenotypes. Yet consistency in organization is also observable in that one or a few species usually have disproportionate importance for MSG formation and/or maintenance. We propose a two-dimensional framework for understanding this diversity and consistency, concentrating on the types of interactions possible between two individuals, usually of different species. One axis represents the similarity of benefit types traded between the individuals, while the second axis expresses asymmetry in the relative amount of benefits/costs accrued. Considering benefit types, one extreme represents the case of single-species groups wherein all individuals obtain the same supplementary, group-size-related benefits, and the other extreme comprises associations of very different, but complementary species (e.g. one partner creates access to food while the other provides vigilance). The relevance of social information and the matching of activities (e.g. speed of movement) are highest for relationships on the supplementary side of this axis, but so is competition; relationships between species will occur at points along this gradient where the benefits outweigh the costs. Considering benefit amounts given or received, extreme asymmetry occurs when one species is exclusively a benefit provider and the other a benefit user. Within this parameter space, some MSG systems are constrained to one kind of interaction, such as shoals of fish of similar species or leader-follower interactions in fish and other taxa. Other MSGs, such as terrestrial bird flocks, can simultaneously include a variety of supplementary and complementary interactions. We review the benefits that species obtain across the diversity of MSG types, and argue that the degree and nature of asymmetry between benefit providers and users should be measured and not just assumed. We then discuss evolutionary shifts in MSG types, focusing on drivers towards similarity in group composition, and selection on benefit providers to enhance the benefits they can receive from other species. Finally, we conclude by considering how individual and collective behaviour in MSGs may influence both the structure and processes of communities.

Large frugivores matter: Insights from network and seed dispersal effectiveness approaches.

While large avian frugivores are known to be key dispersers for large-seeded tree species, their role in community-wide plant-disperser networks is still poorly known. Large avian frugivores are also among the most threatened due to anthropogenic impacts. We evaluated the role of large avian frugivores in a plant-disperser community by (a) determining whether the plant-disperser community was modular, with a distinct community of large frugivores (thereby highlighting their importance), (b) determining relative qualitative and quantitative roles played by large-bodied frugivores vis-à-vis other frugivores and (c) determining impacts of large-bodied frugivore loss on the plant-disperser community. The study was carried out at a tropical forest site in north-east India, which is part of the Eastern Himalaya Biodiversity Hotspot. We collected tree watch data (20:55 hr) from 46 tree species, which represented 85% of tree species that are predominantly bird-dispersed in the area. We found that the plant-disperser community was modular, with a distinct module of large-seeded tree species and large frugivores. Intermediate-sized frugivores such as barbets and bulbuls were the most connected, while large-sized frugivores, such as hornbills and imperial pigeons, were moderately well connected. Qualitative and quantitative roles played by different dispersers varied across the gradient of frugivore body size. Hornbills, the largest avian frugivores, consumed a significantly greater number of fruits and swallowed larger proportions of fruits compared with other avian groups. In comparison with similar-sized frugivores, imperial pigeons fed on larger-sized fruits, highlighting their importance for dispersal of large-seeded plants. Under simulated extinction scenarios, larger extinction cascades were not necessarily caused by larger frugivores; however, extinctions of certain large-bodied frugivores (hornbills, imperial pigeons) caused extinction cascades. Integrating information from networks and seed dispersal effectiveness approaches enabled a better understanding of large frugivore role in a plant-disperser community. While large-bodied frugivores may not be playing a central role in plant-disperser communities, they are crucial as seed dispersal service providers for large-seeded plants. In conjunction with the reported local extinctions of large frugivores like hornbills from the south Asian region, this study's findings highlight the irreplaceable quantitative and qualitative impacts that tropical plant communities are likely to experience in the future.

Friendship across species borders: factors that facilitate and constrain heterospecific sociality.

Our understanding of animal sociality is based almost entirely on single-species sociality. Heterospecific sociality, although documented in numerous taxa and contexts, remains at the margins of sociality research and is rarely investigated in conjunction with single-species sociality. This could be because heterospecific and single-species sociality are thought to be based on fundamentally different mechanisms. However, our literature survey shows that heterospecific sociality based on mechanisms similar to single-species sociality is reported from many taxa, contexts and for various benefits. Therefore, we propose a conceptual framework to understand conspecific versus heterospecific social partner choice. Previous attempts, which are all in the context of social information, model partner choice as a trade-off between information benefit and competition cost, along a single phenotypic distance axis. Our framework of partner choice considers both direct grouping benefits and information benefits, allows heterospecific and conspecific partners to differ in degree and qualitatively, and uses a multi-dimensional trait space analysis of costs (competition and activity matching) and benefits (relevance of partner and quality of partner). We conclude that social partner choice is best-viewed as a continuum: some social benefits are obtainable only from conspecifics, some only from dissimilar heterospecifics, while many are potentially obtainable from conspecifics and heterospecifics.This article is part of the theme issue 'Collective movement ecology'.

Importance of intraspecifically gregarious species in a tropical bird community.

In both single- and mixed-species social groups, certain participants are known to play important roles in providing benefits. Identifying these participants is critical for understanding group dynamics, but is often difficult with large roving social groups in the wild. Here, we develop a new approach to characterize roles in social groups and apply it to mixed-species bird flocks (flocks hereafter) in an Indian tropical evergreen forest. Two types of species, namely intraspecifically gregarious and sallying species, are thought to play important roles in flocks because studies have shown they attract other flock participants. However, it is unclear why these types are attractive and whether they are essential for flock formation. We address these questions by focusing on the composition of the subset of flocks containing only two species each. In two-species flocks, it is reasonable to assume that at least one species obtains some kind of benefit. Therefore, only those species combinations that result in benefit to at least one species should occur as two-species flocks. Using data from 540 flocks overall, of which 158 were two-species flocks, we find that intraspecifically gregarious species are disproportionately represented in two-species flocks and always lead flocks when present, and that flocks containing them are joined significantly more by other species. Our results suggest that intraspecifically gregarious species are likely to be the primary benefit providers in flocks and are important for tropical flock formation. Our study also provides a new approach to understanding importance in other mixed-species and single-species social groups.

Positive relationships between association strength and phenotypic similarity characterize the assembly of mixed-species bird flocks worldwide.

Competition theory predicts that local communities should consist of species that are more dissimilar than expected by chance. We find a strikingly different pattern in a multicontinent data set (55 presence-absence matrices from 24 locations) on the composition of mixed-species bird flocks, which are important subunits of local bird communities the world over. By using null models and randomization tests followed by meta-analysis, we find the association strengths of species in flocks to be strongly related to similarity in body size and foraging behavior and higher for congeneric compared with noncongeneric species pairs. Given the local spatial scales of our individual analyses, differences in the habitat preferences of species are unlikely to have caused these association patterns; the patterns observed are most likely the outcome of species interactions. Extending group-living and social-information-use theory to a heterospecific context, we discuss potential behavioral mechanisms that lead to positive interactions among similar species in flocks, as well as ways in which competition costs are reduced. Our findings highlight the need to consider positive interactions along with competition when seeking to explain community assembly.