Composition and foraging behaviour of mixed-species flocks led by the Grey-cheeked Fulvetta in Fushan Experimental Forest, Taiwan

Thirty-two species were recorded in mixed-species bird flocks led by the Grey-cheeked Fulvetta Alcippe morrisonia in Fushan Experimental Forest, Taiwan. Flocks averaged (± se) 5.8 ± 0.2 species and 51.4 ± 2.7 birds. Most participants were resident species (86.3%), some were elevational migrants (12.6%) and a few were latitudinal migrants (1.1%). Flock size was determined primarily by the abundance of Grey-cheeked Fulvettas, the most abundant species (68.1%). Flocks moved at an average rate of 10.8 ± 0.7 m/min, with larger flocks moving faster than smaller flocks. In moving flocks, canopy species were usually near the front, while understorey species usually followed. Fulvettas gave higher-intensity alarm calls and dived down more frequently in response to avian threats, especially raptors, than to non-avian threats. The overall foraging niche-breadth of the fulvetta was greater than that of any attendant species. Each species in a flock had a unique foraging niche. Most attendant species exhibited low foraging niche-overlap with the Grey-cheeked Fulvetta. Both the predator avoidance and the foraging efficiency hypotheses for mixed-species flocking were supported. The Grey-cheeked Fulvetta plays a critical role in the function of mixed-species flocks. A large flock formed around the Grey-cheeked Fulvetta provides attendant species with numerous opportunities for obtaining food and protection from predators.     

The influence of species composition on behaviour in mixed-species cyprinid shoals

Adult dace, minnows and gudgeon (50 of each) were studied in a fluvarium. Each possible paired-and three-species combination was compared with single-species controls. Species position in the fluvarium and the behaviour of marked individuals were used to measure interactions. Individual behaviours were standardized for day-to-day changes in activity level. The presence of a second species caused significant increases in the frequency and total time of behaviours associated with positional change. Such behavioural changes may reduce competition by adjusting individual position within the shoal. Minnows moved higher in the water and downstream in the presence of dace. Gudgeon moved lower in the water and downstream in the presence of dace and of minnows. Dace made slight, but significant, movements downstream and higher in the water column in response to both minnows and gudgeon. These shifts were very distinct, and the resulting shoal structure mirrors the positions occupied in mixed-species shoals of these species in the wild.     

Associations among Antarctic seabirds in mixed-species feeding flocks

We studied ten mixed-species feeding flocks of seabirds and seals off South Georgia (55̀S, 35̀W) to assess the factors that influenced the species composition of the flocks. The ten flocks were distributed between two oceanographic regions; four flocks were observed off the northwestern end of South Georgia, and six off the island's southeastern end. The flocks differed dramatically in size and species composition. The northwestern flocks were ten to 20 times larger and contained more seals and penguins. We tested whether these differences were a consequence of location, because the two regions were populated by different assemblages of species, and determined that this was not the case. Differences in flock size and species composition reflected differences in the depth distribution of the birds' and seals' prey. The northwestern flocks were associated with deep swarms of Antarctic krill Euphausia superba with a significant quantity of krill also present at the surface. We also tested for differential flock participation by the species found in the surrounding waters and classified species by their proclivity to participate in the flocks. Our results indicate that Black-browed Albatross Diomedea melanophris have a strong tendency to join, and diving-petrels Pelecanoides spp. to avoid, mixed-species feeding flocks. We postulate that Black-browed Albatrosses serve as visual cues to the presence of food for one another and for other species, and that diving-petrels avoid mixed-species flocks in part because of the risk of predation.     

Individual-level personality influences social foraging and collective behaviour in wild birds

There is increasing evidence that animal groups can maintain coordinated behaviour and make collective decisions based on simple interaction rules. Effective collective action may be further facilitated by individual variation within groups, particularly through leader–follower polymorphisms. Recent studies have suggested that individual-level personality traits influence the degree to which individuals use social information, are attracted to conspecifics, or act as leaders/followers. However, evidence is equivocal and largely limited to laboratory studies. We use an automated data-collection system to conduct an experiment testing the relationship between personality and collective decision-making in the wild. First, we report that foraging flocks of great tits (Parus major) show strikingly synchronous behaviour. A predictive model of collective decision-making replicates patterns well, suggesting simple interaction rules are sufficient to explain the observed social behaviour. Second, within groups, individuals with more reactive personalities behave more collectively, moving to within-flock areas of higher density. By contrast, proactive individuals tend to move to and feed at spatial periphery of flocks. Finally, comparing alternative simulations of flocking with empirical data, we demonstrate that variation in personality promotes within-patch movement while maintaining group cohesion. Our results illustrate the importance of incorporating individual variability in models of social behaviour.     

Intraspecific differences in benefits from feeding in mixed-species flocks

The Madagascar Paradise Flycatcher Terpsiphone mutata and Common Newtonia Newtonia brunneicauda frequently form two-species flocks in the deciduous dry forest of western Madagascar. In T. mutata , some males have long tails, while other males and females have short tails. When foraging in mixed flocks, each type of bird captured prey more rapidly than otherwise, but the degree of increase in feeding rate was smaller in long-tailed males. When in mixed flocks, all T. mutata caught prey on leaves in the canopy where N. brunneicauda foraged. Long-tailed males changed feeding habits from sallying when not in mixed flocks, whereas short-tailed birds showed no change of feeding habit. The elongated tails of long-tailed males may have made their foraging less efficient owing to decreased agility in the canopy. N. brunneicauda is monomorphic and often formed groups of three to five individuals. In monospecific flocks, subordinates fed at low rates on branches owing to frequent hostile encounters. When foraging in mixed flocks, however, subordinates foraged among leaves, and their feeding rates increased because the frequency of intraspecific interference decreased greatly. Dominants did not show any difference in feeding pattern with social situation. Thus, heterospecific flocking was more advantageous for subordinates.     

Edge effect and structure of mixed-species bird flocks in an Afrotropical lowland forest

Avian mixed-species flocks are a dominant feature of tropical moist forests, yet their cost–benefit balance and habitat dependence in Africa are not fully documented. We recorded the composition of mixed-species bird flocks in a pristine Afrotropical lowland forest site in Salonga NP, DRC. Our data showed that at least four types of flock existed, one of which specialized on edge habitats. We used multivariate analyses to further characterize edge effect on the most documented mixed-species flock type and found a significant effect on flock composition. While neither species guild nor preferential foraging stratum played an important role in flock participation, the nucleus role was played by a different species at the edge and in the interior, and both species abundance and associations were habitat-dependant, suggesting “domino effects” on the structure of mixed-species flocks.     

Competition in foraging flocks of migrating semipalmated sandpipers

I examined the effect of competitor density on foraging success in staging semipalmated sandpipers (Calidris pusilla) foraging on a burrowing amphipod (Corophium volutator) in each of two study years. Little is known about the effect of competitor density when predation attempts disturb prey, causing a temporary decrease in food availability. Controlling for Corophium density and other potentially confounding factors such as temperature, pecking rate and capture rate increased linearly with sandpiper density. Success rate, the ratio of captures to pecks, was not influenced by sandpiper density. The effect of sandpiper density was similar in each of the two study years and was documented early and late in the low tide period. The increase in foraging rate is argued to be a response to increased competition for rapidly depleting prey at the temporal scale of exploitation by a flock. Potential fitness costs associated with higher foraging costs may include decreased ability to distinguish between the profitability of different prey and reduced vigilance against predators.     

Diffusion of individual birds in starling flocks

Flocking is a paradigmatic example of collective animal behaviour, where global order emerges out of self-organization. Each individual has a tendency to align its flight direction with those of neighbours, and such a simple form of interaction produces a state of collective motion of the group. When compared with other cases of collective ordering, a crucial feature of animal groups is that the interaction network is not fixed in time, as each individual moves and continuously changes its neighbours. The possibility to exchange neighbours strongly enhances the stability of global ordering and the way information is propagated through the group. Here, we assess the relevance of this mechanism in large flocks of starlings (Sturnus vulgaris). We find that birds move faster than Brownian walkers both with respect to the centre of mass of the flock, and with respect to each other. Moreover, this behaviour is strongly anisotropic with respect to the direction of motion of the flock. We also measure the amount of neighbours reshuffling and find that neighbours change in time exclusively as a consequence of the random fluctuations in the individual motion, so that no specific mechanism to keep one''s neighbours seems to be enforced. On the contrary, our findings suggest that a more complex dynamical process occurs at the border of the flock.     

The ecological significance of extremely large flocks of birds

Population size is generally limited by resource availability during and outside the breeding season. Therefore, maximum size of flocks may provide important information on population regulation and the influence of diet and trophic level on maximal degree of sociality. We hypothesized that (a) flock size should increase with nutrient availability; (b) flock size should decrease with latitude because productivity is higher at lower latitude; (c) aquatic habitats should have larger flocks than terrestrial habitats because the former are less accessible; (d) smaller species should have larger flocks because they require overall less food; (e) human‐impacted species that live in perturbed habitats should have smaller flocks than other species; (f) flock size should decrease with increasing trophic level because there is a reduction in biomass due to conversion at each trophic level; and (g) flocks of species depending on ancestral landscapes should have decreased in size in recent years due to human impact (e.g., land‐use). We obtained 1564 observations of flocks that exceeded 100,000 individuals in order to test the predictions listed above. Most effect sizes were small to medium accounting for 1%–9% of the variance, while large effects accounting for 25% or more were only found for total nitrogen used per km² and area used for agriculture. Changes in large bird flocks were caused by habitat degradation and persecution, and temporal decline in size of large flocks revealed changes in nutrient use, reductions in nutrient cycling, and changes in flock size linked to trophic level.     

Flocking behaviour of foraging birds in a neotropical rain forest and the antipredator defence hypothesis

Randomly encountered foraging birds were recorded in a primary rain forest of French Guiana (13,550 records of 216 species), together with their size, diet and habitat use, to assess the relative frequencies of different types of flocking behaviour and some of their ecological correlates. Overall, 42% of birds foraged singly, primarily carnivores (raptors), nectarivores (hummingbirds) and lek-mating frugivores (manakins, some cotingas). For-aging in pairs (26.6%) was widespread, notably among insectivores in the 17–32-g size class. The remaining 31.4% of records were birds in groups of different composition and function, including, in order of decreasing frequency, (1) multispecies upper canopy flocks (83 member species identified)—the largest and most mobile associations of small insectivores, nectarivores and frugivores, mostly tanagers; (2) understorey mixed species flocks of small insectivores, at midlevels of closed forest interior, with 12 core, obligate members and 74 occasional species, mostly active foliage or bark gleaners and probers sharing a unique set of ecological characteristics; (3) monospecific groups (29 species), either gregarious foragers but solitary breeders (large frugivores in canopy) or also breeding colonially or several permanently group living cooperative breeders; (4) opportunistic gatherings of frugivores at fruiting trees (at least 40 species); (5) army ant followers near ground of closed understorey (29 species of mid- to large-sized insectivores); (6) followers of Red-throated Caracaras Daptrius americanus (23 species, usually canopy frugivores entering understorey with caracaras); (7) two raptors following monkeys. Attributes of vulnerability to predators defined by habitat structure (vegetation density or openness) and foraging behaviour (conspicuousness, speed, degree of vigilance) were important determinants of flocking propensity, at least in flocks that were not attracted by a particular food source. The results suggest that the permanent mixed-species flocks in the mature forest under-storey may be an antipredator defence to compensate for the conspicuousness and reduced vigilance resulting from active foraging behaviour in semi-open vegetation, where early detection of predators is difficult.     

Prey selection by plovers: Optimal foraging in mixed-species groups

Prey (earthworm) size selection was investigated in lapwings (Vanellus vanellus) and golden plovers (Pluvialis apricaria) feeding in mixed species flocks and compared with that predicted by an optimal foraging model based on energy intake. As well as the usual constraints of searching and handling time, our model incorporated the difficulty of capturing concealed prey, the orientation time needed to locate prey and the risk of theft by gulls (Larus ridibundus). When costs were taken into account, small worms turned out to be the most profitable. The relative profitability of size classes changed when gulls were present and birds shifted their intake accordingly so that they always took mainly the most profitable worms. Birds were expected to do best by taking the three most profitable size classes and the size range taken was consistent with this. In addition there was an inverse relationship between the probabilities of taking profitable and unprofitable worm sizes. Observations of birds were supported by field enclosure experiments which prevented birds feeding in certain areas. Departures from predictions of the model are interpreted as sampling errors due to birds using depth as an approximate indicator of worm size.     

Interaction networks of avian mixed-species flocks along elevation in the tropical Andes

Ecological communities are comprised of species that interact with each other and those interactions ultimately generate community structure. Network theory provides a useful framework to study communities, by simultaneously considering species composition and the interactions among species. In this study, I use mixed-species flocks as model systems to gain insights on community and network structure. Specifically, I use co-occurrence network analyses to explore if avian mixed-species flocks change in richness and composition and/or in network structure and pair-wise associations, across elevations in the tropical Andes of Bolivia. Networks of flocking species changed both in composition and in the frequency and realization of pair-wise interactions across elevations, but changes in pair-wise associations explained most of network turnover along elevation. Pair-wise interactions changed rapidly, with shared species changing in position and importance within the network. Network dissimilarity was mostly explained by changes in the nature of associations rather than by differences in composition. Altogether, results show that montane mixed-species flocks are composed of loosely connected species and that most species have the potential of switching associations, often increasing in association strength at high elevations (up to 3150 m). Networks increased in connectivity and cohesion with elevation; flocks in lower elevations had more connections and these were less even. Above 3150 m a.s.l., there was rapid decay suggesting that flocks above this critical point are less connected and cohesive. This study exemplifies how combining community, network and pair-wise analyses can provide a more holistic view on the responses of species and assemblages to environmental gradients.     

Forest structure predicts species richness and functional diversity in Amazonian mixed-species bird flocks

Secondary forest has the potential to act as an important habitat for biodiversity and restoring ecological benefits. Functional diversity, which includes morphological and behavioral traits that mediate species interactions with the surrounding environment, relates to the resilience of ecosystems. To assess the relationship between habitat structural differences in primary and secondary forest and the resultant differences in functional diversity of avian species, we followed 11 mixed-species flocks at the Biological Dynamics of Forest Fragments Project, near Manaus, Brazil. We used remote sensing LiDAR to assess which three-dimensional forest structural features are most closely associated with variation in species richness and functional diversity in secondary and primary tropical forest flocks. The species richness of flocks in primary forest increased in areas with higher elevation and higher leaf area density in the understory and subcanopy but was not correlated with habitat structure in secondary forest. Functional diversity increased at lower elevations and with a denser subcanopy in both primary forest and secondary forest but only increased with greater understory leaf area density in primary forest. Together, these results indicate that a dense subcanopy and understory can be important for mixed-species flocks and that flock richness and functional diversity can be predicted by vegetation structure.     

Observations of mixed-species bird flocks at Kichwa Tembo Camp,Kenya

Mixed-species foraging flocks were studied at Kichwa Tembo Camp on the edge of the Masai Mara National Reserve in Kenya between July and September 2004. Observations were made on 29 mixed-species flocks, in which 24 species participated. African Paradise-Flycatcher Terpsiphone viridis, Black-backed Puffback Dryoscopus cubla, Grey-backed Camaroptera Camaroptera brachyura, Collared Sunbird Hedydipna collars and Cabanis's Greenbul Phyllastrephus cabanisi were the most common participants in mixed-species flocks, as well as among the most frequently encountered bird species overall. The Black-backed Puffback was identified as the nuclear species in flocks due to their abundance and frequency with which they were followed by other species. Mixed-species flocks represent another niche dimension in this diverse bird community, but few of these species could be described as flock specialists; most of the birds observed in mixed-species flocks in this study were opportunistic attendant species, including the African Pygmy-Kingfisher Ispidina picta, not previously described as joining mixed-species flocks.     

Visual perception and social foraging in birds

Birds gather information about their environment mainly through vision by scanning their surroundings. Many prevalent models of social foraging assume that foraging and scanning are mutually exclusive. Although this assumption is valid for birds with narrow visual fields, these models have also been applied to species with wide fields. In fact, available models do not make precise predictions for birds with large visual fields, in which the head-up, head-down dichotomy is not accurate and, moreover, do not consider the effects of detection distance and limited attention. Studies of how different types of visual information are acquired as a function of body posture and of how information flows within flocks offer new insights into the costs and benefits of living in groups.     

First nest description, breeding, ranging and foraging behaviour of the Short-legged Ground-Roller Brachypteracias leptosomus in Madagascar

The secretive, endemic Short-legged Ground-Roller Brachypteracias leptosomus was studied from October 1996 to February 1997 on the Masoala Peninsula, northeast Madagascar. Several vocalizations were associated with contact, courtship feeding and food solicitation. One study pair ranged within an area of 19.1 ha and spent 90% of their time together. They used small trees for foraging and resting, and durations of perch time averaged 9.8 min. Of the 229 identified prey items recorded, 88% were invertebrates and 12% vertebrates. The first described nests for this species were observed in December 1996 and January 1997. The first nest was in a natural tree cavity 18.1 m above the ground in a 133-cm diameter-at-breast height (dbh) Weinmannia sp., and it contained at least one egg. This nest failed on 1 January 1997 when a swarm of Honey Bees Apis mellifera took over the cavity. On 7 January, the pair began excavating another nest 22 m above the ground in a 174-cm dbh Canarium madagascarense , in the root mass and decayed material of epiphytes and below a 1 -m diameter forked branch. Incubation lasted between 22–26 days and the nestling period was 30 days. One young fledged in March 1997.     

Space use and agonistic behaviour in relation to sex composition in large flocks of laying hens

Some authors have found indications of subgroup formation when domestic fowl are forced to live together in large flocks, while others have not. In this study experiments were carried out to test the hypothesis that hens in large flocks have home ranges in parts of the pen and that they form subgroups. We also studied if this is influenced by males. In a tiered aviary system (density averaged 16 hens/m(2) of floor area) eight flocks of 568+/-59 ISA Brown laying hybrids were kept in pens. Half of the pens contained 1 male per on average 24 females (mixed flocks). At peak production (36-53 weeks of age) four females roosting closely together for about 14 days and four females roosting far apart from each other were taken out from each flock and put together in separate groups in small pens. Their agonistic behaviour was studied for 2 days before they were put back. This was repeated with new birds, resulting in 16 small sample groups being studied. At 70 weeks, three groups of 10 females per flock roosting closely together in different parts of the pen were dyed with different colours and their locations were observed for 2 nights and 2 days.The incidence of aggressive pecks during day 1 among birds that had been roosting close to each other tended to be lower (P=0.05) than among birds that had been roosting far apart. This effect was not significant among birds from all-female flocks, but among birds from mixed flocks (P<0.05). However, this indicates a recognition of roosting partners and possibly also a rebound effect of the males' reduction of female aggressiveness towards strangers. Irrespective of sex composition in the flocks, birds marked while roosting at the ends of the pens were significantly more often observed within these areas than in other areas of the pen during daytime and came back to the same roosting sites at night (P<0.05-P<0.001). This was not the case for birds from the middle of the pens, where the distribution in the pen in most cases did not differ from random. These results show that laying hens in large groups are rather constant in their use of space, which indicate the presence of home ranges. However, environmental features that facilitate localisation may be important. In summary, we think that these findings indicate the existence of subgroup formation.