The effect of social dominance on fattening and food-caching behaviour in Carolina chickadees, Poecile carolinensis

Subordinates often have to wait for dominants to obtain food. As a result, their foraging success should be less predictable and they should therefore maintain a higher level of energy reserves compared with dominants. A corollary of this prediction is that subordinates should gain mass earlier in the day and maintain higher mass than dominants. We tested these predictions with captive Carolina chickadees. In two different experiments (one where birds were given ad libitum access to food and the other with food access limited to 60 min/day), we formed social flocks of two previously unfamiliar birds and compared their energy management (body fat and food caches) while they were in the flock with energy management when housed alone. Results from both experiments failed to support the predictions. Of all the parameters of body mass and food caching we measured only the following results were significant: (1) On the ad libitum food schedule, both subordinates and dominants accumulated more mass over the day when in a flock compared with when they were solitary, and there were no differences in mass gain between dominants and subordinates. (2) When analysed separately, dominants showed a higher evening mass in the flock compared with the solitary condition, a trend that runs opposite to the prediction. Our results suggest that when in favourable foraging conditions, social interactions might cause dominant and subordinate birds to accumulate more energy reserves as a result of competition. On the other hand, if food supply is limited, both dominants and subordinates may be forced to maintain similar fat reserves as an insurance against increased risk of starvation. Copyright 2000 The Association for the Study of Animal Behaviour.     

The Effects of Food Deprivation in Dyadic Encounters in Apodemus sylvaticus

The nature of dominance in relationships among male Apodemus sylvaticus was investigated using multivariate analysis and by experimental manipulation of the behaviour of dominants. The behaviour of dominant mice was typified by greater frequencies and durations of exploration, approach and pursuit of subordinates in short encounters in a small arena. The direction of encounters normally remained constant over 48 h. When the dominant mouse in a dyad was deprived of food there was a change in its behaviour and an immediate change in the behaviour of the subordinate. Subordinates showed greater interest in starving, though known, dominants, explored more freely and indulged more in pursuit than control subordinate mice i. e. those matched with dominants on an adequate diet. Results suggest that in A. sylvaticus, dominance is associated with the inhibition of the activity of a subordinate and the relationship is maintained by the behaviour of the dominant rather than the deference of the subordinate.     

Flock size and habitat-dependent food and energy intake of foraging Goldfinches

Summary During the breeding season Goldfinches (Carduelis carduelis L.) feed on milky ripe seeds of about 20 food plants. Individual Goldfinches joining a flock reduce the time spent vigilant with increasing flock size. Therefore birds feeding in flocks get an increased intake of kernels per time unit. This was measured for five different food plants (Dactylis glomerata (Gramineae), Knautia arvensis (Dipsacaceae), Senecio vulgaris, Taraxacum officinale, Tragopogon pratensis (Compositae)). In large-sized flocks, birds fed up to 2.3 times more kernels, than when feeding solitarily. In addition, visibility in the vegetation leads to a further increase of kernel intake. Thus feeding under good conditions as in recently mown areas, can raise kernel intake to the seven fold per time unit as compared to solitary feeding. The maximum ingestion rate of kernels was 98 per min which implies a head up-and-down movement every 0.6 s. The calculated energy intake of birds per time unit is lowest in Senecio and highest in Tragopogon. Thus the birds, when feeding on Tragopogon in larger flocks, can gain an energy intake 16 times higher than that reached when feeding on Senecio, despite of a higher kernel intake rate. The energy intake individual Goldfinches gain at the particular plant species is markedly increased with flock sizes up to eight birds, with larger flocks the intake increases only slightly.     

ASPECTS OF THE BEHAVIOUR AND ECOLOGY OF MIXED-SPECIES BIRD FLOCKS IN KASHMIR

Mixed-species flocks of birds were observed between the end of July and late August, principally at Daksum, Kashmir, 2250 m. The species composition and the numbers of individuals in flocks changed during this period; these changes are attributed to resident territory holders and migrant birds joining the flocks. Within the flock different species showed some differences in foraging stations, but nevertheless often appeared to be taking the same type of food. Participant species had different roles in the flock organization. Behaviours involving the entire mixed-flock acting as a unit included path reversal after encounters with avian predators and a tendency to follow set routes. The mixed- species flock exerted an attractive influence on aggregations of species not normally participant.
Similarities between the flocks described in this study and those recorded by other workers are discussed. While different species may derive different benefits from joining these flocks, advantages that could benefit some or all participants include the receipt of information on good feeding areas in an unfamiliar locality, the avoidance of time wasted on feeding on substrates which have been very recently harvested, the beating effect to increase prey availability, and enhanced safety from predators, perhaps through differential alertness of different species and specialized anti-predator behaviour.     

Social stability and daily body mass gain in great tits

This paper presents the results of an experiment on the influenceof changes in flock composition and social rank on the patternof daily mass gain in captive wintering great tits. We createdflocks of three male great tits and scored dominance when rankshad stabilized after a few days. We then moved the dominantbird from each flock into a neighboring flock, thus creatinga certain amount of social instability and unpredictable resourceaccess. Both before and after moving the dominants, we collectedindividual body mass data three times a day. When the hierarchieshad stabilized in the second flocks, individual ranks were scored.After the dominant birds were moved, the rate of daily massgain increased for the average individual. There was also adecrease in average morning body mass, so the birds startedthe day at a lower mass and ended at a slightly higher massthan previously. Dominants were on average heavier than subordinates,but we found no statistically significant effect of rank changeon body mass. The pattern of daily mass gain was linear overtime both before and after treatment. We conclude that an impressionof unpredictable resource access can be created by disturbingthe social structure in a flock, with similar effect as fromrestricting individual access to feeders. Within a flock, instabilityof the social hierarchy thus seems to have an influence on bodymass management in addition to that of environmental stochasticity.     

The relationship between dominance behaviour, bill size and age group in Greater Sheathbills Chionis alba

A dominance hierarchy developed within a wintering flock of Greater Sheathbills which fed regularly at a concentrated artificial food source. Each bir's dominance position was determined from the proportion of interactions in which it was displaced, and from the number of other birds which it was seen to displace.
Most dominant birds were known to be over 6 or 7 years of age. They had larger bills than their subordinates had and initiated displacement interactions more often. After each interaction the displacer resumed feeding more quickly than the bird it displaced. Although dominant birds had a higher average peck rate, they did not maintain a higher body mass throughout the winter, nor did they show a higher survival rate than their subordinates.     

Common Myna Roosts Are Not Recruitment Centres

We studied communal roosting in the Common Myna (Acridotheres tristis) in the light of the recruitment centre hypothesis and predation at the roost. The number and sizes of flocks departing from and arriving at focal roosts were recorded over a two year period. We also recorded the sizes and behaviour of foraging flocks. We found that flock sizes of birds departing from roosts at sunrise were larger than those at the feeding site, suggesting that there was no recruitment from the roosts. Flocks entering the roosts during sunset were larger on average than those leaving the following sunrise, suggesting no consolidation of flocks in the morning. Flocks entering the roosts at sunset were also larger on average than those that had left that sunrise, although there was no recruitment at the feeding site. There was no effect of group size on the proportion of time spent feeding. Contrary to expectation, single birds showed lower apparent vigilance than birds that foraged in pairs or groups, possibly due to scrounging tactics being used in the presence of feeding companions. Thus, the recruitment centre hypothesis did not hold in our study population of mynas. Predation at dawn and dusk were also not important to communal roosting: predators near the roosts did not result in larger flocks, and resulted in larger durations of arrival/departure contrary to expectation. Since flock sizes were smallest at the feeding site and larger in the evening than in the morning, but did not coincide with predator activity, information transfer unrelated to food (such as breeding opportunities) may possibly give rise to the evening aggregations.     

Physical predictors,behavioural/emotional attributes and neurochemical determinants of dominant behaviour

Significant differences in physical and behavioural/emotional/cognitive predictors and attributes, as well as of neurochemical inducers of behaviour, between dominant and subordinate animals are discussed. It is still unknown whether these factors are the causes of differences between dominants and subordinates, or vice versa whether the differences between dominants and subordinates are the origin of differences in these factors. The possibility is discussed that no differences exist among juveniles in the concentrations of neurochemical agents (known in the literature as determinants of dominance) between the brains of future dominants and future subordinates. We describe a study design that makes the assessment of the ‘original’ neurochemical profile of the brain possible.     

Social rank and habitat use in willow tit groups

One view depiets dominance and subordination as roles with equal fitness played as parts of a mixed behavioural strategy, but subordination can also be maintained as a transient strategy of ‘hopeful dominants’. Resource access in groups of non-kin willow tits, Parus montanus, was related to social rank. Dominants excluded subordinates from the preferred microhabitat. Fitness in terms of survival was correlated with access to the preferred foraging sites, indicating that a mixed-strategy explanation is inadequate. Social rank was linked to age, and first-year subordinates behaved as predicted for hopeful dominants. They were not ‘herded’ by dominant group mates because subordinates actively maintained contact with dominants. One possible reason for subordinates joining winter groups might be the opportunity to reduce dominants' pay-off from aggression.     

On the feeding and social behaviour of the laying hen

This paper describes a study of the influences of early husbandry conditions, social attraction and social rank on various aspects of the feeding behaviour of laying hens.Birds were raised in flocks of 10, 60 or 500. Groups of 3 birds, selected from flocks of the same size, were then housed in pens. Some groups consisted of hens raised in the same flock, and some of birds raised in different flocks. The feeding and agonistic behaviour of each group at each of 5 types of feeder was observed and compared with the behaviour shown when the birds had free access to a 1-m long food trough. Each of the 5 feeders offered the same area of feeding space, but differed in its partitioning and spatial distribution. One feeder had a single unpartitioned feeding space. The other 4 feeders had 3 partioned feeding spaces which were adjacent, or separated by distances of 10, 20 or 40 cm, respectively.For the 5 feeders, total feeding times and lengths of feeding bouts were greatest, and the number of feeding bouts least, when the feeding space was unpartitioned. Synchrony of feeding behaviour was low when the feeding space was unpartitioned or the partitioned spaces adjacent, but was comparable to that at the 1-m long food trough when the distance between partitioned feeding spaces was 10 cm or greater. When feeding space was partitioned, the likelihood that 2 birds would eat together at the same site increased with the distance between feeding space. Dominant birds always exhibited the longest feeding bouts and greatest total feeding times, but were less likely to feed in the same space as another bird, and exhibited less synchrony of feeding behaviour than subordinates. The size of the flock in which the birds were raised, and whether or not the birds in a group had been raised together or apart, had no clear effect on behaviour.These results indicate that, within the limits of this experiment, early husbandry conditions do not influence behaviour shown during feeding in later life, and that social attraction has a greater influence on the feeding behaviour of hens than is generally assumed. In view of this latter finding, it is postulated that in attempting to determine the requirements of laying hens for feeding space, attention must be paid to social attraction as well as to competition at the feeder.     

Social dominance in free-living Willow Tits Parus montanus: determinants and some implications of hierarchy

Several features of social dominance among Willow Tit Parus montanus winter flocks were examined during a four-winter study. Birds of both sexes were evenly distributed over the 33 flocks studied. In nearly half of the flocks there was an adult pair accompanied by yearlings, but one-third of the flocks consisted of more than two adults with yearlings. Males were found to be dominant over and larger than females. Within a sex, yearlings were usually subordinate to adults. The effect of size on dominance, after controlled for sex and age, remained obscure in our field data. The hierarchical status of an individual was found to rise or at least stay the same in different years, which supports the "hopeful dominants" hypothesis, i.e. birds stay in flocks hoping to achieve a higher status in the future. The ranks of mates correlated highly significantly, implying that high-ranking birds were paired with other high-ranking birds and low-ranking birds with other low-ranking birds. Birds of different age and sex did not show any differences in the proportion of initiated aggressive encounters directed at other individuals. However, males were more aggressive to other males than to females and also tended to behave less aggressively towards their own mates than towards other individuals in the flock. This could be a male strategy to strengthen the pair-bond and to enhance mate protection described earlier in     

Piping as a display of dominance by wintering Oystercatchers Haematopus ostralegus

Although Oystercatchers Haematopus ostralegus wintering in the Exe Estuary performed the piping display in any type of aggressive encounter, it was predominantly observed in two contexts. ‘Solitary piping’ occurred when one individual piped without an obvious opponent being in the immediate vicinity. In ‘piping ceremonies’, two or more individuals piped at each other for prolonged periods at close distance. Prolonged contests over mussels were usually won by the individual that had initially started piping. The most aggressive and successful birds (the dominants), engaged most often in solitary piping and piping ceremonies, using up to 10% of their time this way. This suggests that the behaviour was associated with dominance rank. The hypothesis is that the two birds are likely to start piping if their relative status has not already been established, so that a quick end to a dispute over a mussel or feeding site is not achieved. A dominant individual performs solitary piping from a distance to suppress the aggressive activities of other birds within its feeding range. If this signal fails, the dominant may join the ceremony, explaining why three was the most frequent number of birds involved in piping ceremonies. The ‘butterfly’ flight, used mainly by apparently dominant birds as they arrived on the feeding grounds, may signal their high status to birds already there. The outcome of several piping ceremonies involving birds of known dominance was consistent with this explanation. The location of piping ceremonies involving only two birds (interpreted as piping ceremonies not joined by dominants) at the extremities of the feeding ranges of the most dominant individuals is also consistent with this hypothesis, because this is where their dominance would be expected to decline and would therefore be more likely to be contested. This explanation of piping and butterfly displays assumes that individuals can recognize other individuals or quickly assess their dominance. Since dominant individuals were attacked less often than subdominants and attacking birds were rebuffed less than would be expected if recognition did not occur, we suggest that some form of dominance or individual recognition did exist. The width of the white collar did not correlate with dominance, but the individual variations in this feature may enable birds to distinguish each other and thus learn their relative status. It is intriguing that the diplomatist posture, which is typically used to rebuff misguided attackers, seems almost designed to give the opponent a particularly clear view of the white collar.     

Effect of the Presence of Subordinates on Dominant Female Behaviour and Fitness in Hierarchies of the Dwarf Angelfish Centropyge bicolor

In many hierarchical animal societies, dominant individuals control group membership owing to their power to evict subordinates. In such groups, the presence of subordinates, and therefore group stability, is continually dependent on subordinates being tolerated by dominants. The dominant decision to tolerate or evict is, in turn, dependent on the costs and benefits to dominants of subordinate presence. We investigated the effect of subordinate presence on dominants in the female dominance hierarchy of the dwarf angelfish Centropyge bicolor, using both observations of natural groups and experimental removals of subordinates. We found that the presence of subordinates had no effect on dominant access to resources, as measured by dominant foraging rates and home range areas, nor on dominant fitness, as measured by growth rates and spawning frequencies. Our results suggest that the presence of subordinates has a neutral effect on the current fitness of dominants, so that dominants have no great incentive to evict subordinates. We discuss the possibility that tolerance of subordinates might be further explained by considering future fitness, as dominant females in these haremic protogynous angelfish stand to inherit the male position, whereupon subordinate females change from potential competition to useful mates.     

Age bias in the bag of pink-footed geese <Emphasis Type="Italic">Anser brachyrhynchus</Emphasis>: influence of flocking behaviour on vulnerability

In pink-footed goose (Anser brachyrhynchus) wintering in Denmark, The Netherlands and Belgium, the proportion of juveniles in the hunting bag is consistently higher than that observed in the autumn population. Such juvenile bias in the bag is usually ascribed to young geese lacking experience with hunting or disruption of juveniles from families. An alternative explanation may be that flocking behaviour of families make juveniles more vulnerable. Observations of morning flights of pink-footed geese to the feeding grounds from two of the major autumn-staging areas showed that geese were distributed in many small flocks (median flock size = 9). This was not significantly different from the flock size distribution shot at by hunters (median = 8), suggesting that hunters targeted goose flock size in proportion to the general probability of encounter. The rate at which hunters downed geese was independent of flock size. The ratio between juveniles and adults in flocks decreased with flock size and flocks of <60 individuals primarily comprised family groups. The likelihood of being shot at was 2.4 times higher for juveniles and 3.4 times higher for older birds in small flocks (<10 individuals) compared to larger flocks. The observations suggest that both juveniles as well as successful adult breeding birds were more vulnerable than non-breeding/failed breeding birds as a result of flocking behaviour.     

Effect of Group Size on Feeding Rate when Patches are Exhaustible

One benefit of group foraging is that individual foragers can join the food discoveries of companions and thus increase encounter rate with food patches. When food patches are exhaustible, however, individual shares of each patch will decrease with group size negating the effect of increased encounter rate. Mean feeding rate may actually decrease with group size as a result of aggression or time wasted joining already depleted patches, or when searching to join the food discoveries of others, which is referred to as scrounging, precludes finding food. I examined the relationship between mean feeding rate and group size in captive flocks of zebra finches (Taenopygia guttata) foraging for small clumps of seeds. Finches in groups of two or four fared better than solitary birds in terms of mean feeding rate despite the fact that birds in groups scrounged a large proportion of their food. Solitary birds initiated feeding activity after a longer delay, which led to their lower success. Early departures by food finders from food patches joined by others may have lessened the impact of scrounging on mean feeding rate. As a result of benefits from the presence of companions, group foraging in zebra finches appears a viable alternative to foraging alone despite the cost of sharing resources.     

Social influences on food caching in willow tits: a field experiment

We studied the food hoarding behavior of willow tits (Parusmontanus), a scatter-hoarding passerine wintering in dominance-structuredflocks. We examined social influences on microhabitat selectionand spatial cache distribution at temporary feeders. Dominantadult males stored food closer to die feeder and at a greaterrate than did subordinates. When alone, the birds stored foodcloser to the feeder than when accompanied by conspecifics.Conifers were preferred over deciduous trees as cache trees.The subordinates cached more in die outer parts of branchesthan dominants. There were no significant differences in dierelative or absolute heights of die caches, nor in the verticalor horizontal hoarding niche breadths between dominants andsubordinates. We experimentally removed die dominants from dieflock for 90 min and recorded the behavior of die remainingsubordinates immediately after die removal. The removal resultedin a decrease in die hoarding distance of die remaining birds,indicating that die presence of dominants directly affecteddie behavior of subordinates and suggesting that kleptopar-asitismby dominants may be prevented by rarhing farther away. Withdie dominants removed, die subordinates cached at a greaterrate than before die removal. The decrease in die hoarding distanceand increase in die hoarding rate were die only significanteffects of die experiment, perhaps suggesting that, during ashort absence of dominants, die subordinates do not benefitfrom changing dieir caching microhabitat They might be excludedfrom those new, possibly safer, microhabitats after die dominantbird rejoins die flock.     

Daily body mass regulation in dominance-structured coal tit (Parus ater) flocks in response to variable food access: a laboratory study

In a dominance-structured flock, social status may determinepriority of access to food. Birds of low social status mayperceive present and future access to food as less predictable,and so have a higher risk of starvation, than birds of highsocial rank. Theoretical models predict that subordinate birdsshould carry larger fat reserves and incur higher mass-dependentcosts than dominants. However, empirical tests of the assumptionsof these models are still scarce and controversial. We investigatedthe effect of dominance rank on daily mass gain under conditionsof fluctuating food availability in a laboratory experimentusing four flocks of four coal tits (Parus ater) each. Thesame amount of food was delivered in two treatments, but inone treatment the food was offered at a constant rate betweendays (fixed treatment), while in the other treatment the dailyfood supply varied in an unpredictable sequence between days(variable treatment). All birds showed greater variance inbody mass in the variable treatment than in the fixed treatment.Body mass within birds showed the same variability at dawn thanat dusk in the fixed treatment, but less variability at dawnthan at dusk in the variable treatment. This may be a mechanismto reduce the immediate risk of starvation at the beginningof the day, when fat reserves are at their lowest and the aggressionbetween flock members when feeding highest. Subordinate birdswere excluded from the feeders by dominants more often in theearly morning than in the rest of the day, and they showedmore variability in daily mass gain and body mass at dawn thandominant birds. These results support the hypothesis that subordinatebirds have a reduced probability of surviving when food availabilitychanges unexpectedly compared to dominants.     

The effect of dominance rank on fat deposition and food hoarding in the Willow Tit Parus montanus– an experimental test

We studied the effects of dominance rank on fat deposition and hoarding behaviour in Willow Tits Parus montanus . Dominant individuals can displace subordinates which gives them priority to new food sources; they can also pilfer stored food from subordinates. This gives subordinates less certain access than dominants both to their own caches and to new food sources. Theory predicts that subordinates should invest more than dominants both in body fat reserves and stored food. Empirical evidence is equivocal; some studies have shown that subordinates built up larger reserves than dominants, whereas others show the opposite. In an earlier indoor experiment, Pravosudov and Lucas found no effect of rank on either hoarding rate or fat reserves, but the experimental design was such that the results were ambiguous. This paper reports on a similar, but improved, experiment in outdoor aviaries. However, our results agree with the earlier experiment, since we found no effect of rank on either food storing or fat deposition. The reasons for this are explored.     

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.     

Group foraging in wild brown hares: effects of resource distribution and social status

Wild brown hares (Lepus europaeus), though normally comparatively solitary, have the capacity to adjust their behaviour such that they can benefit when foraging in groups. They are able to allocate more time to feeding and have an increased corporate vigilance as group size increases. However, these benefits are conditional upon the food distribution. When food is spaced, all individuals benefit. When it is clumped into a small defendable patch, dominant hares attempt to monopolize the resource. They can successfully exclude subordinates when group size is small but, as group size increases, they must devote more time to defending the patch. Therefore, dominants spend less time feeding with increasing group size, while subordinates spend more, since they have more opportunity to feed while the dominant hare is off chasing other individuals. As a consequence, when more than two hares are present, all individuals do less well when food is clumped than when it is spaced.