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.     

THE FOOD AND FEEDING ECOLOGY OF GREY-HEADED ALBATROSS DIOMEDEA CHRYSOSTOMA AND BLACK-BROWED ALBATROSS D. MELANOPHRIS

The food and feeding ecology of Black-browed Albatrosses and Grey-headed Albatrosses was studied from 1975 to 1978 at Bird Island, South Georgia. Two hundred and seventy food samples (averaging 75–85 % by weight of the mean chick feed) were collected from adults of each species in February and March. Chicks of both species received meals of the same size, of which half consisted of liquid. The three major components of the solid diet (krill, squid and fish) were similar for both albatrosses. By weight, fish represented about 35 % of the diet of both species; squid predominated (50 %) in the diet of Grey-headed Albatross, and krill (40 %) in the diet of Black-browed Albatross. Lampreys were confined to the Grey-headed Albatross and, although squid of similar sizes were taken by both species, Black-browed Albatrosses took a much greater diversity of squid. Each major prey type was associated with a characteristic amount of liquid in the complete samples and only in the case of krill and lamprey was this lipid-rich.
As these two albatrosses are of similar size, breed over the same period and feed meals of equivalent weight to their chicks at similar intervals, the difference in the composition of the diet is possibly the most significant mechanism of ecological segregation (in the breeding season).
Evidence of the effect of krill shortage in drastically reducing Black-browed Albatross breeding success is presented to support this. The two species have largely non-overlapping winter oceanic ranges which are probably also correlated with the distribution of preferred prey.     

Aggression in shorebirds in relation to flock density and composition

The aggressive interactions of Turnstones and Purple Sandpipers feeding in wintering mixed-species wader flocks could be classified into those involving food and those involving space. All observed interspecific encounters were of short duration and were initiated and won by the larger species; the majority did not involve food and were resolved by low-intensity displays. Intraspecific interactions (a greater proportion of which involved food) were also resolved quickly and were usually won by the initiator. Space-related encounters between conspecifics were more likely to be resolved than food-related encounters just by threat displays. Aggression rates increased with flock densities. However, the increase in aggression with density was dependent on the species composition of the flock: both Turnstones and Purple Sandpipers were more likely to be involved in fights (both over food and over space) with conspecifics than with other species, indicating that the aggression costs of flocking were less in mixed-species flocks.     

The influence of flocking on the foraging behaviour of the chough (Pyrrhocorax pyrrhocorax) and the Alpine chough (P. graculus) coexisting in the Alps

We studied the flocking and foraging behaviour of the chough Pyrrhocorax pyrrhocorax and the Alpine chough P. gruculus coexisting in the south-western Italian Alps in order to evaluate the costs and benefits of foraging in single- and mixed-species flocks.
In the single-species context, flock size significantly affected the foraging behaviour of the Alpine chough; in larger flocks, the birds stayed for a shorter time in a patch and fed more quickly than in smaller flocks. Flock size did not significantly affect the foraging behaviour of the chough, probably because of the small number of individuals per flock.
The propensity for mixed-species flocking was rather low. The observed frequencies of single-species flocks of choughs and Alpine choughs were significantly higher than those expected on the basis of random flocking, whereas the observed frequencies of mixed-species flocks of the two species were lower than those expected. The stay times became significantly shorter for the chough in the presence of the Alpine chough. Moreover, feeding rates of the Alpine chough were significantly lower in the presence of the dominant chough.
The present study does not confirm the hypothetical foraging advantages of flocking. In single-species flocks, the benefits for the Alpine chough (higher feeding rates in larger flocks) were roughly compensated by the costs (shorter stay times in larger flocks), whereas the chough apparently neither gained benefits nor endured costs.
In mixed-species flocks, the Alpine chough sustained costs due to a reduction of feeding rates and the chough suffered costs due to a reduction of stay times. Hence, on average, single-species flocking gives no evident foraging advantages to either the chough or the Alpine chough, whereas mixed-species flocking provides some disadvantages for both species.     

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.     

Effects of supplemental food on intra-and inter-specific behaviour of the Varied Tit Parus varius

Varied Tits Parus varius lived in flocks containing a pair, or a pair with one or two unpaired birds. To test whether the sociality of Varied Tits or their participation in mixed-species flocks is sensitive to additional food (sunflower seeds), we compared the following parameters between fed and unfed periods at the individual level: population size, degree of site fidelity, mono-specific flock size and the stability of its membership, home-range size and distribution, attendance rate with mixed-species flocks, and flock size. Neither the size of the mono-specific flock nor its membership was affected by food supply. Whether food was added or not, individuals exhibited a strong site fidelity resulting in stable population size. Supplemental feeding had no effect on home-range size or distribution. In the presence of extra food, Varied Tits were observed in mono-specific flocks but rarely in mixed-species flocks. However, when we stopped feeding, they shifted to mixed-species flocking. After adding food, mixed-species flocks were significantly smaller than in the control samples. Varied Tits were more likely to join mixed-species flocks as temperatures dropped and wind speed increased. These results suggest that intra-specific sociality of Varied Tits is relatively insensitive to food supply, but they easily shift to mixed-species flocking in relation to food and weather conditions. We conclude that Varied Tits participate in mixed-species flocks to obtain short-term benefits e.g. increased foraging efficiency but they also obtain long-term benefits from stability of pair bonds and strong site fidelity, which did not respond to supplemental food.     

RELATIVE ABUNDANCE OF LARGE WHALES AROUND SOUTH GEORGIA (1979–1998)1

To assess large-whale stocks following the cessation of land-based South Georgia whaling in 1965, we report three independent sighting databases: a cruise in 1997, observations from Bird Island (NW of South Georgia) between 1979 and 1998, and mariner sightings between 1992 and 1997. All species were rare, with sightings of southern right whales being the most common event. Two right whales photographed off South Georgia matched animals known from Peninsula Valdés, Argentina, a population known to be growing at 7%per annum. In contrast, blue and fin whales appeared to be less abundant. A single blue whale mother-calf pair was observed off the Shag Rocks in February 1997. Extirpation of animals from this particular feeding ground is the most likely reason for ongoing low numbers of all species. Other factors may include competition for krill by traditional predators such as penguins and seals and more recently by humans, an unusually high rate of natural mortality, habitat change such as alteration in sea ice coverage, and/or the impact of ongoing whaling. The history of this critical area of large-whale habitat and this report demonstrate the need for improved, consistent longterm monitoring of population trends for these depleted stocks.     

THE COMPOSITION AND BEHAVIOUR OF SOME MIXED-SPECIES BIRD FLOCKS IN SARAWAK

Mixed-species flocks of birds feeding on insects were observed mainly in forest in Sarawak at a time of year when insect availability is known to be near its annual minimum. The approximate individual and specific composition of most flocks were noted. Twenty-six species occurring in 25% of flocks were regarded as occasional members, 15 species occurring in 40% of flocks being classed as regular members. Recording species' feeding behaviour was the main priority and few observations of inter- and intra-specific interactions within the flock, horizontal distribution or vertical stratification were made, although the last proved to be of potential significance. From the analysis of feeding behaviour two groups of species were distinguished. The larger group contained those usually having an exclusive common feeding pattern and showing very little overlap with the other such species. Fewer species used a wide range of feeding methods, all of which were shared with other species, but the least overlap was with other members of the same category. The observed composition of these flocks, in terms of regular members, might be interpreted as ensuring a low level of inter-specific competition and it is suggested that the less specialized foragers may occur in the flocks by utilizing the ‘gaps’ between, and absences of, specialists. The possible advantages of membership of mixed-species flocks are briefly considered and the likelihood of the selective advantage in any situation being the result of a balance of factors emphasized. In the apparent absence of regular potential predators the existence of these flocks is interpreted primarily as an adaptation for augmenting available insect food, particularly perhaps at critical times of year, by flushing insects as a result of the foraging activities of flock members. It is suggested that the varied responses of insects on being disturbed coupled with the different and fairly specialized feeding techniques of the birds could ensure benefit for all members of the flock.     

Characterizing complex mixed-species bird flocks using an objective method for determining species participation

We developed sampling methods to characterize the participation of bird species in foraging flocks led by the Eastern Tufted Titmouse (Baeolophus bicolor) in North-central Florida during winter, because standard field methods, developed primarily for permanent resident Neotropical flocks, were intractable in our system. During January–February 2004 and November 2004–March 2005, we observed 55 mixed-species flocks, recorded 40 potential flocking species [mean of 12.4 species (SD = 3.8; range 3–20), 26.3 individuals (SD = 12.2; range 8–60), and 3.1 titmice (SD = 1.4; range 1–7), per flock]. Twenty-six species were observed frequently enough (>10% of observations) to be included in analyses. We paired 60-min flock observations with 10-min point counts conducted in locations used by flocks, but after flocks had moved more than 100 m away. This method yielded a measure of flocking propensity: the ratio of the number of individuals observed in the flock versus during the point count for each species. We used regression tree (RT) analysis to classify species into groupings according to their levels of flock participation, and to investigate relationships between flocking propensity and various environmental and social factors that we measured. Our analysis identified three clear species groups; “Nuclear/Regular Associate” (12 spp.; high/moderate), “Occasional Associate” (four spp.; moderate/low), and “Non-joiner/Accidental” (ten spp.; low/no flocking propensity). Groupings were similar to schemes produced via more time-intensive field methods. In order to contextualize grouping categories, we conducted a review of flocking group definitions and relevant autecological information (e.g., interspecific sociality) about our study species. We found this method to be useful for geographically extensive sampling of species’ participation in mixed-species flocks, despite high inter-flock variability in species composition and limited labor.     

云南省纳帕海自然保护区越冬黑颈鹤的集群特征

2004年10月-2005年5月,在云南纳帕海自然保护区采用定点扫描法对越冬黑颈鹤(Grus nigricollis)的集群类犁和集群大小进行了观察.结果表明黑颈鹤夜间集群夜栖,形成较大的夜栖群,平均群体大小为67.9只(16-157,n=17):按照有无灰鹤加入,又将其分为同种集群和混种集群两种类型,其中同种集群的黑颈鹤数量占整个越冬种群的65.3%.在白昼,黑颈鹤以家庭鹤、集群鹤及特殊群体3种类型活动,家庭鹤和集群鹤的平均大小分别为2.7只(2-4,n=145)和16.1只(3-65,n=1017).黑颈鹤的集群大小并不稳定,在日内和月份间均有明显变化(P=0.000＜0.05).存越冬期,最大集群形成于12月,其次为11月和1月;在日内,早上8时集群最大,随后减小并保持相对稳定,18时黑颈鹤开始向夜栖地靠拢,使得集群再次开始增大.随后观察中还发现,黑颈鹤的家庭解体过程开始于3月底,当幼鹤被成鹤驱逐离群后,逐渐加入集群鹤活动,从而使得家庭鹤和集群鹤的大小和组成发生改变.黑颈鹤的集群大小和组成受自身状况、种内关系、天气、食物等多种因素的共同影响,随时间和季节变动而发生变化,是对自身、种群和环境条件变化的综合反映.     

Advantages of social foraging of Willow Tits Parus montanus

The mean number of Willow Tits Parus montanus in single-species flocks was significantly larger than in mixed-species flocks of Willow and Coal Tits P. ater. Both flock size and the tendency of Willow Tits to join mixed-species flocks were negatively correlated with ambient temperature, probably because each bird, when the metabolic rate of the birds increased, could allocate more time to foraging due to improved predator detection by many eyes. The vigilance time of Willow Tits decreased with flock size and was determined by the total number of individuals in a flock rather than by the number of Willow Tits in mixed-species flocks of Willow and Coal Tits.     

The diet of Antarctic fur sealsArctocephalus gazella during the breeding season at South Georgia

 The diet of lactating female Antarctic fur seals Arctocephalus gazella at South Georgia was investigated during the pup-rearing period (January/March) of 1991–1994. Antarctic krill Euphausia superba was the main prey item, occurring in 88% of all scats (n=497), whereas fish occurred in 47% and squid in 5% of all scats. There was considerable intra- and inter-annual variation in the characteristics of krill taken by fur seals. The distribution of krill sizes taken suggests that fur seals are not actively selecting particular sizes of krill and, therefore, that the krill in the diet reflects the krill available around South Georgia. The absence of group 3 krill (44–48 mm in length) in the South Georgia area, as indicated by their absence in the diet of seals, is suggested as a possible reason for low availability of krill and the subsequent reproductive failure among krill predators. The frequency of occurrence of fish was much higher than in previous studies; the pattern of fish consumption showed a consistent seasonal pattern in 3 of the 4 years studied. Of the total number of the myctophid Protomyctophum choriodon, the most numerous fish taxon, 98% were taken between early February and the middle of March. Champsocephalus gunnari and Lepidonotothen larseni agg., which both feed on krill, dominated the fish component of the diet outside this period and together constituted 94% of the total estimated biomass of fish consumed. The intra- and inter-annual variability in the diet of Antarctic fur seals emphasise the need for diet studies to be conducted during the entire pup-rearing periods of several years. Received: 15 March 1995/Accepted: 13 May 1995     

First documentation of leopard seal predation of South Georgia pintail duck

Leopard seals are regular winter visitors to Bird Island, South Georgia, where they mostly prey on fur seals and penguins, and to a lesser extent on Antarctic krill and fish. Leopard seals can exploit many different species, but there are no records of predation on flying shorebirds in the wild. On 4 October 2008, an individually identified juvenile leopard seal female was observed killing and eating a South Georgia Pintail duck. It also preyed on Antarctic fur seals and gentoo and macaroni penguins during its 2-month temporary residency around the island. The varied diet of this seal exemplifies the generalist prey utilization typical of its species. Long-term diet studies at Bird Island and the published record suggest that predation on ducks is a rather exceptional finding; individual ducks are more likely to escape leopard seal attacks than penguins and provide a far less substantial ration. This note documents the first observation of this species of duck in the diet of leopard seals.     

Multi-species territoriality and dynamic of neotropical forest understorey bird flocks

1. Eleven contiguous mixed-species bird flocks, with colour-banded individuals, were monitored continuously during 3 years in a 132-ha study area of primary rainforest in French Guiana.
2. Flock members were divided into six categories according to their flocking propensity and occurrence: 10 core or permanent species and 56 regular, occasional or incidental species. Each core species was represented by a single breeding pair with their fledglings and extra 'floaters' (unmated subadults and adults).
3. Flock home ranges overlapped slightly, but were communally defended by all core species in areas of overlap. Their size varied from 3·2 to 14·3 ha and was inversely correlated with vegetation density, but not flock size or species composition.
4. Flock number, size and composition, as well as boundaries were highly stable between seasons and years. Each flock had a single permanent gathering site and bathing site in late afternoon, the latter sometimes shared by 2–3 flocks.
5. Core species produced 0·18–0·73 fledglings per pair per year, which stayed in their natal flock for 200 to over 421 days. Then, these individuals usually moved between two and six different flocks, sometimes for up to 3 years, before finding a mate and a flock where they could settle and breed. Once breeding, they probably remained for life in the same flock. The mean annual survival rate was at least 0·75.
6. This highly evolved and stable organization, associated with a low breeding success and high survival rate was a critical factor maintaining low species density, delayed reproduction and a proportion of floating individuals buffering population fluctuations.
7. These social groups with their multi-species territoriality and co-evolved roles of flock members were similar to those described elsewhere in South America. They seem to be a general phenomenon in neotropical lowland rainforests.     

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.     

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.     

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.     

Ecological drivers of change at South Georgia: the krill surplus,or climate variability

Macaroni penguins Eudyptes chrysolophus are thought to be one of the most important mesopredators in the Southern Ocean having a greater impact on prey availability and abundance than any other seabird species. Their population centre has long been held to be South Georgia where populations were thought to comprise many million animals. Here we report the results of a recent census of the macaroni population at South Georgia undertaken using aerial survey methods. We report dramatic declines in numbers (～1.0 million breeding pairs) compared to numbers observed in the late 1970s (～5.4 million pairs), but show that these reductions have occurred principally at sites where numbers had previously been very large. During the breeding season, the main foraging grounds of birds from these sites overlap with the foraging grounds of Antarctic fur seals Arctocephalus gazella, a major competitor for their principal prey, Antarctic krill Euphausia superba. We suggest that the redistribution of the macaroni penguin population at South Georgia reflects the recent recovery of fur seal populations and thus the ongoing consequences of human intervention at South Georgia, a process which started more than 2 centuries previously. The implied resource competition and the observed population changes may also be exacerbated by recent reductions in Antarctic krill abundance which have been linked with reductions in seasonal sea ice following recent, rapid, regional warming in the Antarctic; however, the recovery of fur seal populations, and the ongoing recovery of krill‐eating whale populations argues that tropho‐dynamic interactions may be sufficient to explain the observed changes.     

Relationships between frequency of mixed-species flocks, weather and insect activity in a montane cloud forest in Ecuador

The frequency of mixed-species flocks of birds, weather and insect activity were studied in a high-altitude cloud forest in Ecuador. It is generally accepted that participation in mixed-species flocks improves foraging efficiency. If this is true, more flock activity may be expected when food is less available, which may happen during long-lasting periods of rain and otherwise adverse weather conditions. The total number and mean size of flying insects decreased as rainfall increased. The number of flocks observed increased with decreasing number and biomass of insects. Relatively more flocks were seen during rain than during dry weather. No flocks stopped foraging during rain. The flock activity pattern appeared to be the opposite of that found in humid lowlands, probably because of different weather regimes in the two zones. In the lowlands, heavy rain is typically of short duration. In high-altitude cloud forest, the rainfall is often less intense but persists for prolonged periods. Hence, from an energy point of view, cloud forest birds cannot afford to stop foraging during adverse weather conditions when insect availability is low. The results suggest that some mixed feeding parties have evolved in response to low tropical insect availability, necessitating long feeding excursions outside the territory.