Social dominance in communal Mexican jays Aphelocoma ultramarina

Anim . Behar ., 1986 . 34, 175-187

Social dominance in communal Mexican jays

Aphelocoma ultramarina

CHRISTOPHER P . L . BARKAN*, JOHN L . CRAIGt, STUART D . STRAHL*, ANNE M . STEWARTt & JERRAM L . BROWN* *Department of Biological Sciences, State University of New York at Albany, 1400 Washington Avenue, Albany, New York 12222, U .S .A . tZoologv Department, University of Auckland, Private Bag, Auckland, News Zealand

Abstract . Social dominance among members of seven social units of communal Mexican jays (Aphelocoma ultramarina) was studied at feeding stations during the non-breeding season . Hierarchies were generally linear . Males were usually dominant to females and there was a tendency for the oldest male in each unit to be the most dominant adult . An unusual pattern of high social status of yearlings was observed . We attribute the high social status of yearlings and adult males to different causes . Adult male dominance is probably related to competition for breeding opportunities within each social unit . We tentatively interpret the dominance of the yearlings in terms of payoff asymmetries and parental facilitation .

The complexity of communal social structures has fascinated and challenged evolutionary biologists since Darwin's era . The presence of competitive and aid-giving behaviour between the same individuals within the same social units is particularly interesting. Although competition among potential breeders within the social units of communal birds has been known since the earliest studies of colourbanded individuals (Brown 1963a ; Rowley 1965) . the importance of rivalry has recently been reemphasized for jays (Woolfenden & Fitzpatrick 1977 ; Trail et al . 1981 ; Craig et al . 1982), gallinules (Craig 1979), babblers (King 1980), anis (Vehrencamp 1978) and other birds (Brown 1978) . Consequently, we are acquiring a greater appreciation of the role of dominance in a variety of cooperative social structures . The Mexican jay (Aphelocoma ultramarina) differs from other communally breeding species in which dominance has been studied, by the combined presence of plural breeding, separate nesting and long deferred breeding (Brown, in press) . These factors result in the presence of several mature adults along with their non-breeding or breeding offspring in the same social unit (Brown & Brown 1981, 1984) . To investigate competitive interactions in this species we undertook a study of social dominance at winter feeding sites in seven social units . In this paper we describe the dominance relationships among males and females and also the relationships between age and social status, in particular, the status of yearlings .

METHODS

Observations Observations were made on a population of Mexican jays that has been studied since 1969 . The study area is located at and around the Southwestern Research Station (SWRS) in the Chiricahua mountains of southeastern Arizona at 5400 ft (1646 m) elevation . Data on social dominance in seven social units were collected during January-March 1980 and 1981 . All individuals were colour-banded to enable individual recognition (69 in 1980 and 66 in 1981) . Most were banded within the first 2 years of their life, enabling accurate estimation of age thereafter . A few had been banded as adults and hence were of uncertain age : eight in 1980 . and nine in 1981 . In each territory jays were fed from a partiallyburied can . 10 cm in diameter and I I cm deep, the rim of which extended about 2 cm above ground level . A half-inch-mesh wire screen was sometimes installed in the can to limit access to the food, thereby increasing the likelihood of interactions . A crude three-sided ring of dead wood or stones, approximately 60 cm in diameter, was placed around each can . This arrangement tended to standardize the physical environment of individuals waiting to feed at the can . The rocks and larger pieces of wood provided anvils against which the jays could brace seeds while pecking at them . The feeders were centrally located within each 175

1 76

Animal Behai'iour, 34 . /

social unit's territory and baited with a mixture of bread and sunflower seed . The data were obtained using ad libitum sampling (Altmann 1974) . We attempted to identify the winner and loser of each interaction and to categorize the type of behaviour involved . All interactions were observed from blinds and recorded verbally on a tape recorder . The data were later transcribed and entered into a cumulative wins-losses matrix for each social unit . Feeders were empty except during observation periods . Although only a few observation sessions were made for a given unit each week, members of all units learned to come to the feeder quickly in response to a whistle signalling the presence of food .

Age and Sex Classification We divided individuals into one of three general age categories . (1) Yearlings are individuals who were approximately 1 year old (8-11 months) in JanuaryMarch, when our observations were made . (2) Two-year-olds are individuals who were approximately 2 years old (20-23 months) in January-March . (3) Adults are individuals who were approximately 3 years of age or older (32 months or more) in January-March . The sex of individual jays was determined using sex-specific behavioural characteristics observed during the breeding season . Sex of some of the yearlings is not known because Mexican jays rarely attempt to breed before the age of 3 years and some of the yearlings described in this paper disappeared from the study area before reaching that age .

Statistical Analysis of Social Status for Age and Sex Classes The basic unit of our analysis is a dyad . A dyad is the pair of cells in a dominance matrix that indicates the number of interactions won by each of two birds in encounters with each other . Each dominance matrix consists of N(N- 1)/2 dyads . We use the phrase `significant dyad' to designate those which depart significantly (a=0 . 05) from equal wins by each individual (binomial test : Siegel 1956) . The significance of a dyad is independent of the ranking procedure used . Hierarchical rankings were obtained by arranging the dominance matrices so as to minimize the number of entries

falling below the diagonal (Brown 1975) . The relative status of different age and sex classes was determined in two ways . (1) Individuals were ordinally ranked according to position in their social unit's hierarchy . In the analysis, we make a distinction between relative ordinal rank among adults only, and absolute ordinal rank which includes the yearlings, 2-yearolds and adults in a particular unit . In some cases the ordinal rank of a particular individual may be misleading because that individual may not have actually dominated an individual lower in the hierarchy . (2) We compared the number of significant dyads won and lost between different age and sex classes . Significance of patterns of social dominance was tested using the chi-squared statistic . If the win-loss ratio was significantly different from 1 : 1, the null hypothesis of no relationship between age or sex class, and the probability of winning dyads against another age or sex class, was rejected . Because many of the male versus female dyads consisted of the same pairs of individuals during the 2 years of the study, the combined data cannot be considered independent, therefore, we analysed the inter-sex data separately for each year . In statistical analysis of the data for yearlings versus adults we have combined the data for the 2 years . (A particular bird can be a yearling for only 1 year, thus none of the dyads in different years could be between the same two individuals .) Among adults of each sex we looked for a relationship between age and dominance . We determined the age of the winner relative to the loser for every significant dyad and then tested for independence between relative age and probability of winning, with a similar null hypothesis of a winloss ratio of 1 : 1 . These data were not combined for the 2 years .

Linearity The linearity or transitivity of the order in each social unit's hierarchy was calculated using both Landau's index (h) (Landau 1951) and Kendall's coefficient (K) (Appleby 1983) : 312 N -N i=i

(N- 1)]2

h= - N Y

and :

2

Barkan et al . : Social dominance in communal, jays for even values of N, K= I-

24d N 3 -N

for odd values of N, k-

= 1-

24d 3 N -4N

where N equals the number of jays in the unit and V equals the number of jays individual i dominates . N(N- 1)(2N- 1) 1 N , d= 12 - 2~(VY and represents the number of circular triads . Both indices range from 0 to + 1, with + 1 indicating perfect linearity . In order to calculate these indices we had to decide on some criteria for determining whether the dominance relationship of each dyadic pair was known . A conservative estimate is one in which only significant dyads were considered known . Use of this criterion caused us to exclude many dyads . These included jays who were ranked low in the hierarchy and visited the feeder infrequently when others were present, thereby avoiding contact with most of the more dominant individuals . Consequently we have fewer significant dyads involving these individuals . To obtain an estimate of the linearity that includes these relationships, we also calculated the indices using a different criterion ; the dominant-subordinate relationship was determined simply on the basis of which individual had more wins than losses in a particular dyadic pair . The significance of the linearity of each hierarchy was determined using the technique described by Appleby (1983) .

Interaction Frequency Within Age and Sex Classes To get some indication of the frequency of interactions of different age and sex classes, a comparison of intra-class interaction frequency was made . Because different numbers of individuals from the various classes occurred in different social units, we had to correct for this factor in statistical tests . The expected value for the intraclass interaction frequency for each class was obtained using the following formula derived from Altmann & Altmann (1977) : C

E, = lc-SD I ./

D,

C-1

177

where E, is the expected number of intra-class interactions for class c, ICs is the total number of intra-class interactions for C classes in S social units, C is the total number of classes being compared, S is the total number of social units with individuals of class c present, and D, is the number of intra-class dyads for class c . s s.(n 2 -1) ~n D =

where n 1 ,1 is the total number of individuals of class c in social unit s . The expected values for each class were compared to the observed values and heterogeneity was tested using the chi-squared statistic . If a significant chi-squared value was obtained, the null hypothesis, that individuals within different classes were interacting with equal frequency, was rejected .

RESULTS

Behaviour at Feeder An observation period began with filling the feeder and signalling the jays with a whistle . Members of the social unit usually responded within 5 min and frequently in less than I min . The jays would arrive and perch in the trees around the arena and then fly down to begin feeding . Interactions began to occur almost immediately. Usually, all members of the social unit were observed at the feeding arena within the first 10-15 min after observations began . As the observations continued individual birds would come and go independently . At any given time, 1-10 individuals might be within I m of the feeder. Most of them would stand on the perches provided, waiting for a feeding opportunity, opening seeds, interacting with other jays present, or engaging in some other activity such as preening or looking around . Some individuals would wait in nearby trees . Once an individual gained access to the feeder, it usually remained there for 5-30 s . Two basic modes of behaviour occurred at the feeder . (1) The jay pecked seeds open one at a time and swallowed them . The jay either remained on the feeder while it did this or moved to one of the nearby perches . (2) Alternatively, the jay stuffed its mouth and upper throat with unopened seeds . If uninter-

1 78

Animal Behaviour, 34, l

Table I . Distribution of types of dominance interactions expressed as a percentage of the total for each different age and sex class in 1980 Type of interaction

Yearling

Aggressive Fight Major supplantation Minor supplantation Peck Threat

41 10

Adult female

Total

0

7 66 8 5 1

7 59 10 7 2

7 60 9 6 1

Total aggressive

65

87

86

83

Passive Passive supplantation Interrupt First

27 7

lI 1

Il 2

13 2

1

1

1

I

Total passive

35

13

14

l7

1045

3370

2037

6442

Total number of interactions

7

Adult male

6

rupted, it would do this until it could no longer close its bill . It then either hopped to a nearby perch, opened the seeds and swallowed them or, more typically, flew off. When we were able to observe the jays after such a flight, they nearly always cached the load as described in Brown (1963a) and then returned to the arena .

Behaviours Recorded During Interactions Several behavioural activities were used as criteria for dominance . Most were some form of `supplantation' as described by Brown (1963b, 1975) . Our primary (and most frequently observed) criterion for determining dominance relationships was the 'major supplantation' (Table I) . The outcome of interactions involving the other criteria was consistent with the outcome of 'major supplantations' . The criteria are listed in order of decreasing intensity . At all times an interaction was classified according to the highest order of interaction known to have occurred . For instance, pecks and threats often accompanied interactions of higher intensity . When doubt existed about the outcome, the interaction was not included in data . We divided the interactions into two types, aggressive and passive . Aggressive interactions were those in which some overtly aggressive behaviour, such as a peck or threatening posture, accompanied the supplantation . In passive interactions a supplantation occurred without any obvious aggressive

behaviour by either individual . Interactions considered to be 'aggressive' included . (1) Major fight : birds exchanged two or more pecks each and the loser withdrew . (2) Minor fight : same as major fight except only one return peck by loser . (3) Major supplantation : winner advanced with an overt aggressive action and loser withdrew, by moving both feet and sometimes leaving the arena . (4) Minor supplantation : same as major supplantation except that loser moved only one foot back . (5) Peck : winner pecked loser, who neither withdrew nor returned the peck . (6) Threat : winner postured aggressively at loser, which did not withdraw but typically adopted a submissive posture . Interactions considered 'passive' included . (1) Passive supplantation: winner advanced without noticeable overt aggressive action, and loser withdrew by moving both feet . (2) Interrupt : same as passive supplantation except loser moved only one foot . (3) First: two birds simultaneously moved towards the feeder can but one gained undisputed access . The sample used for the final determination of dominance relationships between members of a pair of individuals included all aggressive and passive interactions observed .

1 79

Barkan et al. : Social dominance in communal jays

Contexts of Interactions Interactions were observed in four contexts . (1) Two jays simultaneously moved toward the unoccupied feeder, and some interaction occurred that resulted in one obtaining access and the other not, These were classified as major or minor supplantations, firsts, pecks, or threats . (2) One jay was perched on the rim of the feeder and another arrived and attempted to gain access, either aggressively or passively . This could result in nearly any of the interaction types . (3) Interactions frequently occurred among individuals waiting for another to vacate the feeder . These interactions could be any of the aggressive types . (4) Interactions were occasionally observed among individuals not in the feeding arena . These usually occurred when one individual with a mouthful of seed left to cache it . Sometimes another would follow and either interfere directly with the caching activity or wait and attempt to uncover the seed after the cacher left . If the individual who cached the seeds saw the filcher, an interaction sometimes occurred .

When we were not collecting data at the feeders, we occasionally observed aggressive interactions among jays in a more natural context . These data were not included in our analysis but in nearly all of these cases the outcome of the interaction was the same as what we would have predicted based upon our observations at the feeder .

Insignificant Dyads In 1980, 61% of all possible dyads were significant and in 1981, 78% were significant . Among those dyads in which no significant dominantsubordinate relationship could be determined using the dyadic data alone, three patterns existed . (1) Individuals interacted infrequently because one of the pair spent little time at the feeder . (2) Both individuals were frequent visitors to the feeder and frequently interacted with other individuals but not with each other . Often such individuals were far apart in the hierarchy . (3) Both individuals won an approximately equal number of encounters .

Table II . Landau's index (h) and Kendall's coefficient (K) of linearity in Mexican jay social units in 1980 and 1981 Year Social unit

1980

1981

Wins > losses

Significant dyads

h

K

h

K

TK CO HI SW BY UC RC

8 10 10 6 10 9 13

0 . 833 0 . 764 0 . 952 1000 0 . 861 0967 0876

0825** 0869** 0950** 1 .000* 0856** 0967** 0876**

0 . 524 0303 0715 0629 0 . 552 0858 0729

0 . 500 0 . 281 0. 706* 0. 594 0538* 0858** 0696*

r

9.4

0893

0906

0-616

0596

TK CO HI SW BY UC RC

10 8 14 8 7 10 14

0958 0952 0 . 949 0 . 810 1000 0873 0910

0 . 956** 0 . 950** 0949** 0625* 1 . 000** 0 . 869** 0 . 908**

0721 0905 0 . 732 0798 0893 0642 0745

0 . 713** 0 . 900* 0728** 0788** 0. 893 0631** 0 . 741**

X

10 . 1

0922

0894

0777

0771

Four birds were not included in our estimations of linearity for 1980 because data were insufficient to estimate their position in the hierarchy . N=number of individuals in social unit . *P<0 . 05 ; **P<0. 01 .

Animal Behaviour, 34,

1 80

Table III . Sex and relative rank among adult Mexican jays in seven social units 1980

1981

Rank

Male

Female

Male

Female

x

6 6 5 2 4

1 1 1 4 19

6 4 5 4 10

I 2 2 3 18

S a co w=N in social unit .

Linearity We constructed a total of 14 hierarchies for seven social units in 1980 and 1981 . In Table II we present statistics on the linearity of dominance hierarchies for each social unit in both years . Using the wins greater than losses criterion, all but one of the hierarchies observed over the 2 years had indices above 0 . 8 . Depending upon which index of linearity is used, seven or eight had a value greater than 0 .9, which is considered `highly linear' by Chase (1974) . All of these values were significant at the 0 .05 level or better . When only significant dyads were included the linearity values generally went down . In 1980 the number of significantly linear hierarchies dropped to four, while all seven of the 1981 hierarchies remained significant . The difference between the 2 years may be the result of a smaller sample size in 1980, and may not reflect any real difference in the average degree of linearity between the 2 years . Ten of the 14 dominance hierarchies had no significant non-linear dyadic pairs ; the remaining four had one each . Thus, out of 471 significant dyadic relationships, only four (1%) fell below the diagonal . Overall, 8% of all interactions in 1980 fell below the diagonal and 13% in 1981 . These data indicate that while dominance relationships were clear for most individuals, reversals were not uncommon .

in the lower ranks . Nineteen of 25 males in 1980, and 19 of 29 males in 1981, were ranked among the top four adults in their unit's hierarchy . Nineteen of 27 females in 1980 and 18 of 26 females in 1981 were ranked fifth or lower among adults in their unit's hierarchy . In both years males won significantly more dyads against females than would be expected if the piobability of winning inter-sexual dyads was independent of sex (Table VII) . Alpha adult status Among adults, males held the highest relative position in the adult hierarchy in six of seven social units in both years (Table III) . In 1980, four of the six alpha males were the oldest male in their unit while in 1981 three of the six were the oldest (Table IV) . In both years there were two social units in which the precise age of the dominant male was not known because he immigrated into the study area as an adult . All three cases in which the oldest male was not the dominant adult in the hierarchy were in social units in which no male in the unit was older than 4 years . In two of these units the dominant adult was a female and the oldest male was 3 years old . Male Mexican jays are known to live at least 17

Table IV . Sex and relative age of the highest ranking adult Mexican jay in seven social units

Dominance Among Different Age and Sex Classes Sex and social status

In our analysis of male rank relative to female rank we included only adult jays because yearlings never breed and 2-year-olds rarely do . We determined the rank of each adult relative to other adults in the social unit (Table III) . Males predominated in the higher ranks of the hierarchy, females,

Relative age : Oldest Sex : Male 1980 1981

4 3

Other* Male '

Younger Male Female 0 1

1 1

* Other refers to individuals whose age is uncertain because they were first banded as adults .

Barkan et al . : Social dominance in communal jars

was shared with the oldest adult male . Two other yearlings held the beta position in their unit's hierarchy . Only one of the yearlings was ranked below fourth in 1980 while the 2-year-olds were distributed throughout the hierarchies. The 2-yearold who held the alpha rank was a member of the HI unit in which the oldest male was only 3 years old . He was ranked number one in the hierarchy but he was involved in a triangular relationship, so he was actually subordinate to another 2-year-old . In contrast to 1980, when the six yearlings were in four different social units, in 1981 all of the yearlings were members of the same social unit, H I . The highest ranked yearling was third in the HI hierarchy . Five of the 2-year-olds present in 1981 were individuals who had been in the study area in 1980 . All of them retained approximately the same high rank they had held the previous year . The pattern of yearling dominance was also tested using the dyadic data . These data are summarized for all units in Table VII . In 1980, yearlings were significantly more likely to win in dyads against adults than expected if the probability of winning was independent of yearling status . In 1981, when yearling-adult interactions were restricted to one unit (HI) . there was no significant relationship between being a yearling and the probability of winning in dyads against adult males or females . However, one yearling in this unit dominated all adult females and was out-ranked by only two of the four older males in its unit . When we combined the data for both years there was a total of 62 significant dyads between adults and yearlings . Of these the yearling was dominant in 50 and the adult was dominant in 12 (X== 12 . 85 . P < 0 . 001) . The significance of this result is largely

Table V . Relative age of winner in all significant dyads between adults of known sex Relative age of winner Year

Older

Younger

23 23

6 12

5 . 45 1 . 77

0-02 0 . 18

8 9

11 17

0 . 24 1 . 26

0 . 63 0 . 26

P

Males 1980 1981

Females 1980 1981

181

years, females I 1 years, and 44°~; ; of the population is older than 3 years of age (Brown, unpublished data), so social units without adults older than this arc somewhat unusual . We tested the general relationship between an individual's age and its probability of dominating others of the same sex using the dyadic data between adults of known sex (Table V) . In both years, older males tended to dominate younger males . However, this was significant only in 1980 . There was a slight tendency for younger females to dominate older ones but this did not approach significance in either year . Status o/ yearlings

An unexpected pattern of yearling dominance over adults was observed . Table VI shows the age class of the individuals holding the top four positions in the seven social units in 1980 and 1981 . In 1980 three of the alpha individuals were yearlings although in two units (SW and CO) that rank

Table VI . Age and absolute rank of Mexican jays in seven social units 1980

Young Rank

1 year

a Q

3* 2 0 0 1

S E--(L)

2

1981

Adult years 1 0 0 1 4

>_ 3

years

4* 3* 7 6 33

Young 1 year 0 0 I 0 4

2

Adult years 2 0 1 4

>_ 3

years 5 5 6 6 35

* In two units there was probably a tie between the eldest male and a yearling for alpha position in the hierarchy .

Animal Behaviour, 34, 1

182

Table VII. Age and sex of winner and loser in all significant dyads

Loser Winner

Yearling

Adult male

Adult unsexed

Adult female

1980 Yearling Adult male Adult unsexed Adult female Total adults

2 0 0 0 0

8* 34 0 1

3NS 9* 0 1

23*** 60*** 6Ns 23

1981 Yearling Adult male Adult unsexed Adult female Total adults

7 6 3NS 3 12

7NS 44 3 15

Total adults

34***

134

0 7NS 0 2

9NS 72*** 5NS 31

16Ns

179

*P<0 . 05 ; **P<0 . 01 ; ***P<0 .001 .

Table VIII. Comparison of the number of male and female intra-sex interactions

Males

Year

Females

1980

Observed Expected yz=40 . 37

688 587 P<0 . 001

481 689

1981

Observed Expected x2=45 . 90

2399 1921 P<0 . 001

1336 1486

Table IX . Comparison of the number of yearling and adult intra-age class interactions

Year

Yearlings

Adults

1980

Observed Expected y2=4349

96 25 P<0-001

1343 1414

1981

Observed Expected y2 =3-79

251 217 0 . 1 > P > 0 . 05

421 455

dueto the yearling's dominance over adult females, where yearlings won 32 of 35 dyads (x2=14 . 50,

16 and adult males five (x2=2 . 02, P=0 . 16) . Data

P<0-001) between the two classes . There was a

collected by Kent Johnson and Lee Elliott in the subsequent winters of 1982 and 1983, support our

non-significant trend for yearling dominance over

contention that yearling Mexican jays are fre-

adult males overall . Out of the 21 significant dyads

quently dominant to adult males and females during the non-breeding season .

between yearlings and adult males, yearlings won

Barkan et al. : Social dominance in communal, jars None of the 11 yearlings, and only three of 14 2-year-olds, even attempted breeding in the reproductive season immediately following the collection of social dominance data . None of these attempts was successful . Interaction frequency within sex and age classes We compared the observed numbers of malemale and female-female interactions to the number expected by chance (see Methods) . We found that males interacted with each other more frequently than expected ; females, less . This result was significant in both years (Table VIII) . A similar comparison of intra-class interaction rate between adults and yearlings showed that yearlings interacted with each other significantly more frequently than expected in 1980 but not in 1981 (Table IX) .

183

aggressive interactions in our analysis, our estimation of the rank of the dominant yearlings remains unchanged . Among mammals with well defined group home ranges the young appear to be generally subordinate to the breeders, but interesting exceptions occur . For example, in the African wild dog (Lvcaon pictus), despite the fact that the breeding pair 'has priority over the preferred parts of food' (Frame et al . 1979), 'the alpha female and yearlings had prior access to kills' (Malcolm & Marten 1982) . In certain monkeys and baboons, young may achieve high rank provided they have high ranking mothers (Hausfater et al . 1982 and earlier studies listed in Brown 1975) . Young may also rank high in certain primates that do not appear to have this 'parent-dependent' system (Presbvtis, Hrdy & Hrdy 1976 ; .Alouatta, Jones 1980) .

DISCUSSION Comparison of Mexican and Florida Scrub Jays Yearling Dominance Our finding that some yearling Mexican jays occupy the highest positions in their dominance hierarchies, contrasts with all previous reports for group territorial birds . In the Florida scrub jay (Aphelocoma c . coerulescens, Woolfenden & Fitzpatrick 1977), grey-crowned babbler (Pomatostomus temporalis, King 1980), pukeko (Porphyrio porphyrio melanotus, Craig 1979), and other groupterritorial species for which at least some information exists (reviewed in Brown 1978), breeders have invariably been reported to dominate the nonbreeders, who are commonly the younger individuals . Adult dominance over young has also been widely reported in non-communal breeders (Wilson 1975 ; Gauthreaux 1978) . We were therefore rather sceptical of our results in 1980, especially since the results of 1981 provided no data on this point for six of the seven social units . In 1982 and 1983, however, the pattern of yearling dominance was repeatedly observed . It has also been observed in a captive group of Mexican jays (Ligon, personal communication) . Interactions between adults and yearlings show greater restraint than interactions among adults, as is indicated by the fact that the more aggressive types of encounters are less frequently observed between yearlings and adults than between adults (Barkan, Craig & Brown, unpublished) . However, the high social status of yearlings is robust in this regard ; if we use only

In the Florida scrub jay the dominance order in a social unit is as follows : male breeder > male helpers > female breeder > female helpers > juveniles (Woolfenden & Fitzpatrick 1977) . The Mexican jay differs from this pattern by (I) the dominance of some yearlings over some or all breeders, (2) the presence of more than one breeder of each sex in most units, and (3) more overlap in rank between the sexes allowing some females to dominate some or even all of the males in their unit . A direct comparison of the difference in the pattern of ranking of Mexican and Florida scrub jays is not possible with available data because the two studies were conducted at different times of the year, late winter and late summer, respectively . Consequently the precise ages are also different in the two studies . Woolfenden & Fitzpatrick (1978) present data on juveniles (3-4 months old), helpers, and breeders . The ages of the helpers and breeders were not given but it can be inferred from studies conducted during the breeding season that approximately 64,o of the helpers were about 14-15month-old yearlings, about 36 were 2 years old or older and breeders were 2 or more years old (Woolfenden 1975 ; Woolfenden & Fitzpatrick 1978) . None the less certain comparisons are possible . Male Florida scrub jay helpers were invariably subordinate to breeding males but dominant to breeding females . Female helpers were weakly subordinate to breeding females, Among Mexican jays yearlings were frequently dominant

184

Animal Behaviour, 34, 1

to adults of both sexes. (We were able to determine the sex of three of the dominant yearlings : two were male and one was female.) We suspect that the underlying cause of this difference is ultimately related to the higher degree of sociality observed in Mexican jays . Florida scrub jay juveniles were subordinate to breeders and helpers of both sexes . Woolfenden & Fitzpatrick observed that juvenile Florida scrub jays sometimes supplanted breeders by gently landing on their backs but this behaviour was apparently not aggressive and was infrequently observed . Other than these occasions they never observed a juvenile dominate an older bird of either sex . Although the Mexican jay's pattern of adult male dominance over adult females resembles that of the Florida scrub jay, it differs in that more overlap appears to exist between the sexes . Florida scrub jay female breeders are almost completely subordinate to breeding and non-breeding males ; in Mexican jays there are instances of female dominance over adult males in winter, and even of a breeding female being the dominant individual in its group . There is also more aggression between females in the Mexican jay than the scrub jay . Woolfenden & Fitzpatrick (1977) reported that female-female encounters were very rare in the latter . Although we did find proportionately fewer intra-sex interactions among females than among males, interactions between females were by no means rare . A well defined dominance hierarchy did exist among the females in most of the social units . This is probably a result of the presence of more than one breeding female within a social unit of Mexican jays in contrast to the Florida scrub jay in which there is generally only one breeding female per social unit (Woolfenden 1974, 1976) . The greater amount of aggression observed among female Mexican jays may be partly due to this difference . Parental facilitation In the genus Aphelocoma duration of association

between young and their parents within the natal territory varies across a spectrum of sociality from a few months in the California scrub jay to many years in the Mexican jay . Yearling dominance is known only in the latter species where jays of both sexes commonly acquire breeding status on their natal territory (Brown & Brown 1984) . In the Florida scrub jay, which is intermediate in this spectrum, both sexes may stay a year or two but

usually only males breed near enough to obtain a part of the natal territory (Woolfenden & Fitzpatrick 1978) . A key factor for a yearling in choosing how long to associate with its parents is `the probability . F. of obtaining a mate and being able to breed if an individual opts against helping and for breeding' (Brown 1978) . In general, F is influenced by the factors that are responsible for delayed breeding, such as shortage of females (for males), shortage of suitable undefended space, and inadequacy of individual skills (reviewed in Brown 1983) . Probably for a combination of reasons, young Mexican jays appear to have a lower F than young scrub jays (Woolfenden & Fitzpatrick 1984 ; Brown, in press) . In this perspective Brown & Brown (1984) suggested that yearling dominance in the Mexican jay be interpreted as an extreme form of tolerance that has been attained only at the most social end of the spectrum of sociality in Aphelocoma . Specifically, they suggested that parent Mexican jays facilitate the acquisition of breeding status for their offspring by tolerating them for extended periods and by tolerating their aggressive behaviour over food in winter . Such tolerance would tend to keep the resources of the territory under the control of the genotypes in the group as hypothesized by Brown (1974) . Consistent with this interpretation is the fact that four of the five dominant yearlings present in 1980 were offspring of the dominant adult male in their unit . The father of the fifth was unknown, and there was no clearly dominant older male in its group (HI) . In 1981, when yearling dominance was not so much in evidence, all five yearlings were in the HI unit, and there was still no clearly dominant patriarch among the adult males . The only yearling from 1980 who disappeared in the following year was one whose father, a 13-year-old dominant male, also disappeared . Yearling jays do not appear to pose a threat to adults in a breeding context . We are not sure why, but there are at least two possibilities . Yearlings . for reasons unrelated to social status, may not be motivated to breed, or may not be able to dominate adults in a sexual context . Yearlings have never been observed to participate in the contests that occur between males over access to females or between females over nest ownership . Tolerance may also benefit older jays who are not the parents since the yearlings are likely to share costs of territory defence, vigilance and

Barkan et al . : Social dominance in communal jays parental care (as modelled by Brown 1982) without reducing the reproductive success of adults . Payoff asymmetry .' With the above perspective we can better appreciate the potential payoffs for yearlings and adults in winter dominance encounters . A complementary explanation of yearling dominance in Mexican jays is that the utility of access to the feeder might be different for adults and yearlings . If, as a result of their inexperience, yearlings are less efficient foragers than adults, there might be a payoff asymmetry (Maynard Smith 1974 ; Parker 1974 ; Maynard Smith & Parker 1976) such that access to food has greater utility to yearlings than to older, more capable adults . An adult faced with a hungry yearling opponent might simply retreat rather than escalate the contest . Consistent with this are the data on the qualitative nature of Mexican jay dominance interactions which indicate that adults interact less aggressively with yearlings than they do with other adults (Barkan et al ., unpublished data) . Dominance of young over adults has been described in two other bird species, the Pinon jay (Gymnorhina cyanocephalus) (Balda & Balda 1978) and in migrating semipalmated sandpipers (Calidris pusilla) (Harrington & Groves 1977) . We hypothesize that these instances are most likely explained by a payoff asymmetry between the adults and young with respect to a food resource . In addition to an asymmetry in the utility of access to feeders . there may also be long-term fitness advantages for breeders and other adults in tolerating yearling aggression over food in winter . These are discussed above under `parental facilitation' . Sex and Social Status The pattern of male dominance over females is not an unusual result in jays (Brown 1963b ; Woolfenden & Fitzpatrick 1977) and other birds (Brown 1975 ; Wilson 1975), especially in the nonbreeding season (Smith 1980) . Adult male Mexican jays average 4`;0 larger in wing and tail length, than adult females (Pitelka 1951) . This slight dimorphism could contribute to greater male `Resource Holding Potential' (RHP) and hence dominance over females . Part of the reason for the overlap in the inter-sexual dominance pattern might be a result of the fairly large overlap in morphological characteristics .

185

We observed that most interactions were initiated by the eventual winner ; therefore, the significant difference in number of interactions won by males implies a more aggressive nature . If we look at intra-sexual interactions, where no sexual asymmetries can occur, we find significantly more male-male interactions per individual than female-female interactions . Male Mexican jays appear to be more aggressive, both intra- and inter-sexually . Why should selection for social dominance operate differently on males and females? During the non-breeding season, fitness is primarily a matter of survival in good condition . If there is an effect of social status on survival it would be of approximately equal value to members of either sex . Therefore, the ultimate explanation for male dominance over females is probably related to breeding activity . Each breeding season most adult male Mexican jays attempt to build a nest and attract a female . Competition among males manifests itself in three ways, depending on the stage of the nest . (1) Individuals are frequently seen disturbing each other's nests and robbing them of lining material (Brown 1963a) . This activity sometimes causes a delay in completion, or abandonment of the nest . (2) In some cases a dominant male has usurped a nest built by a subordinate male . In one case we observed, a dominant male accompanied his mate while she visited the nest of a subordinate male ; when the owner returned the dominant male chased him away and then returned and remained perched next to his mate until she left . (3) One Mexican jay may visit the nest of another and eat the eggs (Trail et al . 1981) . If the nest owner is present or arrives during this activity, aggressive interaction often results . Nests subject to such depredations almost invariably fail . We have also observed infanticide upon nestlings . Males generally continue in their efforts to breed after these failures, but once another nest within the social unit has nestlings to be fed, these efforts are usually curtailed and the unsuccessful males begin helping to feed the young . In brief, aggressive interactions among Mexican jays are frequent during the breeding season . The potential effect of dominance on the reproductive success of individuals leads us to suggest that selection for dominance among males is probably important in this season .

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Animal Behaviour, 34, l

Our finding that yearlings may dominate breeding adults in winter was unexpected and leads us to ask how commonly this might occur in other communal breeders . It has been suggested that breeders enforce their prerogative by keeping subordinates in low ranking positions in all respects (Vehrencamp 1983) . This is clearly not valid for all species and all contexts . Our findings have led us to invoke the concepts of payoff asymmetries and parental facilitation as possible explanations .

ACKNOWLEDGMENTS

We thank Esther Brown, Steve Martindale, and Nell Sedransk for helpful discussion and assistance in various phases of this work . For logistical support in the field we thank the staff of SWRS, particularly Vince Roth and Joseph Remington . We are also very grateful to the Southwestern Research Station of the American Museum of Natural History for use of their land and facilities . Special thanks to Ed and Charlotte Bagwell, the late Matthew and Anne Pugsley, Otto and Agnes Bruhlman and Mont Cazier for kindly allowing us to work on their land . R . Kent Johnson and Lee Elliott provided data collected in the 1982 and 1983 field seasons . G . Woolfenden, W . Koenig, and A . Gaston made helpful comments on the manuscript . Finally, C .P .L .B . thanks Libby Lyman Barkan for her kind support during the completion of this research . Financial support was provided by a Sigma Xi Grant in Aid to S .D .S . and NIMH grants MH16345,

MH33498 and NSF grant BNS

811558902 to J .L.B .

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(Received 15 August 1984 ; revised 27 December 1984 : MS . number : .44275)