Alliances in winter flocks of willow tits; effects of rank on survival and reproductive success in male-female associations

Summary The value of being dominant in winter flocks of willow tits may not only be a matter of personal survival for males, although their survival probability was highest among flock members. Winter flocks of willow tits contain male-female subunits, and the dominant pair may be viewed as an alliance based on mutual benefits. Dominant males provide their mates with protection in winter, and females return the help in reproductive success. Survival was directly related to rank only within sexes. Mates of dominant males survived significantly better than low-ranked males even though these females were subordinate to these males in direct interactions. Mates of dominant males were relatively spared from costs of low rank as they were subject to aggression less often than expected. Presumably dominant males prevented such attacks, as they excluded other flock members from high tree sections where their mates fed. Males should gain from improving the survival probability of their mates since it was not always possible to replace lost mates in spring populations having a male-biased sex ratio. Dominance in avian winter flocks may thus have more indirect effects than merely to improve the personal survival probability through resource priority. The benefits of being a dominant suggest that subordinate willow tits join groups because there is no space available for them to take up a territory as dominants.     

Group structure in a restricted entry system is mediated by both resident and joiner preferences

The benefits of grouping behaviour may not be equally distributed across all individuals within a group, leading to conflict over group membership among established group members, and between residents and outsiders attempting to join a group. Although the interaction between the preferences of joining individuals and existing group members may exert considerable pressure on group structure, empirical work on group living to date has focussed on free entry groups, in which all individuals are permitted entry. Using the humbug damselfish, Dascyllus aruanus, we examined a restricted entry grouping system, in which group residents control membership by aggressively rejecting potential new members. We found that the preferences shown by joining members were not always aligned with strategies that incurred the least harm from resident group members, suggesting a conflict between the preferences of residents and preferences of group joiners. Solitary fish preferred to join familiar groups and groups of size-matched residents. Residents were less aggressive towards familiar group joiners. However, resident aggression towards unfamiliar individuals depended on the size of the joining individual, the size of the resident and the composition of the group. These results demonstrate that animal group structure is mediated by both the preferences of joining individuals and the preferences of residents.     

Flock size,food dispersion,and the feeding behavior of crossbills

Summary Feeding rates of five captive red crossbills (Loxia curvirostra) were measured when they were foraging alone, and in flocks of two or four on three seed dispersion patterns. On the most strongly clumped seed dispersion, individuals had higher mean feeding rates and the smallest probability of starvation when in flocks of two than when alone or in flocks of four. Individuals in flocks of four had higher feeding rates on the weakly clumped seed dispersion than on the uniform and more clumped seed dispersions; there were no food finding benefits gained from flocking on the uniform seed dispersion and aggression increased as food became more clumped. Most recent work has assumed that flocking results in higher feeding rates only because time spent vigilant is reduced. Crossbills, however, did not visit more cones per unit time as flock size increased, as would be expected if less time was spent vigilant. Thus, any reductions in vigilance as flock size increased were countered by increases in other behaviours, such as those related to aggression. Consequently, the higher mean feeding rates of crossbills in flocks than when solitary is not attributable to reduced vigilance. The increase in mean and the decline in variance of feeding rates occurred because crossbills in flocks found good patches earlier, and possible by spending less time assessing poor patches.     

Prior residency as a key determinant of social dominance in the willow tit (Parus montanus)

Summary We studied how age, body size and prior residency affected social dominance in the willow tit (Parus montanus) groups. The contribution of each variable was experimentally tested in unisexual two-bird trials, in which the birds were matched for all variables except the one studied. Large birds were dominant over smaller ones (Fig. 1). The effect of body size was more prominent in males than in females. Age had no influence on dominance. Residents became dominant more often than newcomers (Fig. 2). Adulthood or larger body size did not override the advantage of prior residency (Fig. 2). Therefore, the proximate reason for the age-dependent dominance seen in natural willow tit flocks is most likely the prior residency advantage of the adults. Factors connected with fighting ability (body size and age) seem to be less important than the time of establishment of rank, which may reflect the importance of resource value differences between residents and newcomers in this context. The advantage of residency might make it advantageous to be a member of a flock even as a subordinate, rather than being solitary.     

Rank-dependent hoarding effort in willow tits (<Emphasis Type="Italic">Parus montanus</Emphasis>): a test of theoretical predictions

Many birds and mammals store energy as hoarded food supplies. A supply of stored food is beneficial during periods when food is scarce, but building up and managing such a supply also entails costs. The optimal number of caches will be reached when the net benefit is at its maximum. If dominants can steal more stored food from subordinates than the other way around, the optimum will differ between these categories. A previous theoretical model of hoarding in groups with dominant and subordinate members produced three testable predictions: (1) hoarders should store more food as anticipated future conditions get worse; (2) subordinate flock members should store more food than dominants; and (3) dominants should increase hoarding relatively more than subordinates as conditions get worse. Here we present a field experiment on willow tits (Parus montanus) designed to test these predictions. We found support for all three. Hoarding increased as conditions got worse, subordinates stored at a higher rate than dominants, and dominants increased their hoarding effort relatively more than subordinates as conditions worsened. These results support the incorporation of information on dominance and food availability into models predicting food storage behaviour.Communciated by J. Dickinson     

The costs and benefits of territory sharing for the Caribbean coral reef fish,Scarus iserti

Summary Along the Caribbean coast of Panama, groups of unrelated female striped parrotfish, Scarus iserti, co-defend a common feeding territory. Field manipulations of group size and composition were performed to examine the benefits and costs accrued by dominant fish within these shared territories. Dominant fish benefit from the presence of relatively large subordinates because they share in the defense of the territory. Removals of these fish caused increases in defense time and decreases in feeding time for dominant group members. Dominants benefit from the presence of small subordinates because they increase the foraging efficiency of dominants. Removals of smaller subordinates caused reductions in the feeding time of dominant fish, although no changes in defense time occurred. Concurrently, dominant fish reduce costs of resource depletion by displacing subordinate group members from good food patches. Dominance interactions within a group reduce the amount of time subordinates spend feeding (subordinate individuals fed at higher rates following the removal of a dominant) and limit a subordinate's access to high quality resources. This combination of benefits and reduced costs ultimately contributes to the economic defensibility of a striped parrotfish territory and has led to the evolution of group territorial behavior in the absence of kin selection and cooperative parental care.     

Chemical rival recognition decreases aggression levels in male Iberian wall lizards, <Emphasis Type="Italic">Podarcis hispanica</Emphasis>

In an agonistic interaction, the assessment of the probable outcome of future encounters with the same individuals may be the best way of decreasing costs of fighting, but this may only be accomplished if animals are able to recognize individual conspecifics. We staged encounters between male lizards, Podarcis hispanica, to examine whether odoriferous cues are involved in rival recognition during agonistic interactions. We experimentally manipulated the odour of intruding males, creating familiar males with their own odour or bearing odours of unfamiliar males, and unfamiliar males bearing unfamiliar odours or odours of familiar males. The results showed that when familiar males were impregnated with scents from unfamiliar males, they elicited an aggressive response by resident unmanipulated males similar to that observed for a new unfamiliar male with unfamiliar odour. This suggests that resident males were unable to recognize familiar males when their own scents were removed. In contrast, responding males were less aggressive towards familiar males impregnated with their own odour and towards unfamiliar males impregnated with scents of familiar males, suggesting that when two males have already interacted, their scents become familiar for both males, and that the detection in successive encounters of the familiar scent suffices to reduce the aggressive response of territorial males. Therefore, recognition mechanisms based on chemical cues during agonistic encounters may contribute to reducing the intensity and the costs of fighting in P. hispanica and may play an important role in the organization of their social system.     

Singing and the establishment and maintenance of dominance hierarchies in captive brown-headed cowbirds

Summary The importance of singing in the establishment and maintenance of dominance rank was investigated in captive flocks of brown-headed cowbirds (Molothrus ater). Each flock consisted of 3 males and 1 female. In one experiment the two largest males in each of 4 flocks were temporarily devocalized by puncturing the interclavicular air sac just before group formation. The third male in each group was sham-operated. There was a significant tendency for the non-devocalized birds to become the dominant member of their group. Only sham-operated males sang immediately after the operation. In a second experiment, dominant males from flocks with established hierarchies were temporarily devocalized. None of these males lost their dominant positions, despite their inability to sing. These results suggest that song is important in the establishment of dominance rank; however, once established, social hierarchies may be maintained by non-vocal means, such as social inertia or individual recognition.     

Dominance and the dynamics of phenotype-limited distribution in common cranes

We studied the behavior of 13 radiotagged cranes dispersing from a communal roost over days when they changed their main daily foraging area between consecutive days during two winter seasons. Individuals went to a new foraging zone when on the previous day their morning food intake had fallen below their mean morning food intake measured over the whole winter. Food intake on the day before a change in foraging area was positively correlated with dominance rank. Dominant cranes changed to new zones with higher numbers of birds and food density, while subordinate cranes went to new zones with lower numbers of birds. As a result, all birds increased their food intake over that of the previous day. Dominant cranes remained more faithful to their most preferred foraging zone, where they spent 69% of the mornings, while subordinate birds were more mobile, switching among zones frequently. Dominant birds left the roost later than subordinate birds on the days they changed to a new zone, which could be used to track the main departing flows. The results suggest that the dynamics that led to a truncated phenotype-limited distribution were determined by social dominance and food abundance, with dominant cranes shifting to a new zone to maintain their high intake levels and subordinates changing more frequently whenever their daily intake did not reach the minimum metabolic requirements. Received: 16 December 1996 / Accepted after revision: 22 February 1997     

Social influences on the whistled songs of starlings

Summary Playback experiments on individuals singing near their nest show that starlings tend to repeat the theme they have just heard even if it is an unfamiliar dialect and not programmed in their own current sequence. The type of response and its temporal characteristics could be related to internal sequential organization rules. Moreover, starlings discriminate between two dialectal variants of a given theme and respond more often and more quickly to the familiar dialect. A social influence on the choice of theme by the individual can also be observed in foraging flocks and roosts where song is frequently given. Observation of such groups reveal that one theme predominates on each given occasion and that it depends on the number of individuals present and on the type of group (flock or roost). This is probably a function of the tendency to match. Previous observations of flights toward the roost showed that birds originating from different dialectal areas congregate in such groups. A few analyses of songs in flocks and roosts confirm the presence of different dialects simultaneously.     

Social organisation of thornbill-dominated mixed-species flocks using social network analysis

Mixed-species associations are a widespread phenomenon, comprising interacting heterospecific individuals which gain predator, foraging or social benefits. Avian flocks have traditionally been classified as monolithic species units, with species-wide functional roles, such as nuclear, active, passive, or follower. It has also been suggested that flocks are mutualistic interactions, where niches of participating species converge. However the species-level perspective has limited previous studies, because both interactions and benefits occur at the level of the individual. Social network analysis provides a set of tools for quantitative assessment of individual participation. We used mark-resighting methods to develop networks of nodes (colour-marked individuals) and edges (their interactions within flocks). We found that variation in flock participation across individuals within species, especially in the buff-rumped thornbill, encompassed virtually the entire range of variation across all individuals in the entire set of species. For example, female, but not male, buff-rumped thornbills had high network betweenness, indicating that they interact with multiple flocks, likely as part of a female-specific dispersal strategy. Finally, we provide new evidence that mixed-species flocking is mutualistic, by quantifying an active shift in individual foraging niches towards those of their individual associates, with implications for trade-off between costs and benefits to individuals derived from participating in mixed-species flocks. This study is, to our knowledge, the first instance of a heterospecific social network built on pairwise interactions.     

Helping at the nest,allofeeding and social status in immature arabian babblers

Summary Arabian babblers, Turdoides sqamiceps, are cooperatively breeding, group territorial birds, occurring in desert habitats. Non-breeders participate in several types of cooperative behaviour, including care of eggs, nestlings and fledglings, and bringing food to one another (allofeeding). This paper reports on observations of helping with eggs and young, and peer allofeeding, performed by immature babblers, fledged the previous season or earlier in the same season. The birds were from a babbler population studied since 1971 in the Arava valley of eastern Israel. They belonged to groups in which all individuals had been colourringed and were accustomed to human observers. Individuals and nests in the territories of these groups could be watched from distances of 1–2 m without causing alarm, so fine details of behaviour could be observed in the field. In most allofeeding interactions observed, a more dominant individual brought food to a subordinate. Sometimes the subordinate bird avoided accepting the proferred food, and was then hit or chased by the bird that had attempted to feed it. Occasionally a subordinate bird attempted to feed a more dominant individual. Dominant birds always refused to take food offered by a subordinate, and hit or chased any subordinate attempting to feed them. Frequencies of visits to nests were correlated with helper rank for those visits where the incoming bird displaced a previous visiter from the nest. Frequences of visits to unattended nests, however, were not correlated with helper rank. Similarly, frequencies of feeding visits to fledglings were correlated with helper rank until incubation of the next brood began. After this, the correlation disappeared. Numerous instances of aborted nest visits were observed in which a helper, often carrying food, arrived at the nest-tree but left again without visiting the nest. In many other cases, a helper arrived at the nest-tree but delayed visiting the nest for several minutes. Aborted and delayed visits usually occurred when a more dominant bird was in the vicinity of the nest. This suggests interference between helpers. Direct interference between helpers visiting fledglings was occasionally observed. In such cases, a more dominant helper snatched food from a subordinate approaching fledglings, and then fed this food to the fledglings itself. Interference between helpers, and conflict between proferers and recipients of food during allofeeding, are not easily explained by kin selection or reciprocity. On the other hand, such behaviour is readily explained by a hypothesis which suggests that individuals may increase their social status in the group by performing cooperative behaviour. Babblers that establish status by demonstrating ability to bear the short-term costs of cooperative behaviour, rather than through direct aggression towards rivals, are likely to forge collaborative relationships with other group members. Since babblers must collaborate to establish and defend a territory, such relationships are essential to reproductive success.     

On the possible contribution of mixed species flocks to species richness in neotropical avifaunas

Summary Mixed species foraging flocks are a dominant component of the infra-structure of avian communities in neotropical forests. In Amazonia, these flocks consist of pairs of 10–20 species, many of which are permanently associated with mixed flocks. At least half of these flocking species maintain territories that correspond exactly to the flock home range. Small individuals that participate as permanent members of the flocks must adopt the large home range of the larger nucleus species. Therefore, the densities of smaller species are dependent on the availability and density of flocks rather than the availability of food resources. Single pairs of 4 small flocking species with individual body masses of 8 g occupied exclusive territories of 8–12 ha. These were the same exact territories that were defended by at least 6 other flocking species with individual body masses of up to 37 g. Because of their attachment to flocks with large territories, small species are expected to under-utilize available food resources. The under-utilization of food resources is expected to allow smaller species to coexist with greater niche overlap resulting in increased species richness. This hypothesis was tested by quantifying foraging niche in terms of foraging height, foraging maneuver, and prey substrate; and using these values in addition to body mass and bill size (length, depth and width) to determine relative niche overlap between large versus small species pairs.Smaller species had greater foraging overlap than large flocking species and particularly the three smallest species of the genus Myrmotherula; longipennis, axillaris and menetriesii had very high overlap (average foraging niche overlap for the 3 species=0.83±0.12 compared with 0.12±0.19 for all flocking species), similar body sizes (body masses differing by no more then 8%) and similar bill morphologies (maximum ratio in length=1.08, width=1.07, and depth=1.06). These results are consistent with the hypothesis that small species participating in Amazonian mixed flocks can coexist with greater niche overlap because their density is flock dependent rather than resource dependent.     

Behavioral diversification in a young species flock of pupfish (<Emphasis Type="Italic">Cyprionodon</Emphasis> spp.): shoaling and aggressive behavior

Increasing empirical and theoretical evidence supports the idea that sympatric speciation is operating, for example, in species flocks comprising several closely related fish species within one lake. Divergent natural selection (promoting spatial and food niche partitioning) and sexual selection (assortative mating) have been identified as key selection factors in intralacustric adaptive radiations. However, the evolution of social behaviors accompanying such adaptive radiations is less well understood. Using a phylogenetically young species flock of pupfish (Cyprinodon spp.) as a model, we examined differences among six sympatric species and compared their shoaling, aggressive, and territorial behaviors with that of a sister species (C. artifrons). Despite an estimated age of the species flock of less than 8,000 years, pronounced behavioral differentiation was found. C. simus, the smallest species in the flock, shoaled more than the other species and was less aggressive and less territorial than C. beltrani. F₁-hybrids between C. simus males and C. beltrani females showed an intermediate expression of shoaling and aggressive behavior. Niche partitioning among the members of this species flock appears to be accompanied by rapid divergent evolution of social behaviors. We discuss the potential role of phenotypic plasticity and within-species variation of social behaviors for such rapid behavioral diversifications in sympatric speciation processes.     

Kin recognition by juvenile predatory mites: prior association or phenotype matching?

Kin recognition, the biased treatment of conspecific individuals based on genetic relatedness, is a widespread phenomenon in animals. The most common mechanisms mediating kin recognition are prior association (familiarity) and phenotype matching. Recognition based on prior association allows identifying familiar individuals. Recognition based on phenotype matching is an extension of prior-association-based recognition and allows identifying familiar and unfamiliar individuals due to a shared phenotypic trait. I investigated which of the two mechanisms is used by cannibalistic juvenile predatory mites, Phytoseiulus persimilis. Protonymphs that were associated with either siblings or non-kin discriminated familiar and unfamiliar larvae and preferentially cannibalized the latter irrespective of genetic relatedness. In contrast, despite previous association with either siblings or non-kin, protonymphs did not discriminate unfamiliar sibling and unfamiliar non-kin larvae. Association in the larval stage therefore mediated kin recognition based on familiarity, but not phenotype matching in cannibalistic P. persimilis protonymphs. Furthermore, in the presence of a familiar prey individual, sibling cannibalism occurred significantly sooner than non-kin cannibalism. This quick sibling cannibalism may have been the consequence of preferential association of siblings and/or may indicate the occurrence of an alternative cannibal phenotype. I discuss the adaptive significance of prior-association-based recognition for P. persimilis juveniles and emphasize the ability of P. persimilis to use multiple recognition mechanisms in dependence of the ontogeny and the ecological context.     

A field test of the effects of familiarity and relatedness on social associations and reproduction in prairie voles

Inbreeding depression is a well-documented phenomenon. In animals, one means of avoiding the costs of inbreeding is through the recognition and avoidance of kin as mates. Prairie voles (Microtus ochrogaster) are short-lived, socially monogamous rodents that demonstrate inbreeding depression in the laboratory. Field data indicate that pair formation in nature is opportunistic but pairing among close relatives seems uncommon. We examined the role of relatedness and familiarity on prairie vole social associations and reproduction by placing adult voles into 0.1-ha enclosures with familiar siblings, unfamiliar siblings, and unrelated, unfamiliar conspecifics. Live-trapping data indicated that indices of social pair bonding were random with respect to relatedness and familiarity. Among females whose litters were sired by a single male, litters were significantly more likely to be sired by unfamiliar than familiar males, but the number of litters sired by males that were unrelated to their partner was not different from the number of litters sired by males that were related to their partner. Additionally, females that produced offspring with familiar siblings were significantly more likely to have litters with multiple paternity than females not producing offspring with familiar siblings. However, multiple paternity was not influenced by relatedness of sires. Finally, older individuals were more likely to produce offspring with each other than with younger individuals. Our findings suggest that prior association is a more important mechanism of inbreeding avoidance than phenotype matching for prairie voles mating under ecologically relevant conditions.     

Use of different signaling modalities to communicate status by dominant and subordinate Heermann’s kangaroo rats (<Emphasis Type="Italic">Dipodomys heermanni</Emphasis>)

Dominance relationships in solitary species may be an important factor in the maintenance of long-term, stable relationships among territorial neighbors. We examined the mediation of intraspecific interactions in a solitary, territorial kangaroo rat, Dipodomys heermanni, and tested whether unfamiliar kangaroo rats establish a dominance hierarchy and then decrease aggression and increase communication (via footdrumming and sandbathing) after initial interactions and the establishment of a social structure. Results revealed that both dominance hierarchies and familiarization with particular individuals are likely to mediate social interactions. After only one pairing per dyad, an almost linear dominance hierarchy emerged, which became perfectly linear after a 90-min familiarization period. During the course of subsequent interactions between dyad partners, fighting decreased and non-agonistic communication increased. Dominant kangaroo rats sandbathed at higher rates than subordinates, possibly to deposit scent to advertise competitive ability, whereas subordinate kangaroo rats footdrummed from inside the burrow, which seemed to indicate an unwillingness to interact. We suggest the kangaroo rats use a conditional strategy when deciding to fight (be dominant) or withdraw (be subordinate) by employing different modes to communicate status and minimize the potential risk of injury during unnecessarily prolonged fights.     

The role of helpers in feeding chicks in cooperatively breeding green (red-billed) woodhoopoes

Summary Observations were made of ten green (red-billed) woodhoopoe Phoeniculus purpureus flocks during the breeding season in order to quantify the relationship between flock size and the amount of food delivered to chicks. The study period was kept short specifically to minimize the effects of environmental stochasticity. Neither woodhoopoe feeding visit rates nor the total amount of food brought to chicks increased with flock size. Although nonbreeders did not increase the net rate of food provisioning to chicks, they reduced parental input in chick rearing, and hence energy expenditure by the breeding pair. However, over an 8-year study period, which includes data for 144 flock years, this did not result in increased breeding frequency or enhanced survival of breeders. There is thus no evidence that helpers' feeding contributions to young per se influence the indirect fitness of helpers.     

The scent of dominance: female field crickets use odour to predict the outcome of male competition

Pheromones may convey information about mate quality and social status. In the field cricket Gryllus integer, females mount the males for copulation, such that males cannot coerce females to mate. We examined whether virgin G. integer females preferred the scent of potentially dominant males to that of subordinate males. First, we collected pheromones by confining males on filter paper. Next, we offered filter paper from each of two size-matched males and control paper to females that had never been exposed to males, and measured the time spent by the female on each kind of paper. Finally, dominance status of the males in each size-matched pair was determined by pitting the two males against one another in agonistic contests. When offered filter paper from subsequently dominant versus subsequently subordinate males, females spent more time on the paper from the dominant male than the subordinate male, and much less time on control paper. Thus, pheromones may inform female G. integer about a male's potential to achieve dominant social status. Male pheromones were also associated with the female's tendency to mount a male. In contrast to cockroaches, where females prefer the scent of subordinate males (presumably to avoid risk of injury), female crickets prefer the scent of potentially dominant males and are more likely than males to wound their mating partners.     

Evidence of kin-selected tolerance by nestlings in a siblicidal bird

Behaviorally dominant members of blue-footed booby (Sula nebouxii) broods can effect siblicide by restricting access of subordinate siblings to parents providing food. In spite of their capacity for siblicide, dominant chicks permit subordinates to feed during short-term food shortage; in fact, the proportion of the food that the dominant takes is independent of the total amount delivered in older chicks. A model of optimal food distribution suggests that dominant chicks maximize their inclusive fitness with this pattern, rather than by satisfying their own food requirements and leaving what remains for the subordinate sibling. The indirect reproductive potential represented by a chick's sibling appears to have influenced the evolution of siblicidal brood reduction in this species.