Flock Cohesion Research Articles

Free Flight Training as a Tool for Psittacine Reintroductions

As habitat loss and other threats accelerate, ecological restoration and reintroduction science are becoming progressively more important. The psittacines are among the most endangered bird groups and are prime candidates for restoration through reintroduction. Unfortunately, post-release survival of captive-raised animals is often quite low because, in part, of high predation rates, low site fidelity, poor flight ability, and low flock cohesion. Current best practices in parrot release hold the birds in captivity for a year or more and include distinct methods to address each of these challenges. Here, we conduct a small-scale, proof-of-concept study using free flight methods and human-socialized trained adult birds to hand raise and release a group of six fledgling Blue-and-yellow Macaws in their historical range in southeastern Brazil. All six released birds showed strong flock cohesion and fidelity to the release site, avoided predation, and survived without supplemental feeding for over one year. One bird was captured by local people but was recovered and rereleased and it has reintegrated into the group and is still alive and doing well. The human-socialized trained adult birds modeled both desirable behaviors (flocking, foraging, reacting to predators) and undesirable behaviors and they were returned to captivity before the conclusion of this study. Our study suggests that free flight training has great potential to help captive-raised young attain a broad array of vital skills needed for survival post-release.

Reintroduction of the Extinct-in-the-Wild Spix’s Macaw (Cyanopsitta spixii) in the Caatinga Forest Domain of Brazil

We describe efforts to reintroduce the extinct-in-the-wild Spix’s Macaw (Cyanopsitta spixii) within its historical range in eastern Brazil. Twenty captive-reared Spix’s Macaws were released, along with 15 Blue-Winged Macaws (Primolius maracana), as heterospecific flocks in two events during the dry and rainy seasons of 2022. We monitored the release groups through daily observation and telemetry tracking from early June 2022 to early June 2023. We documented an overall first-year survival of 58.3% (CI: 37.8–78.7%), with 65% of Spix’s Macaws establishing a stable area of activity within 5 km of the release site, excluding any temporary long-distance forays. Eighty-five percent of released Spix’s Macaws exhibited flock cohesion, including interactions with and integration into wild Blue-Winged Macaw groups at the release site. Several released Spix’s Macaws formed pair-bonds with conspecifics and engaged in nest cavity exploration, breeding behavior, and territorial defense of nest sites, with three females also laying and incubating eggs. One nesting pair successfully hatched and reared chicks in an artificial nest cavity. These releases employed a novel reintroduction strategy using a surrogate species model, the sympatric Blue-Winged Macaw, to “mentor” and facilitate post-release adaptation by the target species and increase the total number of individuals in the release cohort. Lastly, participatory monitoring by local citizens is considered a way to engage and involve local communities in species and habitat conservation and potentially create new employment opportunities in the region.

Mu opioid receptor stimulation in the medial preoptic area or nucleus accumbens facilitates song and reward in flocking European starlings.

It has been proposed that social cohesion in gregarious animals is reinforced both by a positive affective state induced by social interactions and by the prevention of a negative state that would be caused by social separation. Opioids that bind to mu opioid receptors (MORs) act in numerous brain regions to induce positive and to reduce negative affective states. Here we explored a potential role for MORs in affective states that may impact flocking behavior in mixed-sex flocks of nonbreeding European starlings, Sturnus vulgaris. Singing behavior, which is considered central to flock cohesion, and other social behaviors were quantified after infusions of the MOR agonist D-Ala2, N-Me-Phe4, glycinol5-ENK (DAMGO) into either the medial preoptic area (POM) or the nucleus accumbens (NAC), regions previously implicated in affective state and flock cohesion. We focused on beak wiping, a potential sign of stress or redirected aggression in this species, to provide insight into a presumed negative state. We also used conditioned place preference (CPP) tests to provide insight into the extent to which infusions of DAMGO into POM or NAC that stimulated song might be rewarding. We found that MOR stimulation in either POM or NAC dose-dependently promoted singing behavior, reduced beak wiping, and induced a CPP. Subtle differences in responses to MOR stimulation between NAC and POM also suggest potential functional differences in the roles of these two regions. Finally, because the location of NAC has only recently been identified in songbirds, we additionally performed a tract tracing study that confirmed the presence of dopaminergic projections from the ventral tegmental area to NAC, suggesting homology with mammalian NAC. These findings support the possibility that MORs in POM and NAC play a dual role in reinforcing social cohesion in flocks by facilitating positive and reducing negative affective states.

μ-Opioid Receptor Stimulation in the Nucleus Accumbens Increases Vocal-Social Interactions in Flocking European Starlings, Sturnus Vulgaris.

Social connections in gregarious species are vital for safety and survival. For these reasons, many bird species form large flocks outside the breeding season. It has been proposed that such large social groups may be maintained via reward induced by positive interactions with conspecifics and via the reduction of a negative affective state caused by social separation. Moreover, within a flock optimal social spacing between conspecifics is important, indicating that individuals may optimize spacing to be close but not too close to conspecifics. The μ-opioid receptors (MORs) in the nucleus accumbens (NAc) are well known for their role in both reward and the reduction of negative affective states, suggesting that MOR stimulation in NAc may play a critical role in flock cohesion. To begin to test this hypothesis, social and nonsocial behaviors were examined in male and female European starlings (Sturnus vulgaris) in nonbreeding flocks after intra-NAc infusion of saline and three doses of the selective MOR agonist d-Ala2-N-Me-Phe4-Glycol5-enkephalin (DAMGO). DAMGO in NAc dose-dependently increased singing behavior and facilitated social approaches while at the same time promoting displacements potentially used to maintain social spacing. These findings support the hypothesis that MORs in NAc promote social interactions important for group cohesion in nonsexual contexts and suggest the possibility that MORs in the NAc play a role in optimizing the pull of joining a flock with the push of potential agonistic encounters.

Collective attention in navigating homing pigeons: group size effect and individual differences

One prominent feature of collective attention is the so-called ‘group size effect’. Often documented in studies of vigilance by animals foraging gregariously, this effect describes how individual group members are able to reduce their attention to environmental cues (e.g. predators) as group size increases, since the group can pool information from multiple individuals. Despite extensive literature in one context (foraging–vigilance), it remains unclear whether this effect can be observed in other contexts where animals face different informational demands (e.g. navigation). Using a custom-made sensor combining head-mounted inertial measurement unit and back-mounted GPS, we examined head movement, a proxy for visual scanning behaviour, in homing pigeons, Columba livia , flying alone, in pairs and in flocks (ca. 10 individuals). Individual birds' head movements decreased sharply when they switched from solo to paired flights and again when they switched from paired to flock flights. Homing efficiency (or route straightness) did not decrease in parallel with decreasing head movement but continued to increase over repeated flights. We also found systematic individual differences during flock homing flights, where birds positioned more frontally and more centrally in the flock made larger head movements. Moreover, leaders were positioned more frontally and tended to show larger head movements than followers. Decreased head movement by peripheral birds suggested increased attention to the rest of the flock rather than increased individual vigilance (contra ‘the edge effect’). Overall, our results suggest that individual birds reduced their visual scanning during navigation because of the need to attend closely to fellow flock members to maintain flock cohesion. While reduced per capita visual scanning may reduce the amount of environmental information available to individual birds, it is likely to be compensated by the flock's collective attention. • The group size effect in attention was examined in the context of navigation. • We measured head movement of homing pigeons using a custom-made sensor. • Pigeons flew alone, in pairs, and in flocks (∼10 individuals). • Pigeons decreased their head movement sharply as the flock size increased. • This effect was likely driven by individuals' efforts to maintain flock cohesion.

Minimizing Potential Allee Effects in Psittacine Reintroductions: An Example from Puerto Rico

The family Psittacidae is comprised of over 400 species, an ever-increasing number of which are considered threatened with extinction. In recent decades, conservation strategies for these species have increasingly employed reintroduction as a technique for reestablishing populations in previously extirpated areas. Because most Psittacines are highly social and flocking species, reintroduction efforts may face the numerical and methodological challenge of overcoming initial Allee effects during the critical establishment phase of the reintroduction. These Allee effects can result from failures to achieve adequate site fidelity, survival and flock cohesion of released individuals, thus jeopardizing the success of the reintroduction. Over the past 20 years, efforts to reestablish and augment populations of the critically endangered Puerto Rican parrot (Amazona vittata) have periodically faced the challenge of apparent Allee effects. These challenges have been mitigated via a novel release strategy designed to promote site fidelity, flock cohesion and rapid reproduction of released parrots. Efforts to date have resulted in not only the reestablishment of an additional wild population in Puerto Rico, but also the reestablishment of the species in the El Yunque National Forest following its extirpation there by the Category 5 hurricane Maria in 2017. This promising release strategy has potential applicability in reintroductions of other psittacines and highly social species in general.

Vegetation structure drives mixed-species flock interaction strength and nuclear species roles

Abstract Mixed-species flocks are a key facilitative interaction for tropical birds. Forest fragmentation leads to species loss and spatial turnover in these flocks, yet it is unknown how these changes to composition influence within-flock species interactions. We used network analysis to characterize flocking interactions along a fragment-size gradient in the Colombian Western Andes. We asked 1) how patch size, edge density, and vegetation structure explained network measures indicative of flock cohesion, 2) whether changes were driven by flocking species turnover or changes to the frequency of species co-occurrence, and 3) whether nuclear species, those that maintain flock stability and cohesion, changed in importance across the gradient. We constructed weighted social networks from flock compositions observed on 500-m transects, and then calculated global network measures and the centrality of six nuclear species. Patch size and edge density did not correlate with interspecific co-occurrence patterns, but interaction strength increased with canopy height. Flocks contained numerous, weak interactions, and there were no flock subtypes, suggesting flock composition was dynamic and unstructured. Several redundant nuclear species were present and varied in importance based on ecological conditions. A chlorospingus (Passerellidae) was most central in old-growth forest, whereas several tanager (Thraupidae) species became more central in smaller fragments and disturbed forest. When partitioning network dissimilarity, we found that 66% of dissimilarity resulted from species turnover, whereas only 34% resulted from changes to species co-occurrence. This finding suggests that coherence of flocking behavior itself is maintained even as extensive species turnover occurs from continuous forest to small fragments.

Carolina chickadee (Poecile carolinensis) calling behavior in response to threats and in flight: Flockmate familiarity matters.

Signalers can vary their vocal behavior, depending on the presence or absence of conspecific group members, and on the composition of the group. Here we asked whether Carolina chickadee (Poecile carolinensis) signalers varied their vocal behavior, depending on whether they were in the presence of familiar or unfamiliar flockmates. We sorted 32 Carolina chickadees into 4 groups with 4 familiar birds each and 4 groups with 4 unfamiliar birds each and recorded their behavior in seminatural aviary settings. We presented the familiar and unfamiliar aviary groups with a variety of stimuli ranging in level of threat and assessed birds' calling behavior in these contexts. Birds housed with familiar flockmates called sooner in threatening stimulus presentations compared with birds housed with unfamiliar flockmates. Call rates and note compositions of calls produced did not differ for the 2 types of flock. Upon release from the captive aviaries, birds from familiar flocks were also more likely to call in flight than were birds from unfamiliar aviaries. These findings suggest that chickadees vary their calling behavior as a function of experience with their surrounding social audience and provide insight into mechanisms of flock cohesion and relationship formation and maintenance within flocks. (PsycINFO Database Record

Tail Movements in Birds—Current Evidence and New Concepts

Abstract Birds of a wide range of species show characteristic movements of their tail, often called tail flicking, tail wagging or tail flashing. Tail flicking refers to vertical up-and-down movements of the tail, while tail flashing is defined as a horizontal movement, often including tail spreading. Here, I review proposed functions of such behaviour. Most relate to communication with conspecifics, predators or prey. Tail flashing may induce movement of the bird's prey that makes the prey more vulnerable to capture (‘prey-flushing’). Tail movements may signal to a predator that the signaller has detected it (‘perception advertisement’), or that the signaller is particularly alert or otherwise difficult to catch (‘quality advertisement’). Further, it may warn conspecifics of predators (‘alarm signal’), or it may advertise quality as a mate, signal social status, or aid in flock cohesion. This behaviour may, possibly, though it seems unlikely, represent a cue rather than a signal in that it benefits the r...

Forest fragmentation in an African biodiversity hotspot impacts mixed-species bird flocks

The effect of forest fragmentation on mixed-species foraging bird flocks has been poorly evaluated, particularly in African forests. We examined the consequences of forest fragmentation on such flocks in the East Usambara Mountains, Tanzania, by addressing ecological as well as behavioral components. We counted these flocks in five small (<31ha), isolated fragments and six widely spaced continuous forest sites. Given that flock cohesion and stability might be facilitated via positive interactions of leader or nuclear species, we focused on one putative nuclear species, the square-tailed drongo (Dicrurus ludwigii), and using playback of its vocalisations, we evaluated (i) if its vocalisations attracted flocking birds as expected by a nuclear species, and (ii) if loss of this species in fragments contributed to flock declines. We found that flocks in forest fragments had smaller sizes, reduced species richness, a reduced proportion of understorey specialists, a higher proportion of forest generalists and non-forest species, and were more variable in size and composition compared to flocks observed in continuous forest. Furthermore, flocks in fragments were composed of a higher proportion of omnivores than insectivores. Despite lower absolute abundance in fragments, D. ludwigii was observed in 86% of natural occurring flocks in both fragments and continuous forest, and flocks with D. ludwigii were significantly larger than those without it––especially in the continuous forest. Playback vocalisations of D. ludwigii attracted flocks of similar abundance and species richness in fragments and continuous forest, but a vastly different composition, confirming (i) its nuclear role and (ii) that, in fragments, flock assemblages had a distinct composition, with a high proportion of forest generalists and non-forest species. Loss or diminished abundance of nuclear species due to fragmentation may be an important factor that affects the presence and composition of mixed-species flocks. Given that 67% of flocking species forage in the understorey, and that insectivores were more negatively affected in fragments than omnivores, the implications of our findings are relevant for conservation in tropical forests.

Decay of interspecific avian flock networks along a disturbance gradient in Amazonia

Our understanding of how anthropogenic habitat change shapes species interactions is in its infancy. This is in large part because analytical approaches such as network theory have only recently been applied to characterize complex community dynamics. Network models are a powerful tool for quantifying how ecological interactions are affected by habitat modification because they provide metrics that quantify community structure and function. Here, we examine how large-scale habitat alteration has affected ecological interactions among mixed-species flocking birds in Amazonian rainforest. These flocks provide a model system for investigating how habitat heterogeneity influences non-trophic interactions and the subsequent social structure of forest-dependent mixed-species bird flocks. We analyse 21 flock interaction networks throughout a mosaic of primary forest, fragments of varying sizes and secondary forest (SF) at the Biological Dynamics of Forest Fragments Project in central Amazonian Brazil. Habitat type had a strong effect on network structure at the levels of both species and flock. Frequency of associations among species, as summarized by weighted degree, declined with increasing levels of forest fragmentation and SF. At the flock level, clustering coefficients and overall attendance positively correlated with mean vegetation height, indicating a strong effect of habitat structure on flock cohesion and stability. Prior research has shown that trophic interactions are often resilient to large-scale changes in habitat structure because species are ecologically redundant. By contrast, our results suggest that behavioural interactions and the structure of non-trophic networks are highly sensitive to environmental change. Thus, a more nuanced, system-by-system approach may be needed when thinking about the resiliency of ecological networks.

Non‐Redundant Social Information Use in Avian Flocks with Multisensory Stimuli

AbstractAnimals generally live in multisensory worlds; however, our understanding of multisensory perception is rather limited, despite its relevance for explaining the mechanisms behind social interactions, such as collective detection while foraging in groups. We tested how multisensory stimuli affected the antipredator behavior of dark‐eyed juncos (Junco hyemalis) using alarm calls as an auditory signal and flushing behavior as a visual cue. We varied the degree of risk within the group by manipulating the number of group mates alarm calling and/or flushing using robotic birds. We assumed that alarm calling and flushing were redundant stimuli and predicted that they could generate one of three types of responses (enhancement, equivalence, or antagonism) depending on the mechanism of multisensory perception. We set up an artificial flock with three robotic juncos surrounding a live junco and controlled for multiple confounding factors (e.g., identity of the focal, body mass, food deprivation time). We found that the degree of alarm of live juncos increased when at least one robot flushed. However, the time it took the live individuals to react to the robots' behavior increased, rather than decreased, with at least one alarm call. This could be the result of an orienting response or sensory overload, as live juncos increased scanning behavior after being exposed solely to alarm calls. Contrary to some theoretical assumptions, alarm calling and flushing behavior elicited independent unimodal responses, suggesting that they are non‐redundant stimuli and that together they could reduce the occurrence of false alarms and facilitate flock cohesion.

High throughput analysis reveals dissociable gene expression profiles in two independent neural systems involved in the regulation of social behavior

BackgroundProduction of contextually appropriate social behaviors involves integrated activity across many brain regions. Many songbird species produce complex vocalizations called ‘songs’ that serve to attract potential mates, defend territories, and/or maintain flock cohesion. There are a series of discrete interconnect brain regions that are essential for the successful production of song. The probability and intensity of singing behavior is influenced by the reproductive state. The objectives of this study were to examine the broad changes in gene expression in brain regions that control song production with a brain region that governs the reproductive state.ResultsWe show using microarray cDNA analysis that two discrete brain systems that are both involved in governing singing behavior show markedly different gene expression profiles. We found that cortical and basal ganglia-like brain regions that control the socio-motor production of song in birds exhibit a categorical switch in gene expression that was dependent on their reproductive state. This pattern is in stark contrast to the pattern of expression observed in a hypothalamic brain region that governs the neuroendocrine control of reproduction. Subsequent gene ontology analysis revealed marked variation in the functional categories of active genes dependent on reproductive state and anatomical localization. HVC, one cortical-like structure, displayed significant gene expression changes associated with microtubule and neurofilament cytoskeleton organization, MAP kinase activity, and steroid hormone receptor complex activity. The transitions observed in the preoptic area, a nucleus that governs the motivation to engage in singing, exhibited variation in functional categories that included thyroid hormone receptor activity, epigenetic and angiogenetic processes.ConclusionsThese findings highlight the importance of considering the temporal patterns of gene expression across several brain regions when engaging in social behaviors.

Oblique color vision in an open-habitat bird: spectral sensitivity, photoreceptor distribution and behavioral implications

Color vision is not uniform across the retina because of differences in photoreceptor density and distribution. Retinal areas with a high density of cone photoreceptors may overlap with those with a high density of ganglion cells, increasing hue discrimination. However, there are some exceptions to this cell distribution pattern, particularly in species with horizontal visual streaks (bands of high ganglion cell density across the retina) that live in open habitats. We studied the spectral sensitivity and distribution of cone photoreceptors involved in chromatic and achromatic vision in the Canada goose (Branta canadiensis), which possesses an oblique rather than horizontal visual streak at the ganglion cell layer. Using microspectrophotometry, we found that the Canada goose has a violet-sensitive visual system with four visual pigments with absorbance peaks at 409, 458, 509 and 580 nm. The density of most cones involved in chromatic and achromatic vision peaked along a band across the retina that matched the oblique orientation of the visual streak. With the information on visual sensitivity, we calculated chromatic and achromatic contrasts of different goose plumage regions. The regions with the highest visual saliency (cheek, crown, neck and upper tail coverts) were the ones involved in visual displays to maintain flock cohesion. The Canada goose oblique visual streak is the retinal center for chromatic and achromatic vision, allowing individuals to sample the sky and the ground simultaneously or the horizon depending on head position. Overall, our results show that the Canada goose visual system has features that make it rather different from that of other vertebrates living in open habitats.

Dynamical Modeling of Collective Behavior from Pigeon Flight Data: Flock Cohesion and Dispersion

Several models of flocking have been promoted based on simulations with qualitatively naturalistic behavior. In this paper we provide the first direct application of computational modeling methods to infer flocking behavior from experimental field data. We show that this approach is able to infer general rules for interaction, or lack of interaction, among members of a flock or, more generally, any community. Using experimental field measurements of homing pigeons in flight we demonstrate the existence of a basic distance dependent attraction/repulsion relationship and show that this rule is sufficient to explain collective behavior observed in nature. Positional data of individuals over time are used as input data to a computational algorithm capable of building complex nonlinear functions that can represent the system behavior. Topological nearest neighbor interactions are considered to characterize the components within this model. The efficacy of this method is demonstrated with simulated noisy data generated from the classical (two dimensional) Vicsek model. When applied to experimental data from homing pigeon flights we show that the more complex three dimensional models are capable of simulating trajectories, as well as exhibiting realistic collective dynamics. The simulations of the reconstructed models are used to extract properties of the collective behavior in pigeons, and how it is affected by changing the initial conditions of the system. Our results demonstrate that this approach may be applied to construct models capable of simulating trajectories and collective dynamics using experimental field measurements of herd movement. From these models, the behavior of the individual agents (animals) may be inferred.

Mixed bird flocks: patterns of activity and species composition in a region of the Central Andes of Colombia

Mixed bird flocks are groups of individuals from different species that travel and forage together. Such groups are common in several bird communities around the world. We present species composition and activity patterns of mixed bird flocks in a region of the Central Andes of Colombia. We compared the number of species per flock, as well as the number of flocks among 3 different habitats. We tested hypotheses concerning the flocks daily activity and the co-occurrences of species within them. We recorded 75 species, and the species number per flock varied from 4 to 21. Our data suggest that habitat affects the number of flocks but not their species number, and that the activity of flocks is similar throughout the day. In addition, the association of birds in flocks is affected by interspecific facilitation, with some species co-occurrences found more times than expected by chance. We hypothesize that some tanager species could have a role in flock cohesion. We witnessed 2 predator attacks upon flocks, a number of agonistic interactions among flock members, and squirrels following bird flocks. Our results meet some general patterns described for mixed bird flocks.

Seasonal changes in courtship behavior, plasma androgen levels and in hypothalamic aromatase immunoreactivity in male free-living European starlings ( Sturnus vulgaris)

In songbirds from temperate latitudes, singing during spring has an essential role in mate attraction, while during the non-breeding season it is connected to territorial aggression and/or maintaining dominance hierarchies or flock cohesion. Courtship behavior is regulated by plasma testosterone (T) levels. Other androgens, like dehydroepiandrosterone (DHEA) could be responsible for aggression. The aromatization of androgens in the brain is an essential step in mediating their effects on behavior. Our goal was to determine whether the seasonal changes in male courtship behavior (measured by average song bout length and wing-waving/flicking) are related to seasonal changes in androgen activity (measured by plasma T, DHEA levels) and aromatase (ARO) immunoreactivity in the preoptic area/medial preoptic nucleus (POA/POM) of free-living male starlings. DHEA increased during pair formation, decreased at nesting and remained at low levels. The number of ARO cells – in line with the T levels – increased during the courtship and nesting periods, but outside the breeding season it was low. Song bout length showed a similar pattern, namely the peak was reached during the courtship period, and after that males stopped singing when chicks started to hatch. Short and fast wing-flicking and wing-waving behavior was observed only during the breading season. Summarizing, we have found that song bout length of male starlings changes parallel with plasma T levels and ARO immunoreactivity in the POA/POM. Furthermore, DHEA levels were low during the sexually inactive period which suggests that other mechanisms could be involved in the aggressive non-courtship behavior/vocalization in these birds.

Steering self-organized robot flocks through externally guided individuals

In this paper, we study how a self-organized mobile robot flock can be steered toward a desired direction through externally guiding some of its members. Specifically, we propose a behavior by extending a previously developed flocking behavior to steer self-organized flocks in both physical and simulated mobile robots. We quantitatively measure the performance of the proposed behavior under different parameter settings using three metrics, namely, (1) the mutual information metric, adopted from Information Theory, to measure the information shared between the individuals during steering, (2) the accuracy metric from directional statistics to measure the angular deviation of the direction of the flock from the desired direction, and (3) the ratio of the largest aggregate to the whole flock and the ratio of informed individuals remaining with the largest aggregate, as a metric of flock cohesion. We conducted a systematic set of experiments using both physical and simulated robots, analyzed the transient and steady-state characteristics of steered flocking, and evaluate the parameter conditions under which a swarm can be successfully steered. We show that the experimental results are qualitatively in accordance with the ones that were predicted in Couzin et al. model (Nature, 433:513–516, 2005) and relate the quantitative differences to the differences between the models.

The relationship between leadership and gregariousness in mixed‐species bird flocks

Among the various species that form mixed‐species bird flocks, “nuclear species” are thought to be important in flock formation and maintaining flock cohesion. Such nuclear species have been noted to occur in large groups on their own and to lead flocks, but the relationship between leadership and intraspecific group size has not been quantitatively tested at a large scale. Using a dataset of descriptive studies of terrestrial flock systems collected over 75 y, we found that intraspecific group size was significantly larger in flock leaders than in species that attend the same flocks but do not lead. The relationship held in a reduced dataset with phylogenetically‐independent flock systems. We discuss a framework for explaining the connection between leadership and intraspecific group size, contrasting between those hypotheses that emphasize that gregariousness serves to attract the attention of other species, and those hypotheses that suggest that gregariousness leads to kin‐selected behavior from which other species can also benefit.

Bird mixed-flocks and nuclear species in a tecoma savanna in the Pantanal

In bird mixed flocks, a prominent species, the so-called nuclear species, improves the cohesion and maintenance of the flocks, while other less conspicuous species are assumed as satellite. In this study we described the composition, as well as examined the existence of both nuclear and satellite species in mixed flocks of a savanna in the Pantanal. The observations were developed using three transects during the dry season of 2002. Bird species abundance and respective rate of participation in mixed flocks were surveyed by transects, while intraspecific sociality, communication, foraging maneuvers of species, and responses to predators were sampled by direct observations. These parameters were evaluated to distinguish nuclear from satellite species. We observed 41 bird mixed flocks, which included from 2 to 17 species of which Suiriri suiriri (Vieillot), one of the most abundant species, was present in most flocks, often represented by 2-4 individuals, whereas most other species occurred lone or in pairs. While foraging by acrobatic maneuvers S. suiriri often gave contact calls, as well as earlier giving alarm calls if faced with a risk of predation. In addition, S. suiriri always started mixed flocks movements. Conversely, most other species were silent and closely inspected the vegetation while foraging. Such species always followed S. suiriri and seldom gave contact calls. Hence, the conspicuous traits exhibited by S. suiriri, potentially, are exploited by the other bird species as cues, which are important references for bird mixed flock cohesion in a savanna in the southern Pantanal.