Effects of snow cover on the social and foraging behavior of the great tit Parus major

In the Joetsu region of central Honshu, Japan, snow lies on the ground 2–3 m deep from January to February every year. To test the effects of snow cover on the social and foraging behaviors of the great tit (Parus major Linnaeus) in the region, the following parameters were compared between the pre-snowy period (November and December) and the snowy period (January and February) at the individual level: population size, monospecific flock size and the stability of its membership, home range size, attendance rate with mixed-species flocks, and foraging height. Great tits lived alone or in pairs, but often joined mixed-species flocks. Throughout the study period, individuals exhibited strong site fidelity, which resulted in a stable population size. Neither the size of a monospecific flock nor its membership was affected by snow cover. The home range expanded when birds joined mixed-species flocks during both periods. Birds more frequently joined mixed-species flocks during the snowy period, and the size of mixed-species flocks was significantly larger than during the pre-snowy period. During the snowy period, birds shifted their foraging position from the ground to the upper parts of trees. They also used the upper parts of trees when they joined mixed-species flocks. These results suggest that the intraspecific sociality of great tits is relatively insensitive to snow cover, but that the home range size and foraging positions are affected by joining mixed-species flocks, rather than by snow cover.     

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.     

FLOCKING AND FEEDING IN THE GREAT TIT PARUS MAJOR–AN EXPERIMENTAL STUDY

Some aviary experiments designed to investigate the survival value of flocking in the Great Tit Parus major are described.
An individual searching for a source of food is less likely to find it if he is on his own or in a pair than when in a flock of four. This is a result of local enhancement: with more pairs of eyes searching, some member of the flock is likely to come across the food sooner than would a lone bird, and once one finds the food, others in the flock more or less immediately assemble at the food source. Once the birds have found a source of food, dominance interactions play a role in determining which individuals obtain the most food.
If one bird finds a single item of food, both it and the other members of the flock rapidly alter their searching behaviour, concentrating their attention in the general area and /or type of place where the food item was found.
Even if a bird searches in a place and does not find food, the others in the group show some increase in searching effort in the same area or type of place. The extent to which they do this depends on whether they are used to finding food in clumps or dispersed.
The relevance of these findings to birds in the wild is discussed. It seems likely that local enhancement does occur in natural flocks, and the flocking in Great Tits is of benefit to the participants because they increase their effectiveness in finding food. However, the question of the survival value of mixed-species flocks of titmice remains unresolved.     

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.     

山地次生林冬季鸟类集群的初步研究

通过山地次生林两冬季鸟类集群的研究表明,集群的鸟类主要是由大山雀、沼泽山雀和普通Ｓｈｉ组成;每群个体数为７．４１－１０．６３只;据群的行为（单种群、多种集群和单独个体）分析,大多数种类是单独个体活动,冬季鸟类集群大小和组成有空间的变化。气候因子影响群的形成和行为。     

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.     

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.     

Mixed species flocking of tits (Parus spp.): a field experiment

Summary We tested two general models of flocking behaviour, namely the antipredation model and foraging efficiency model on mixed-species tit flocks (Parus spp.). After food addition the size of mixed-species flocks was significantly less than in the control samples. In the presence of extra food significantly more birds were observed either in monospecific flocks or solitary, than during the control observations. In the presence of a living predator the birds foraged in larger mixed-specifies flocks than during the control observations. In addition, the social behaviour of Great Spotted Woodpecker, Middle Spotted Woodpecker and Nuthatch shifted to mixed-specific flocking. The size of monospecific flocks was independent of both treatments. The density of birds increased significantly after food addition, while in the predator presence the birds tended to leave the forest. These results support the view that both the antipredation model and foraging efficiency model seem to be valid for mixed-species flocking. However, in the case of monospecific flocks, the territory maintenance could be the most important factor.     

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.     

Foraging strategy in a social bird, the alpine chough: effect of variation in quantity and distribution of food

I conducted feeding experiments to determine the effect of variations in food availability on individual flocking behaviour and foraging efficiency in a social corvid, the alpine chough Pyrrhocorax graculus, which lives in large flocks all year round. In 37 trials I varied both food quantity and the number of food patches. A decrease in the amount of available food reduced the mean flock size, the proportion of birds that had access to food, and their mean pecking rate. A decrease in the number of patches, on the other hand, reduced only the proportion of birds that had access to food. The number of choughs foraging was not influenced by food competition but depended only on the number visiting the site. Females competed less well than males: when food was made scarce, they frequented the site in the same proportion as did males, but had less access to food. I suggest that in this social corvid, long-term advantages to flocking related to social bonds, such as the maintenance of pair bonds, may compensate for short-term costs such as a reduction in foraging efficiency. Copyright 1999 The Association for the Study of Animal Behaviour.     

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.     

Subordination in mixed-age bird flocks–a removal study

During the winters of 1979 to 1986, flocks (7–17 per season, 78 total) of Willow Tits Parus montanus (generally consisting of an adult pair and four juveniles) were studied in a subalpine mixed forest in Central Norway. The mean survival rate between December and early April for juveniles (0.32) was significantly lower than that found for adults (0.74). As it has previously been found that adult Willow Tits, being dominant, prevent the juveniles in a flock from using preferred foraging sites, a removal experiment was made to see if the survival of juveniles improved when living in flocks with no adults present. After removal of the adult pair from 13 flocks in early January, the juveniles from these flocks survived significantly less well, up to early April, than either the juveniles in 11 flocks in which the adults were present (controlled for flock size) or those in 14 flocks consisting of an adult pair and four juveniles. Despite a skewed access to the resources in favour of dominant adults, the cost of being a subordinate juvenile is probably outweighed by the improved predator evasion and improved food finding efficiency due to the greater experience of the adult pair possessing the territory.     

Linking vegetation structure and bird organization: response of mixed-species bird flocks to forest succession in subtropical China

As forests undergo natural succession following artificial afforestation, their bird assemblages also change. However, interspecific avian social organization associated with forest succession has not been fully understood, particularly for mixed-species bird flocks. To disentangle how mixed-species flocks change as a function of local forest structure, we analyzed flock characteristics (particularly species richness, flocking frequency and propensity) and vegetation physiognomies along a presumed successional series (early, middle, and advanced) simultaneously in subtropical forests in southern China. As hypothesized, monthly point counts demonstrated that complexity of flocks increases with the progression of natural forest succession at a local scale. Advanced forests differed significantly from pioneering plantations with respect to vegetation structure, flock characteristics and constituents (especially for understory specialists). Importantly, forest succession affected flock patterns particularly in relation to the flocking propensity of regular species, and the frequency of nuclear species (Huet’s fulvetta Alcippe hueti), which in turn determined flocking occurrence at different successional stands. Canonical correspondence analysis indicated that understory flocking species (mainly Timaliidae babblers) were significantly associated with intact native canopy cover, complex DBH diversity, as well as high densities of dead trees and large trees, representing a maturity level of successional stands. Our study reveals that the effect of natural forest succession on mixed-species bird flocks is species-specific and guild-dependent. From a conservation perspective, despite a high proliferation of pine plantation in southern China, priority should be placed on protecting the advanced forest with a rich collection of understory flocking specialists.     

Mixed-species flocking of forest birds on Little Barrier Island

Abstract

Three aspects of mixed-species flocking of forest birds on Little Barrier Island were investigated. Whiteheads, fantails, parakeets, and grey warblers occurred more often in flocks than in “non-flocking” situations. Whiteheads were the main lead species, although parakeets formed groups within flocks and occasionally appeared to lead. Whitehead clumps defined the flock centre; only fantails were found commonly in the centre with whiteheads. Birds other than whiteheads generally orientated below or to the side of their nearest neighbours. We suggest that mixed-species flocking is a significant factor influencing the structure of forest bird communities in New Zealand during winter.     

STRUCTURE AND FORAGING PATTERNS OF FLOCKS OF TITS AND ASSOCIATED SPECIES IN AN ENGLISH WOODLAND DURING THE WINTER

Blue Tits were the commonest and most frequent members of mixed-species insectivorous flocks during the winter at Wytham Wood, Oxford. Six common flocking species (Blue Tit, Longtailed Tit, Great Tit, Coal Tit, Marsh Tit and Golderest) made up nearly two-thirds of the birds in the study area. A tentative interspecific social dominance hierarehy was constructed: Great Tit > Blue Tit > Marsh Tit > Coal Tit > Long-tailed Tit and Goldcrest. Blue Tits permitted conspecifics to approach them more closely than did other flock members. Blue Tits most frequently led the flocks, but no more than would be predicted by their abundance. Great Tits, and Marsh Tits and Coal Tits to a lesser extent, defended territories during the winter. Two of the six common species frequented the inner parts of branches (Great Tit, Coal Tit), two the outer parts of branches (Blue Tit, Marsh Tit), and two the twigs (Long-tailed Tit, Goldcrest). Members of each of these species-pairs showed marked differences in height of foraging and/or species of tree frequented. Because of their abundance, the impact of Blue Tits outside of their most highly frequented foraging zones may exceed that of species concentrating in these other zones (e. g., twigs high in trees). The Coal Tit foraged most diversely, the Great Tit least diversely. Species that foraged diversely in one of the three foraging categories (species of tree, substrate, height) usually foraged less diversely than most other species in the other dimensions. Species usually overlapped least in the zones exploited (inner parts of branches, etc.). Great Tits overlapped with other species less than did any other common flock member, and Coal Tits were the next lowest in this regard.     

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.     

Bird mixed‐species flock formation is driven by low temperatures between and within seasons in a Subtropical Andean‐foothill forest

According to both the predation avoidance and foraging efficiency hypotheses, birds within mixed flocks increase their foraging efficiency and/or can spend more time feeding and less time looking out for predators. These hypotheses predict that birds in mixed flocks obtain benefits. Thus, mixed flock formation could serve as a strategy to cope with difficult conditions imposed on birds such as climatic conditions that ultimately result in a change in predation pressure or food resources. We evaluate the hypotheses that forming part of a flock confers benefits to its members and the associated prediction that birds will take advantage of these benefits and flock more often under cold and dry weather conditions between and within seasons to cope with such conditions. We surveyed the presence of mixed flocks, flocking propensity, number of species and individuals in mixed flocks in the Subtropical Yungas foothill of Argentina, to examine seasonality, flocking behavior of birds and their responses to two climatic variables: temperature and humidity. Bird species presented a higher flocking propensity and mixed flocks occurred more frequently during the dry and cold seasons than during the more benign seasons, and lower values of temperature within seasons triggered the flocking behavior. Although effects between seasons were expected, birds also showed a short‐term response to small changes in temperature within seasons. These results strengthen the ideas proposed by the foraging hypothesis. Although benefits derived from flocking have yet to be determined, whatever they are should be understood in the context of seasonal variation in life‐history traits.     

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.     

Seeing without being seen: a removal experiment with mixed flocks of Willow and Crested Tits Parus montanus and cristatus

This paper tests the hypothesis that foraging site selection reflects a trade-off between the various needs for concealment from predators, to find food, and for the individual to maintain some view of its surroundings. After removal of Crested Tits Parus cristatus (the dominant species in mixed flocks), Willow Tits P. montanus did not decrease their foraging heights as expected but remained in the most exposed parts of young pines. In contrast, after removal of Willow Tits, Crested Tits increased their foraging height from the sheltered lower canopy to sites previously occupied by Willow Tits. When flock size was reduced, the birds maintained the same high levels of vigilance without concealing themselves in dense vegetation. I suggest that flock members may benefit from foraging in sites that afford good anti-predator vigilance.     

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.