Using intra-flock association patterns to understand why birds participate in mixed-species foraging flocks in terrestrial habitats

High association strengths are linked to phenotypic similarity, including plumage color and patterns, of participants in mixed-species bird flocks of southwestern China

Participants in mixed-species bird flocks (MSFs) have been shown to associate with species that are similar in body size, diet, and evolutionary history, suggesting that facilitation structures these assemblages. In addition, several studies have suggested that species in MSFs resemble each other in their plumage, but this question has not been systematically investigated for any MSF system. During the nonbreeding season of 2020 and 2021, we sampled 585 MSFs on 14 transects in 2 habitats of Tongbiguang Nature Reserve in western Yunnan Province, China. We performed social network analysis and the Multiple Regression Quadratic Assignment Procedure to evaluate the effect of 4 species traits (body size, overall plumage color, distinctive plumage patterns, and diet) and evolutionary history on species association strength at the whole-MSF and within-MSF levels. All 41 significant relationships showed that species with stronger associations were more similar in their various traits. Body size had the strongest effect on association strength, followed by phylogeny, plumage patterns, and plumage color; diet had the weakest effect. Our results are consistent with the hypotheses that the benefits of associating with phenotypically similar species outweigh the potential costs of interspecific competition, and that trait matching can occur in plumage characteristics, albeit more weakly than in other traits. Several explanations exist as to why similarities in plumage may occur in MSFs, including that they could reduce predators' ability to target phenotypically "odd" individuals. Whether trait matching in plumage occurs through assortative processes in ecological time or is influenced by co-evolution requires further study.

Interspecific sociality alters the colonization and extinction rates of birds on subtropical reservoir islands

Island biogeography theory has proved a robust approach to predicting island biodiversity on the assumption of species equivalency. However, species differ in their grouping behaviour and are entangled by complex interactions in island communities, such as competition and mutualism. We here investigated whether intra- and/or interspecific sociality may influence biogeographic patterns, by affecting movement between islands or persistence on them. We classified bird species in a subtropical reservoir island system into subcommunities based on their propensity to join monospecific and mixed-species flocks. We found that subcommunities which had high propensity to flock interspecifically had higher colonization rates and lower extinction rates over a 10-year period. Intraspecific sociality increased colonization in the same analysis. A phylogenetically corrected analysis confirmed the importance of interspecific sociality, but not intraspecific sociality. Group-living could enable higher risk crossings, with greater vigilance also linked to higher foraging efficiency, enabling colonization or long-term persistence on islands. Further, if group members are other species, competition can be minimized. Future studies should investigate different kinds of island systems, considering positive species interactions driven by social behaviour as potential drivers of community assembly on islands. This article is part of the theme issue 'Mixed-species groups and aggregations: shaping ecological and behavioural patterns and processes'.

Neotropical mixed-species bird flocks in a community context

Mixed-species flocks are an important component of bird communities, particularly in the Neotropics, where flocks reach their highest diversity. The extent to which mixed-species flocks represent unique functional or ecological roles within communities, and how these attributes change over environmental gradients, however, is not well understood. We use a trait-based approach to examine functional aspects of flocking assemblages as they relate to those observed in the larger avian community across a 3000 m elevational gradient. Our results reveal similar ecological strategies among flocking species and the communities in which they occur, at the scale of the regional pool and across elevations. Trait variation in flocking and non-flocking assemblages is structured along two major axes defined by size- and resource-related traits. The trait space occupied by flocking species, however, represents only half (51%) that of the larger community. Similarly, the trait space of flocks across elevations is restricted compared to non-flocking species. The shared trait space across flock types represents small-bodied invertivores foraging in lower forest strata, traits associated with increased vulnerability to predation. The concentration of flocking species in functional trait space suggests high niche packing and either more overlap in ecological strategies or more finely divided niches relative to non-flocking species. This article is part of the theme issue 'Mixed-species groups and aggregations: shaping ecological and behavioural patterns and processes'.

Mixed-species flock composition matters: interspecific influences on finding novel food in North American parids

Individuals of a wide range of species are sensitive to the presence of other species, and can often benefit from associations with other species in mixed-species groups (MSGs) through food-finding or avoiding predation. In an earlier field study, we found that both Carolina chickadees, Poecile carolinensis, and tufted titmice, Baeolphus bicolor, were better able to solve a novel feeder task when their MSGs were more diverse in terms of species composition. Like most studies of MSGs, however, that earlier study did not experimentally manipulate MSG size and composition. We did that manipulation here, providing experimental flocks of chickadees and titmice with three novel feeder tasks in semi-natural aviary environments. We found that successful titmouse flocks generally had a higher proportion of titmice in them, going against the findings of our earlier field study. Conversely, successful chickadee flocks solved one of the novel feeder tasks more quickly with a higher proportion of titmice in them, corroborating the findings of our earlier field study. We now need to assess socio-ecological influences on MSG size and composition, and how those relate to individual behaviour. This article is part of the theme issue 'Mixed-species groups and aggregations: shaping ecological and behavioural patterns and processes'.

Scavenging in the realm of senses: smell and vision drive recruitment at carcasses in Neotropical ecosystems

Social information, acquired through the observation of other individuals, is especially relevant among species belonging to the same guild. The unpredictable and ephemeral nature of carrion implies that social mechanisms may be selected among scavenger species to facilitate carcass location and consumption. Here, we apply a survival-modelling strategy to data obtained through the placement and monitoring of carcasses in the field to analyse possible information transmission cascades within a Neotropical scavenger community. Our study highlights how the use of different senses (smell and sight) within this guild facilitates carcass location through the transmission of social information between species with different carrion foraging efficiencies. Vultures with a highly developed sense of smell play a key role in this process, as they are the first to arrive at the carcasses and their presence seems to serve as a visual cue for other species to locate the resource. Our study supports the local enhancement hypothesis within scavengers, whereby individuals locate carcasses by following foraging heterospecifics, also suggesting the importance of the sense of smell in the maintenance of the community structure.

The formation of "mega-flocks" depends on vegetation structure in montane coniferous forests of Taiwan

A mixed-species bird flock is a social assemblage where two or more bird species are moving together while foraging and might benefit from increased foraging efficiency and antipredator vigilance. A "mega-flock," which includes flocking species from different vegetation strata, often exhibits high species diversity. Mechanisms for the formation of mega-flocks have not yet been explored. In this study, we evaluated the influence of vegetation structure and bird species diversity in driving the occurrence of mega-flocks. We investigated the composition of mixed-species flocks, local bird communities, and vegetation structure in five vegetation types of two high-elevation sites in central Taiwan. Mega-flocks occurred more frequently in pine woodland than later successional stages of coniferous forests. However, species richness/diversity of local bird communities increased along successional stages. Therefore, vegetation variables exhibit more influence on the occurrence of mega-flocks than local bird communities. Besides foliage height diversity, understory coverage also showed positive effects on flock size of mixed-species flocks. Our results indicated that pine woodlands with more evenly distributed vegetation layers could facilitate the interactions of canopy and understory flocks and increase the formation of mega-flocks and thus the complexity of mixed-species flocks.

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

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.

The relationship between morphology and behavior in mixed-species flocks of island birds

Understanding how co‐occurring species divide ecological space is a central issue in ecology. Functional traits have the potential to serve as a means for quantitatively assessing niche partitioning by different species based on their ecological attributes, such as morphology, behavior, or trophic habit. This enables testing ecological and evolutionary questions using functional traits at spatio‐temporal scales that are not feasible using traditional field methods. Both rapid evolutionary change and inter‐ and intraspecific competition, however, may limit the utility of morphological functional traits as indicators of how niches are partitioned. To address how behavior and morphology interact, we quantified foraging behavior of mixed‐species flocks of birds in the Solomon Islands to test whether behavior and morphology are correlated in these flocks. We find that foraging behavior is significantly correlated with morphological traits (p = .05), but this correlation breaks down after correcting for phylogenetic relatedness (p = .66). These results suggest that there are consistent correlations between aspects of behavior and morphology at large taxonomic scales (e.g., across genera), but the relationship between behavior and morphology depends largely on among‐clade differences and may be idiosyncratic at shallower scales (e.g., within genera). As a result, general relationships between behaviors and morphology may not be applicable when comparing close relatives.

Shifts in foraging behaviour of heterospecific flocking birds in a lowland Malaysian rainforest

Mixed-species flocks (MSFs) serve important roles in bird communities, especially in tropical forests. Although structure of mixed-species bird flocks and its benefits has been intensively studied globally, the foraging plasticity of a species when joining MSFs has rarely been evaluated. The present study examines foraging strategies of the Rufous-crowned Babbler (Malacopteron magnum), Chestnut-winged Babbler (Cyanoderma erythropterum) and Black-naped Monarch (Hypothymis azurea) when participating in MSFs in the Krau Wildlife Reserve, central Peninsular Malaysia. These species exhibit active foraging shifts in utilisation of vertical strata, foraging substrate, attack manoeuvres and foliage density, when foraging in MSFs, compared to when foraging outside MSFs. While the Rufous-crowned Babbler and Chestnut-winged Babbler commonly used gleaning and stretching (to completely extend the legs or neck to reach the food items) manoeuvres when foraging outside MSFs, respectively, they adopted probing manoeuvre and frequently used higher strata upon joining MSFs. The Chestnut-winged Babbler tended to forage on the underside of leaves and the Black-naped Monarch frequently utilised branches when joining MSFs, while they exclusively used aerial leaf litter and live green leaves, respectively, when foraging with conspecifics. The monarch also adopted the hovering manoeuvre and frequently foraged within denser foliage cover when joining MSFs. This study demonstrated that flock members exhibits foraging plasticity either through an expansion or active shift in foraging niches when participating in MSFs, thus suggesting the occurrence of possible foraging improvement and/or reductions in predation risk.

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.

Alarm communication networks as a driver of community structure in African savannah herbivores

Social information networks have the potential to shape the spatial structure of ecological communities by promoting the formation of mixed‐species groups. However, what actually drives social affinity between species in the wild will depend on the characteristics of the species available to group. Here we first present an agent‐based model that predicts trait‐related survival benefits from mixed‐species group formation in a multi‐species community and we then test the model predictions in a community‐wide field study of African savannah herbivores using multi‐layered network analysis. We reveal benefits from information transfer about predators as a key determinant of mixed‐species group formation, and that dilution benefits alone are not enough to explain patterns in interspecific sociality. The findings highlight the limitations of classical ecological approaches focusing only on direct trophic interactions when analysing community structure and suggest that declines in species occupying central social network positions, such as key informants, can have significant repercussions throughout communities.

Fission-fusion dynamics and group-size-dependent composition in heterogeneous populations

Many animal groups are heterogeneous and may even consist of individuals of different species, called mixed-species flocks. Mathematical and computational models of collective animal movement behavior, however, typically assume that groups and populations consist of identical individuals. In this paper, using the mathematical framework of the coagulation-fragmentation process, we develop and analyze a model of merge and split group dynamics, also called fission-fusion dynamics, for heterogeneous populations that contain two types (or species) of individuals. We assume that more heterogeneous groups experience higher split rates than homogeneous groups, forming two daughter groups whose compositions are drawn uniformly from all possible partitions. We analytically derive a master equation for group size and compositions and find mean-field steady-state solutions. We predict that there is a critical group size below which groups are more likely to be homogeneous and contain the abundant type or species. Despite the propensity of heterogeneous groups to split at higher rates, we find that groups are more likely to be heterogeneous but only above the critical group size. Monte Carlo simulation of the model show excellent agreement with these analytical model results. Thus, our model makes a testable prediction that composition of flocks are group-size-dependent and do not merely reflect the population level heterogeneity. We discuss the implications of our results to empirical studies on flocking systems.

"In a tree by the brook, there's a songbird who sings": Woodlands in an agricultural matrix maintain functionality of a wintering bird community

The agricultural matrix has increasingly been recognized for its potential to supplement Protected Areas (PAs) in biodiversity conservation. This potential is highly contextual, depending on composition and spatial configuration of matrix elements and their mechanistic relationship with biological communities. We investigate the effects of local vegetation structure, and proximity to a PA on the site-use of different guilds in a wintering bird community within the PA, and in wooded land-use types in the surrounding matrix. We used occupancy models to estimate covariate–guild relationships and predict site-use. We also compared species richness (estimated through capture–recapture models) and species naïve site-use between the PA and the matrix to evaluate taxonomic changes. We found that tree cover did not limit the site-use of most guilds of the community, probably due to high canopy cover across all chosen sites. Exceptions to this were guilds comprising generalist species. Shrub cover and bamboo cover had important effects on some woodland-associated guilds, suggesting a change in limiting factors for site-use under adequate tree cover. Site-use across the matrix was high for all analyzed guilds. This was found to be due to three non-exclusive reasons: (i) presence of one or more ubiquitous species (found all across the landscape) within some guilds, (ii) redundancy of species within guilds that buffered against a decrease in site-use, and (iii) turnover in guild composition/abundances to more generalist species from PA to matrix. Estimated species richness was higher in the matrix (107± 11; mean ± SE) than in the PA (90± 7), which may have been in part due to the addition of generalist species in the matrix. Understanding factors that limit biological communities is crucial to better managing the ever-increasing matrix for biodiversity conservation. Our study provides insights into the effects of different components of vegetation structure on the bird community in wooded land-use types in the matrix. We highlight the value of woodlands surrounding PAs in maintaining multiple guilds, and hence, the functionality of a wintering bird community. However, we caution that the matrix may fall short in retaining some specialized species of the community.

Vigilance responding to number of conspecifics among mixed groups of cranes in demilitarized zone

Numerous studies have addressed antipredatory benefits of mixed-species flocks of foragers, but studies on individual's vigilance as a function of group size are limited. In the Cheolwon area of the Korean Demilitarized Zone, vigilance of the subordinate White-naped cranes (Grus vipio) in 11 groups composed of conspecifics and the dominant Red-crowned cranes (Grus japonensis) was examined. Vigilance correlated negatively with group size due to negative correlation with the number of conspecifics, but not the dominant heterospecifics. This is consistent with the hypothesis that a decrease in vigilance in larger groups is due to antipredatory benefits from increased predator detection in larger groups (associated with the presence of a larger number of conspecifics). This suggested that the mechanism leads to canceling out of the otherwise expected antipredatory benefits to the subordinate species from the increased predator detection by larger group size (associated with larger number of dominants). This is one of only a few behavioral studies of these endangered crane species in the relatively inaccessible wintering area of international importance in the areas of high conservation value.

Does mixed-species flocking influence how birds respond to a gradient of land-use intensity?

Conservation biology is increasingly concerned with preserving interactions among species such as mutualisms in landscapes facing anthropogenic change. We investigated how one kind of mutualism, mixed-species bird flocks, influences the way in which birds respond to different habitat types of varying land-use intensity. We use data from a well-replicated, large-scale study in Sri Lanka and the Western Ghats of India, in which flocks were observed inside forest reserves, in 'buffer zones' of degraded forest or timber plantations, and in areas of intensive agriculture. We find flocks affected the responses of birds in three ways: (i) species with high propensity to flock were more sensitive to land use; (ii) different flock types, dominated by different flock leaders, varied in their sensitivity to land use and because following species have distinct preferences for leaders, this can have a cascading effect on followers' habitat selection; and (iii) those forest-interior species that remain outside of forests were found more inside flocks than would be expected by chance, as they may use flocks more in suboptimal habitat. We conclude that designing policies to protect flocks and their leading species may be an effective way to conserve multiple bird species in mixed forest and agricultural landscapes.

Influence of Seasonal Food Availability on the Dynamics of Seabird Feeding Flocks at a Coastal Upwelling Area

The formation of multi-species feeding flocks (MSFFs) through visual recruitment is considered an important strategy for obtaining food in seabirds and its functionality has been ascribed to enhanced foraging efficiency. Its use has been demonstrated in much of the world's oceans and includes numerous species. However, there is scant information on the temporal stability of the composition and abundance of MSFFs as well as the effect of seasonal food availability on their dynamics. Between July 2006 and September 2014, we conducted monthly at-sea seabird counts at Valparaiso Bay (32°56' to 33°01'S, 71°36' to 71°46'W) within the area of influence of the Humboldt Current in central Chile. This area is characterized by a marked seasonality in primary and secondary production associated with upwelling, mainly during austral spring-summer. Based on studies that provide evidence that flocking is most frequent when food is both scarce and patchy, we hypothesized that seabird MSFF attributes (i.e. frequency of occurrence, abundance and composition) will be modified according to the seasonal availability of food. Using generalized linear models (GLMs), our results show that the contrasting seasonality in food availability of the study area (using chlorophyll-a concentration as a proxy) had no significant influence on MSFF attributes, sparsely explaining their variations (P>0.05). Rather than seasonal food availability, the observed pattern for MSFF attributes at Valparaiso Bay suggests a substantial influence of reproductive and migratory (boreal and austral migrants) habits of birds that modulates MSFF dynamics consistently throughout the whole year in this highly variable and patchy environment. We highlight the importance of visual recruitment as a mechanism by which migratory and resident birds interact. This would allow them to reduce resource unpredictability, which in turn has a major impact on structuring seabird's MSFF dynamics.

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.