Agonistic Interactions in Siskin Flocks: Why Are Dominants Sometimes Subordinate ?

Behavioral Ecology Behav Ecol Sociobiol (1989) 25:141-145 and Sociobiology Springer-Verlag 1989

Agonistic interactions in siskin flocks: Why are dominants sometimes subordinate ?

J.C. Senar 1, M. Camerino 1, and N.B. Metealfe 2 1 Museu de Zoologia, Ap. 593, 08003 Barcelona, Spain 2 Department of Zoology, University of Glasgow, Glasgow G12 8QQ, UK

Received July 15, 1988 / Accepted April 18, 1989

Summary. Dominance interactions among captive that individual (Wilson 1975). Early descriptions siskins were examined to see if the behavior of of dominance relationships stressed consistency dominants reduced the risk of subordinates leaving and predictability of encounters between group the flock. The outcome of aggressive encounters companions and related it to fighting ability (see was related to the possession status of the two Schein 1975). However, there is increasing evidence birds (i.e., which bird was first to arrive at the that many other factors can be important in deter- contested resource) and the type of aggression used mining the outcome of a fight; the outcome cannot (i.e., display or attack). More dominant birds were always be predicted accurately on the basis of the successful whether they were possessors or in- relative status of the opponents (Huntingford and truders, and whether they attacked or displayed. Turner 1987). When possessors, they tended to display, presum- Social species that live in groups with stable ably because of the greater cost of attack and the membership are highly relevant to this topic since lack of substantial benefits associated with it. they are potentially faced with the greatest conflict When intruding, they tended to attack, possibly between aggression and gregariousness (Balph because attack is slightly more successful than dis- 1977). Living in permanent groups increases the play. When initiating encounters against domi- risk of localized competition for resources and, nants, subordinates were more successful if they hence, increases agonistic encounters. However, were possessors than if they were intruders. Subor- despotism that leads to the driving off of subordi- dinates tended to use displays whether they were nates may not be advantageous to dominants (Ek- possessors or intruders, even though when the man 1987) since it can lead to group disruption, birds were intruding, displays were less successful the loss of the benefits of grouping, and as Hogstad than attacks. Subordinates may use display when (1987) has pointed out, to the breakdown of the intruding because attack holds a higher risk of re- entire social system. Therefore, the possibility ar- taliation. The fact that siskins can repel more dom- ises for the evolution of mechanisms that reduce inant intruders merely by using displays suggests despotism and increase tolerance in dominants (see that dominants, by respecting possession and al- Rohwer and Ewald 1981; Hogstad 1987; Ekman lowing reversals, are able to reduce the likelihood 1987). that subordinates will leave the flock. This may Cardueline finches are very good subjects for be to the dominants' long-term advantage, since the study of these topics. They flock all year ar- they gain benefits from being in stable flocks. ound and show a high level of integration among the members of their stable groups (Glfick 1980; Senar 1984; Senar and Metcalfe 1988). Dominance relationships in these birds have been widely de- Introduction scribed, and these relationships show frequent "reversals" in which dominants are defeated by lower- An animal that consistently wins agonistic en- ranking birds (i.e., Tordoff 1954; Coutlee 1967; counters with another is said to be dominant over Thompson 1960; Senar 1987). The presence of such reversals suggests that there may be contexts Offprint requests to: J.C. Senar in which dominant finches allow subordinates to 142 win encounters, possibly to reduce the risk of sub- truder, c) type of aggressive behavior used during the agonistic ordinates leaving the flock. The aim of this paper interaction (i.e., Display or Attack; see Senar 1982), and d) is to examine the factors that determine the likeli- winner of the encounter. A bird was considered to be dominant over another if it hood of subordinate siskins' (Carduelis spinus) won significantly more than 50% of the encounters between winning encounters with dominants. We focus on them (Senar 1985a), as indicated by the binomial test (Siegel the relationship between social rank of the oppo- 1956). Since social hierarchies in siskins are nor linear (Senar nents and outcome of aggressive encounters, tak- 1985a), we use the terms "dominant" and "subordinate" to refer only to a bird's relationship with the other bird in the ing into account the form of aggression (whether encounter (the" relative value of dominance," Huntingford and threat or attack) and the possession asymmetry Turner 1987). Dominance relationships were statistically signifi- of contestants (whether possessors or intruders). cant between all but two pairs of birds; agonistic interactions within these pairs were excluded from the analyses. The two groups had three birds in common; therefore, the duplicated Material and methods dyads were combined to produce one data set per pair of birds. This produced a total of 15 separate dominant-subordinate The study was carried out with two captive groups of siskins. dyads, which formed the basis of the analyses. Each group consisted of five adult birds, color-ringed for indi- The influence of possession, type of aggression used, and vidual identification. Each group was housed in an outdoor dominance status on the success rate of agonistic encounters cage (100 x 50 x 60 cm) with three perches, two food containers, were analyzed using a trifactorial ANOVA. Success rates were and one water source, and was observed through a one way calculated as arcsine transformations of the percentage of ini- glass panel. Only one bird at a time was able to feed or drink. tiated encounters won by each individual in each dyad; these The feeders were replenished once a day, and observations were calculated only when there were more than 10 interactions on interactions were made immediately afterwards. The experi- in each situation. ment was conducted during June and July 1986 for one group The relative use of displays and attacks was assessed in and during October and November 1986 for the other; the a similar way, by calculating the (arcsine transformed) percent- mean duration of each daily recording session was half an hour. age of interactions that were displays for each bird in each A total of 46 (19+27) h of observations were made. In all dyad, when both possessor and intruder. interactions, the first bird to display or attack was considered the Actor, and the bird receiving the display the Reactor (Senar 1985a). The first bird to arrive at a resource (food container, Results perch, or water) was considered the Possessor, and birds that came later and challenged the Possessor were called Intruders. Dominants won 88% (SD=9.27, n= 15 dyads) of An individual was considered to have won an encounter if its their agonistic encounters and subordinates 12% opponent demonstrated a submissive posture or withdrew = 9.20, n = 15). Dominants initiating the inter- (Senar 1982, 1985a). Winners of encounters gained or retained (SD access to the feeder, water, or perch. Since all birds were highly action (Actors) won on 98% of the occasions, and familiar with the resources and would be equally hungry, we subordinate actors on 61% (Table 1); this differ- assumed that there would be no payoff assymmetries (Maynard ence was highly significant (t=8.74, dr=24, P< Smith and Parquer 1976) for the contestants. 0.001). A total of 3149 agonistic interactions were studied, and for each one the following data were recorded: a) idemity of While siskins won the majority of interactions that the Actor and Reactor, b) identity of the Possessor and In- they initiated, they were more likely to win if they

Table 1. Trifactorial table showing the success rate of dominant or subordinant siskins when initiating encounters, according to their Possession status and the Type of Aggression used. The percentage of acts that were displays (as opposed to attacks) is shown in the right hand column, in relation to dominance and possession status. The standard deviation and sample size (no. of dyads) is given in parentheses. Significance levels of student's t-tests; * P < 0.05; * * P < 0.01 ;* * * P < 0.001 ; ns: not significant

Dominance Possession % % won according to Type of Aggression % Status Status won Displays Attack Display

Dominant Possessor 99% 100% ns 99% 90% 98% (1,4 15) (1,9) (1,5 15) (8,6 15)

(1,7 15) Intruder 96% 99% ** 95% 31% (4,9 15) (2,5 15) (2,1 11) (15,8 15)

Subordinate Possessor 89% 87% ns 91% 95% 61% (0,2 9) (23,1 3) (11,0 6) (7,2 6)

(11,7 15) Intruder 51% 75% ** 33 ~ 59% (16,2 9) (12,5 5) (16,9 6) (16,3 9) 143

Table 2. Three-way ANOVA of the effect of dominance, posses- Discussion sion status, and type of aggression used on success rate (arcsine- transformed percentage of encounters won) in agonistic en- Success rate oJ" dominants and subordinates." Does counter it vary with possession ? Source of variation F df P Dominants can displace another individual with- Dominance 92.39 1,62 < 0.001 out difficulty due to their bigger Resource Holding Possession 42.13 1,62 < 0.001 Power (RHP; Parker 1974). Although their success Type Aggression 17.02 1,62 <0.001 rate is significantly higher when possessors than Dom. x Possession 20.00 1,62 <0.001 Dora. x Type Aggression 1.72 1,62 0.195 when intruders, they are very successful in either Possession x Type Agg. 7.53 1,62 0.008 situation (99% success vs 96%, respectively). For Dora. x Poss. x Type Agg. 1.78 1,62 0.187 subordinates the situation is quite different: they have a much greater success rate when initiating encounters against dominants if they are posses- Table 3. Two-way ANOVA on the effect of dominance and sors than if they are intruders (success rate of pos- possession on the (arcsine-transformed) percentage of acts that were displays (rather than attacks) sessors = 89% ; of intruders = 51%). This could be due to the Prior Ownership effect, whereby the Source of variation F df P first of two individuals arriving at a resource is predicted to have an increased probability of suc- Dominance 12.55 1,43 0.001 cess in encounters, irrespective of the individual's Possession 155.14 1,43 <0.001 Dominance x Possession 1.43 1,43 0.239 relative Resource Holding Power (RHP) or social rank (Maynard Smith 1982). Prior ownership has been shown to be a significant factor in territorial were possessors than if they were intruders, and defense (Brown 1963; Davies 1978; Krebs 1982; if they used attacks rather than displays (Tables 1 Nelson 1984) and in the integration of bird flocks and 2). This effect of possession is even more im- where resident individuals may be dominant over portant for subordinates than for dominants (Ta- immigrants (Balph 1979; Yasukawa and Bick bles 1 and 2), subordinates experiencing a greatly 1983). It is clear that possession is far more impor- increased success rate when possessors. In spite of tant for subordinate siskins than for dominants; this higher success rate, subordinates were less lik- dominants may alter their behavior according to ely to initiate encounters when they were posses- whether they are possessors or intruders (see be- sors than when dominants were possessors (per- low), but they tend to win the encounters that they centage of encounters initiated when possessors, initiate regardless. subordinates: 43 %, sd = 16.62, n = 11 ; dominants : Since subordinates are far more successful 58%, sd=18.9, n=15; t student=2.129, df=24, when they are possessors, one might predict that P < 0.05). The significant interaction between pos- they would mainly initiate encounters when in that session and type of aggression indicates that the situation. However, this prediction is not sup- behavior that leads to success depends on posses- ported by the data, which suggests that other fac- sion: to be successful, possessors should use dis- tors are also important (see below). plays whereas intruders should attack (Tables 1 and 2), this relationship being true for dominants To attack or to display? as well as subordinates (Table 2). Analysis of the type of aggression used (i.e., the percentage of en- In general, dominants and subordinates mainly use counters that were displays) shows that this predic- displays. If physical attacks are unnecessary, dis- tion holds (Tables i and 3). plays are to be expected since they have a lower Dominance also had a significant effect on the type energetic cost and carry less risk of injury than of aggression used, subordinates using more dis- attacks (Maynard-Smith 1982; Popp 1987 a). plays than dominants (Tables 1 and 3). Possessors The success rate of dominants is quite high in that initiated interactions exhibited similar types all situations, whether they attack or display, and of aggression, regardless of dominance status (t= whether they are the possessor or the intruder. 1.15, df= 19, ns). However, dominance status had However, when intruding, they are slightly (but a significant effect on the behavior of Actors when significantly) more successful when performing at- they were intruders; dominants often attacked, tacks than when displaying. This may explain why whereas subordinates mainly used displays (Ta- they are more likely to attack than display in this ble 1; t=4.18, df=22, P<0.001). context, the greater probability of success possibly 144 outweighing the increased cost of attack. In con- that they thereby retain "useful" subordinates trast, when dominants were possessors, they won within the flock (Rohwer and Ewald 1981; Senar nearly all the encounters that they initiated wheth- 1988). Nevertheless, they are capable of driving er they displayed or attacked. Given the greater off subordinates should the need arise (e.g., in cost of attacks and the lack of substantial benefits times of food shortage; Rohwer and Ewald 1981). associated with them, it is, therefore, not surprising In this way, this respectful behavior would reduce that they performed many more displays than at- the level of conflict in what would otherwise be tacks when in this situation. competitive situations. It has an adaptive value for The behavior used in initiating an aggressive subordinates as well as dominants since it main- interaction is even more important for subordi- tains group cohesion with consequent advantages nates. When the subordinate bird was the posses- such as reducing predation, finding food, etc. sor, displays were as successful as attacks. How- (Senar and Metcalfe 1988). ever, when the subordinate was the intruder, it was "Respectful" behavior results in the appear- far more effective if it attacked. These success rates ance of reversals between the members of a group did not, however, correctly predict the relative fre- in some agonistic encounters (in the sense given quencies of attacks and displays. While subordi- by Sabine 1949). The fact that reversals appear nates favored displays when possessors (as pre- mainly in species that are socially organized in dicted), they continued to use predominantly dis- flocks (B. Ens pers.comm.) supports the hypothesis plays when intruding, despite the lower success of of birds showing "respect. "Instances of such floc- this behavior. This may possibly be due to the risk king species that show reversals in a relatively high of retaliation: attacks on dominant birds have a proportion of agonistic encounters are Junco hye- higher success rate but are also associated with malis (Sabine 1949), Quelea quelea (Shawcross a significantly higher risk of a retaliatory attack, 1982), Parus atricapillus (Hartzler 1970), Carduelis which can lead to an escalated fight (Hinde 1981 ; tristis (Coutlee 1967), Carpodacus mexicanus Senar 1982). This risk is possibly taken only when (Thompson 1960), Sturnus vulgaris (Ellis 1966), Ar- the value of the resource is higher for the subordi- enaria interpres (Metcalfe 1986), and Tadorna ta- nate than for the dominant, so that the low-rank- dorna (Patterson 1977). Species whose aggrega- ing bird is willing to incur a greater cost in order tions do not have a stable membership, or which to obtain this value (Popp 1987b). are aggregated merely as a result of overlapping home ranges, do not appear to show "respectful" behavior, e.g., Gallus domesticus (Schelderup-Ebbe The concept of respect 1922; Fischel 1927), Phasianus colchicus (Collias Maynard Smith and Parker (1976) suggested that and Taber 1951), and Haematopus ostralegus (Ens the outcome of encounters should largely be de- and Goss-Custard 1984; B. Ens pets.comm.). In pendent on asymmetries between the protagonists. these species dominants are recorded as winning These asymmetries are of three types. Differences all their encounters with subordinate individuals. in RHP (i.e., fighting ability or dominance) and Respectful behavior may, therefore, have evolved in the pay-off (i.e., the value of the resource) are in social species in order to retain subordinates thought to be most important, and uncorrelated in the flock. asymmetries (such as prior ownership (Stevens Acknowledgements. We are most grateful to P. Monaghan, L. 1988)) are used only if RHP and pay-off asymme- Arias de Reina, F. Uribe, A. Omedes, T. Borras, B. Ens and tries are not easily recognizable. In this study there P.W. Ewald for critical comments on the manuscript or parts are clear RHP asymmetries, but nevertheless domi- of it. We also would like to thank J. Nadal for his help, and nants give way to subordinates in some circum- R. Nos for her continuous encouragement and advice. This research was supported by a grant of the Obra Social de la stances. Although dominants win nearly all en- Caixa de Barcelona to JCS. counters, they can lose when faced with a displaying subordinate that is a possessor. This has also been found in baboons contesting for females or References food (Kummer etal. 1973; Sigg and Falet 1985) Balph MH (1977) Winter social behaviour of dark-eyed juncos: and could be interpreted as the showing of respect Communication, social organization, and ecological impli- by dominants, which allows low-ranking group cations. Anim Behav 25 : 859-884 members to feed unaffected by, and in the vicinity Balph MH (1979) Flock stability in relation to social dominance and agonistic behaviour in wintering dark-eyed juncos. Auk of, those of higher rank (Senar 1985b; Sigg and 96:714~722 Falett 1985). The benefit to dominants of showing Brown JL (1963) Social organization and behavior of the mexi- such "respect" and reducing despotism might be can jay. Condor 65:126-153 145

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