DOMINANCE

HIERARCHIESAND HELPER
HARRIS'HAWKSCONTRIBUTIONSINJAMESW. DAWSON AND R. WILLIAM MANNAN
Schoolof Renewable
Natural Resources,
Universityof Arizona,
Tucson,Arizona 85721 USA

ASSTR•CT.--Weexamineddominancehierarchies,mating relationships,and helping by
individuallymarkedHarris' Hawks(Parabuteo
unicinctus)
at 64 nestsin Arizona(1984-1988).
Dominancehierarchieswere linearand alwayscontainedan adult femaleasthe mostdominant member.We placed hawks into three dominancecategories(alpha, beta, and gamma)
basedon frequenciesof supplanting.Pairswithout helpers(16%, n = 64) and groupsthat
containeda beta male helper (63%)were the mostcommonsocialstructures.Somegroups(11%) containeda secondalpha female (alpha-2) that was subordinateto the alpha female
and dominant over the alpha male. Of 68 observationsof copulatorybehavior,45 (66%)were
copulationsbetweenalpha malesand alpha females.Betamalesattemptedto copulatewith
alpha females(23 observations;34%) but were unsuccessfulbecauseof a lack of femalecompliance.
Observationsof movementsby markedhawkssuggested
that mostgammahelperswere offspringof the alphapair and that betamalesand alpha-2femaleswere probably
unrelatedto the alpha pair. Alpha hawksrestrictedaccessto the nestby both betaand gamma
helpers,but beta maleswere toleratedcloserto the nest than were gammahelpers.Conse-

quently,betaand gammahelpersrarely participatedin the direct careof eggsand young
but were activein procuringprey, transportingprey to the nestarea,and defendingthe nest
from predationby Great Horned Owls (Bubovirginianus).
Received
29 May 1990,accepted
15
January1991.

COOPERATIVE
breeding occursin diverse avian
groups but is unusual among Falconiformes(Newton 1979,Brown 1987,Skutch 1987).Only
the GalapagosHawk (Buteogalapagoensis)
andHarris' Hawk (Parabuteo
unicinctus)
are reported

toward reproductionvary. Alloparentalbehavior(i.e. parental behavior by a nonparent) comprisesall parental-type behaviorsin somespecies(e.g. Woolfenden and Fitzpatrick 1984)but
is apparently lacking in others (e.g. Carrick

to breedregularlyin cooperativeunits (de Vries
1973,Mader 1975b,Faaborget al. 1980,Faaborg
and Patterson1981). Sociality in the latter species extendsto breeding and nonbreeding periodsand hasbeen the subjectof severalstudies

1972). The role of extra hawks at Harris' Hawk
nests is not well documented. Bednarz (1987)(Mader1975a, Griffin

1976, Brannon 1980,

Whaley 1986,Bednarz1987).Harris' Hawks reside on and defend their territories year round(Dawson and Mannan 1991). In Arizona, breeding is seasonaland occursprimarily in springand summer.Socialityduring nonbreedingperiods includes the formation of social aggregationscomposedof > 1 breeding group in undefended zones between territories. Breedinggroupsrange in size from 2 to 7 individuals andaverage3.8 hawksper nestin Arizona (Dawson
and Mannan 1989). Group composition isskewed toward adult males (Mader 1975a, Bed-narz 1987),although adult femalesregularly occur ashelpersin the Arizona population (Dawson and Mannan 1991).

Among cooperatively breeding birds, thekinds and amountsof aid provided by helpers
649

usedtime-lapsephotographyto recordthe presence of hawks

on the nest. He concludedthat

adult males must "actively assist"breedersat
nests.Mader (1979) reportedthat an adult male
aided the adult pair by incubating eggs anddelivering food to the nest. His observationsof
nesting behavior were detailed but were limited to only a single nest in Arizona.
Similarly, the role of dominancein the socialorganization of breeding groups of Harris'

Hawks has never been studied. Aggressionis
relatively restrained in speciesthat breed incooperative units, and dominance hierarchies

have been describedfor most speciesthat havebeen studied in detail (reviewed in Brown 1987).

Breedingstatusis invariably linked with dominance statusin cooperativegroups,prompting
the suggestion that social competition influences cooperation (Emlen 1984). Concepts ofparent-offspring conflict (Trivets 1974) havebeen applied to cooperativebreeding and modThe Auk 108: 649-660. July 1991

650

DAWSONANDMANNAN[Auk,Vol. 108

els of conflictbetween breederand helper have common nesting behaviors(e.g. brooding, shading,been proposed(Brown 1978,Vehrencamp1977, and feedingyoung) or socialbehaviorsof this species
Emlen 1984). Manipulation or suppressionof (e.g. group hunting, communal perching, and backhelpers by breedershasbeen proposedas a so- standing).
We placedcoloredflaggingat known distances(25,cial factorunderlying nonbreeding and aid-giv50, tOO,150, 200, 250, 300, and 400 m) from nestsanding by helpers (e.g. Craig 1980).
measureddistancesfrom preferred saguaroperches
We studied the socialorganization of Harris' to neststo help observersin estimatingdistancesof
Hawks in Arizona (Dawson and Mannan 1989)

perch sites to the nest. During behavioral observa-

and report here on dominance hierarchies at
the nest, helper relatednessto breeders,copulatory behavior, and the influence of dominance on helping. We also report on potential
contributionsby helpers toward reproduction.
Intergroup behaviorssuchas territoriality andsocial aggregations are discussed elsewheretions, we scanned the area around the nest (a radius(Dawson and Mannan 1991).
STUDY AREA AND METHODS

of 375-600m) every5 min and recordedthe behaviors
and positionsof groupmembersrelative to the nest.
We usedall-occurrencessampling(Altmann 1974) torecord selected behaviors associatedwith mating,dominance,and nesting(e.g. food deliveriesand exchanges).Prolonged behaviors such as incubating
eggs,feeding young, hunting, and predatorharassment were continuouslymonitored.
Obvious aggressiveinteractions were uncommon
between Harris' Hawks of the same group but sup-planting was common near the nest (Dawson and
We studied Harris' Hawks in a 48-km2 study area
Mannan 1991).Supplanting
occurredwhen a hawk ap50 km north of Tucsonin Pinal County,Arizona, fromproacheda secondhawk, which left its perchsiteas
1984-1988.Topographyand vegetationin the study the first hawk arrived. We constructed a dominancearea are typical of the Arizona Upland Subdivisionof the Sonoran Desert (Turner and Brown 1982) andwere describedby Dawsonand Mannan (1991).
In this paper, groupdescribesa breeding unit thatwas presentin the study area for at least t yr. We
considereda group to be the same group between
yearsif one or more of the top three ranking hawks
remained present.Nest describesa breeding attemptdefinedby the presenceof eggsor young.We studied
behaviorat the nest in 29 groupsthat produced64
nests from 1984 to 1987, and we monitored member-ship in groupsfrom 1984to the present.
We arbitrarilynamedeachgroupafter a distinctive
landmark or habitat feature in its territory. Methods
used to estimategroup sizes,evaluate group affiliations, locatenests,and trap hawks were describedby
Dawsonand Mannan (1989).Hawks were categorized
as malesor femalesbasedon body weight and werecategorizedas adult or immature,basedon plumage
characteristics(see Dawson and Mannan 1989 for de-

scription of methods). We color-banded all groupmembersand nestlingsusing3 plasticleg bandsandt numbered metal band in 91% of 64 nests; one hawk
was not marked at 6% of nests and 2 hawks were notmarked

at 3% of nests.

hierarchyfor eachgroupby recordingthe outcomesof supplantinginteractions.Supplantingprovided ausefulway to assess
dominancebecausesupplanting
was frequent and usually occurred on the tops ofsaguarocacti(-<80 m from the blind) where participants could be easily identified.

Harris'Hawks
usually
copulatedonsaguaros
at'10
to tOOm from the blind. During copulationapparent
cloacalcontactis accompaniedby the male leaning
backand positioninghis tail to one sideunderneaththe tail of the female (Mader 1975a). We used thispostureto assess
the outcomeof copulatorybehavior
and recordeda copulation
when a male positionedhis
tail under the tail of the female. When a male mount-ed a female,performedcopulatorybehavior,but did
not positionhis tail under the tail of the female,werecordedan attemptedcopulation.
The distributionsof reported variableswere tested
for normality and were, in some instances,not normal. Therefore,we have reported median valuesand
usedMann-Whitney-Wilcoxontestswhen distributions were not normal. Mean values were reported
andcomparisonsweremadeusingt-testsfor variables
with normal distributions (Gibbons 1985).

We observedbehavior at nests from April to August,1984-1987,for a minimum of t0 h at eachnest(œ= 48.8, n = 64 nests,total observations= 3,125 h).

We used30 x spottingscopesand t0 x 50 binoculars
to observe nesting behavior from elevated blinds(Dawson and Mannan 1989). We used watches,taperecorders,and an ethogram codethat consistedof 48
alphanumericelementsto recordbehavior.The code

RESULTSDOMINANCEAT THE NEST

Supplanting was frequent near the nest andoften occurred in bouts that involveda series ofhelped to simplify data collectionon severalhawks

supplantationsof a subordinateby a dominant
until the subordinate left the nest area. Sup-at once and to obtain consistent data from different

planting ratesin boutsbetweenthe sametwo

observers.Most elements in the code represented

hawks ranged from 1 to 21 per minute (œ= 7.7

July1991]

Dominance
andHelping
inaHawk

651

T^I3LE
1. Supplantingmatrixfor Harris'Hawksin the Gato4 group,1986,that containedan adult maleas
the mostdominant helper. Data taken from 79 hours of observation.
Supplantee

Supplanter

Sex

XWGR

WRXW

GKXK

WXRR

BXWK

XWGR
WRXW
GKXK
WXRR

Adult
Adult
Adult
Adult

Age

Female
Male
Male
Male

-0
0
0

1
-0
9

10
39
-0

7
16
4
--

3
7
1
0

BXWK

Immature

Female

0

9

0

0

--

supplantations/rain,n = 692supplantingbouts). unit. We groupedtogetherconsecutiveranks
We monitoredsupplantingat 4 neststhrough- with low frequenciesof dominanceinteractions
out the nestingcycleand found that supplant- and placed eachhawk into 1 of 3 levels:alpha,
ing relationships remained consistentduring beta,and gamma(in descendingorderof rank).
the nesting period. Reversalsin the role of an Most groupsof 3 containedalpha and beta levindividual asa supplanteror supplanteedid not els,and mostgroupsof > 4 containedall threelevels.
occurduring the nestingperiod.
Socialorganization
ofgroups.--Analphafemale
Dominancehierarchiesbasedon supplanting
were linear, "peck-right" (Schjelderup-Ebbe was the most dominant member in all groups
1935)systemsin which the top hawk was dom- studied. However, 3 social structures that difinant overthe othergroupmembers,the second fered at alpha and beta levels were evident
from the top was submissiveto the top hawk amongbreeding units attending nests.First, aand was dominant over the other group mem- simple pair with no helpers occurredat 16%ofbers, and so on. We ranked each hawk in a
64 nestsstudied. Percentagesof simple pairs
group accordingto the outcomesof supplant- ranged from 11 to 17 (• = 14) in the 4 yr of
ingsand foundthat frequenciesof supplantings study. Second,the most common social strucbetween ranks differed. Interactions
in whichture (63%of total nests,range over years= 56the dominant hawk (ranked as 1) supplanted 80%, •? = 66%) contained an adult male (betathe secondhawk in the hierarchy(rankedas2), male) that was subordinateto an alpha female
representedhere as 1>2, were relatively infre- and male (Table 1) plus 0 to 4 gammahawks.
quent (median = 0.6 per 10 hours of observa- The third social structure featured a second adulttion; n = 29 groups,64 nests,110observations). female (alpha-2) that was subordinate to the
Supplantingsbetween 4>5-7 were also infre- alpha female,but dominant over the alpha malequent (median = 0.5 per 10 hours of observa- (Table 2). Alpha-2 femaleswere presentat 11%
tion; n = 8 groups,18 nests,174 observations). of the nestsstudied (range over years= 0-20%,• = 9%).
Supplanting was most frequent in the 2>3
dyad (median = 2.3 per 10 hoursof observation;
All alpha and beta hawks were adults withn = 23 groups, 54 nests,636 observations)and the exceptionof one immature alpha female inthe 2>4-7 dyad (median = 5.0 per 10 hours a group of 3. Among gammahawks,22%were
observation;n = 8 groups,18 nests,516 obser- adults and 78% were iramatures. We recorded
vations). Differences in frequencies of sup- only malesat the beta level (Table 3) and, withplanting were significantbetween dyads 2>3
the exceptionof alpha-2 females,extra females
and 1>2 (Mann-Whitney U-testby groups,P = were subordinate to beta males. The beta level
0.000) and between dyads 2>4-7 and 4>5-7
differed from both alpha or gammain that it(Mann-Whitney U-test, P = 0.000). Hawks that wasoccupiedby only one hawk per group.Beta
ranked3 in groupsalsoactivelysupplantedsub- males were absentfrom 6 groups of 3 hawks
ordinates(median= 2.5;n = 8 groups,18 nests, that containedhelpers.We classifiedhelpersin
285 observations),and frequenciesdiffered sig- the latter groupsas gammabasedon supplantnificantly between 3>4-7 and 1>2 dyads ingsandbehaviorat the nest(seenestingroles).
Gamma hawks were adult or immature, male or(Mann-Whitney U-test, P < 0.05).
The differencesin dominance relationships female, and groupscontainedup to 4 gamma
providedobviousgroupingswithin a breeding hawks.Within the gammalevel, dominanceap-

652

DAWSON
ANDM•NAN[Auk,Vol.108

TABLE
2. Supplantingmatrixfor the Gato1 group, 1984,that containedan adult femaleasthe mostdominanthelper. Data taken from 138 hours of observation.
Supplantee

Supplanter
GWXG
WXGG

Age
Adult
Adult

Sex

GWXG

WXGG

BRXR

RWXR

XWBW

FemaleFemale

-0

4
--

2
8

7
28

15
12

BRXR

Adult

Male

0

0

--

12

32

RWXR
XWBW

Adult
Immature

Female
Male

0
0

0
0

0
0

-0

3
--

peared to be age-related (adults dominate im- alphastatusby hawkswithin their natalgroups
matures) and secondarily sex-rated (females or territories.Thus, some gammahelpers anddominate males).
probably most beta helpers were unrelated toRelatednessof helpers.--Themajority of im- alpha members.

mature hawks were offspring(œ--- 76%, range= 75-78%for 1985-1988)that were helping theirparentalgroup.The remainingimmaturehawks(g = 24%,range = 23-25%) were hawksthat left
natal groups and joined unrelated groups as
gammahelpers.We found that both relatedand
unrelatedadulthelpersoccurredin groups.Malehawks color-banded as nestlings often remained as adult gamma helpers in their natal
territories for up to 3 yr. We recordedpresenceof offspring in 17 groups that were monitored

Copulatory
behavior.--Harris'Hawks engaged
in copulatorybehaviorthroughoutthe nesting
cycle, and we observed peaks in copulations
andattemptedcopulationsbeforeand after egg
laying (Fig. 1). We observed44 copulationsand
23 attempted copulations(20 nests, n = 15
groups).Copulationswere obtainedonly by alpha males, but beta males (91.3%) and alphamales (8.7%) attempted copulations.Sexualbehavior was always directed toward the dominant femalein a groupwith oneexception,whenfor 4 yr (1984-1988). Of 21 males color-banded an alpha male copulated with an alpha-2 fein 1984, 16 (58%) were present in their natal male.
groupsthroughtheir firstyear,10(48%)in their
We observedapparentcloacalcontactduringsecondyear, 4 (19%)in their third year, and 0 44 copulations within 0.25-5.00 min (œ= 1.25
in the fourth year. Of 18 females, 5 (28%) were min) after the male perchedon the female. Fepresent in their first year, 1 (6%) in the second malesdid not engagein copulatorybehavior
year, and 0 by the third year.
during attemptedcopulations.Males attemptWe recordeddispersalfrom natal groupsby ing copulation, however, usually continued
6 hawks that assumedgammapositionsin non- copulatorybehavioruntil femalesleft or threatnatal groups. We also observed 3 instancesin ened them. Consequently,the duration of atwhich an adult male (bandedasa nestling)ob- temptedcopulations(œ= 5.33 min) was signiftainedbetastatusby joiningan unrelatedgroup. icantly longer than the durationof copulations
We did not observe

the attainmentof beta or(• = 2.79 min; t = 2.82, df = 65, P < 0.005).

TABLE
3. Sex,age,andstatuslevelof Harris'Hawks(n = 243)in 64 groups,1984-1988.
Capitalized
letters(F, M) are adults and lowercaseletters (f, m) are iramatures.

Group No.of No.of

Alpha

size

groupsindiv.

F

M

f

2

10

20

10

10

.......

3
4
5
6
7

21
15
10
6
2

63
60
50
36
14

20
15
10
6
2

21
15
10
6
2

Total

64

243

63

64

Beta

Gamma

- 2F

M

F

M

f

m

1
-----

1
3
2
0
1

13
13
7
6
2

-1
1
I
1

2
1
4
3
1

1
5
7
6
2

4
7
9
8
3

1

7

41

4

11

21

31

July1991]

Dominance
andHelping
inaHawk
Nest
100

0-50

653
50-tO0IO0-t50150-200

200-300

m

ß

I•

Gamma

Weeks

Fig. 1. Ratesand frequencyof copulatorybehavior by Harris' Hawks in relation to stageof nestingat 20 nests,15 groups,1984-1986.Open bars show
copulations(n = 44), and hatchedbarsshowattempted copulations(n = 23).

CONTRIBUTIONSTO NESTING

Fig. 2. Relationshipbetweenperching distancefrom the nestand supplantingfor beta and gamma
helpersin 23 Harris'Hawk groups.Upperaxisshows
meanpercentagesof perchingevents(n = 1,139)in
which supplantingoccurred,and lower axis shows
meanpercentageof perchingtimenearthe nest(totalof 925.2 h).

Helperaccessto thenest.--In pairs,dominanceby alpha femalesdid not appear to affect pa- plantation were not significantlydifferent forrental behavior by alpha males; supplanting betamalesin groupscontaininggammahawks
boutswere usuallylimited to a singleeventand (n = 322,8 groups)and for betamalesin groups
did not serve to drive the alpha male away. without gamma hawks (n = 164, 15 groups;
Interactions
that were more intense were exMann-WhitneyU-test,P = 0.215).Similarly,timepressedby an alpha female toward an alpha intervalsfor supplantingof gammahawks did
male in only 9 instances(6 groups)when she not differ significantlyin groupswith a beta
forcibly took food from the male while he was male (n = 330 observations,8 groups) and inperchednear the nest. Males in pairs freely en- groupswithout a beta male (n = 120, 14 groups;
tered the nest and participatedin all categories Mann-WhitneyU-test,P = 0.264).Third, alphamales allowed beta males to remain on the nestof parental-type behavior.
Ingroups,
accesstothenes•
bybetaandgam- (9.2%of 710observations)moreoftenthan gamma hawks was directly affectedby dominance. ma hawks (2.7% of 351 observations).
Alpha femaleswere generally lessaggressive
Although beta and gamma hawks often approachedand perchednearor in the nest,alpha toward helpers at the nest than were alphamales.Most instances(66%, n = 90 observations)males--and sometimes alpha females--supplanted them repeatedlyuntil the subordinate of helpers perching uninterrupted on the nest
left the nestarea.Harassmentby dominantswas occurredwhen an alpha femalewaspresentand
relatedto the nestsite.Alpha malesand helpers the alpha male was absent.Alpha femalesusuperched and hunted together in relative har- ally perched alongsideof helpers on the nest
momentarily before supplanting them. Behavmony at locations>400 m from the nest.
Alpha malesshoweda differential tolerance ior of alpha-2 femalestoward helperswas simof beta and gamma hawks near the nest in 3 ilar to that of alpha females.
Nestingbehaviors.--Alphafemalesstayednear
ways.First,supplantingof betamaleswas mostfrequentwhen they perchedwithin 50 m of the the nest most of the time and left for brief penest,and supplantingof gammahawkswasmost riods to take food from helpersor to visit water
frequentwhen they perchedwithin 150 m of sources.Nest attendanceby other group memthe nest (Fig. 2). Supplanting declined when bers (Table 4) reflected dominance relationbetaand gammamembersperchedbeyondthe ships.Alpha-2 femalesalsocloselyattendedthe
50-m and 150-m zones, respectively.Second, nest and rarely joined other membersin hunttime intervals between the arrival of a helper ing foraysaway from the nest area. Although
within 50 m of the nest and supplantingby the alpha males often hunted with helpers, theyalpha malewere higher for betamales(median alsospentthe majorityof time perchednear the= 2.3 rain; n = 636 observations,23 groups)than nest. Attendanceby beta and gamma helpers
for gamma hawks (median = 0.9 min; n = 516 reflectedsupplanting by alpha males.
We found that in pairs, females and males
observations,21 groups;Mann-Whitney U-test,
P = 0.000). Intervals between arrival and sup- participatedin most parental behaviorsexam-

654

DAWSONANDMANNAN

TABLE
4. Percentageof time spentattendingthe nest
(perched -<100 m) by Harris' Hawks (n = 223) in
54 groupsof >2.
Time

Status

Alpha

Sex

n

Observa-neartion (h)nest (%)[Auk, Vol. 108

ble 2), the alpha female performed the majorityof female incubation (66%, n = 12 observations),

and brooding/shadingthe young (51.8%, n =27 observations),whereas the alpha-2 female
performed most of the feeding duties (55%, n= 51 observations).

Incubationand feedingof nestlingsby gam-F

542,620.8

92.3

M

54

2,620.8

64.0

ma members

Alpha-2

F

7

311.0

86.9

Beta

M

41

1,704.0

32.1

alpha male was absent.We observedincubation

Gamma

F
M

25
42

917.5
988.0

10.1
16.0

was rare and occurred

when

the

by a gammaimmaturefemaleon only 1 occasion
for 7 min. She wassupplantedby the alpha malewhen

he returned.

Gammamales tried

to feedyoung on 8 occasionsat 5 nests,but the alphafemale immediately flew to the nest and took
the prey in 6 of the observations.We observed

ined. Hunting and harassmentof predators-behaviors often performed by >1 hawk in immaturegammafemalesfeed nestlingswithbreeding groups--were, in pairs, performed out interruption on 6 occasionsat 3 nests.
Provisioning
the brood.--Harris' Hawks capprimarily by the male alone while the female
remained at the nest. In groups of >-3 hawks, turedprey by hunting aloneor with other hawks
alpha females and males also provided most of in cooperativeunits (Mader 1975a,Bednarz 1988,
the direct parental care such as incubating, Dawson 1988). Helpers sometimesshared food
brooding, shading,and feeding nestlings(Ta- at kill sites and delivered food to the nest area.
ble 5). We observedbeta males participating in Food deliveries to the nest often took a circuiat least one of these behaviors in 54.1% of nests
tous route of prey exchangesthat ascended
attendedby groupsthat containeda beta male. throughthe dominancehierarchybeforebeing
Among all groups,we observedthe highestin- delivered to the nest. We examined the provivolvement by a betamale in direct parentalcare sioning of the brood and alpha female by diin the Ocotillogroup(1984nest).The betamale viding the task into 3 components:capturing
incubatedthe eggs3 times and accountedfor prey, transportingfood to the nest,and feeding
11.3% of total incubation time (n = 18 hours of
nestlings.
At nestsattendedby a simple pair, alpha males
observationduring the incubationperiod)compared with 28.1%for the alpha male and 60.6% usually hunted alone while alpha females refor the alpha female. The beta male fed the mained near the nest. Consequently, cooperayoung on 1 of 7 feeding events observed(n= tive hunting was commononly at nestsattend10 hours of observationwhen nestling age = ed by >-3hawks.We observedboth cooperative
hunting and solohunting by hawksin breeding
11 days).
Groups that contained2 alpha femalesdif- groups.To estimatethe frequencyof cooperafered from other groups,in termsof nest-duty tive versussolohunting by Harris' Hawks durpartitioning,only at the alphalevel. Incubation, ing breeding, we included only observations
brooding,shading,and feedingwere sharedby from groups.We alsoexcludedthoseinfrequentalphaand alpha-2females.For example,in Gato soloattemptsby alpha femaleswhile they weregroup,1984nest(supplantinghierarchyin Ta-

alone at the nest (n = 22 observations) in order

TABLE
5. Observationsof behaviorsassociatedwith nestingby Harris' Hawks in 54 groups,1984-1988.
Alpha

Beta

Gamma

-2

Behaviorn

FemaleMale

FemaleMale

FemaleMale

Incubate

278

194

52

21

10

1

Brood/shade

323

210

83

26

13

0

0

Feed
Maintain nest
Hunt

988
76
2,201

681
26
112

203
22
459

40
17
36

48
9
601

6
2
439

10
0
554

426

110

123

21

75

49

50

Detect/harass
predators

0July1991]
to restrict our estimate

Dominance
andHelping
ina Hawk
to membersthat had the655(1 nest/1.9km2)washigher than that of Harris'

options of cooperative or solo hunting (i.e. alpha malesand beta and gammahawks). Of 204
observationsof prey captureattempts,36 (17.7%)
were solo attempts and 168 (82.3%) were co-

Hawks (Dawson and Mannan 1991). Owls com-

operative attempts.

sequently,incidentsof aggression
by hawksto-

monly nestedwithin 500 m of active Harris'
Hawk nests(in 45 of 64 nesting territories)andsometimes

nested as close as 12 meters. Con-

Harris'Hawksgenerallyfed togetheron large ward owls were common.
We found evidence of 25 instances of Great
prey animals and delivered prey to the nest
only after feeding first at the kill site.In a sam- Horned Owl predation on nestlings. Greatple of 23 kills of desert cottontails (Sylvilagus Horned Owl predation occurred only in the
auduboni)by the North Star group (6 members) sixth week of the nestling stage and early inin 1985, 14 were delivered to the nest. The rethe postfledgling period (up to 6 weeks after
fledging). Owls took the entire brood at 3 nests;
maining 9 were consumedat the kill site.
Although beta and gamma membersoften a nestlingwastakenon consecutivenightsuntil
flew to the nestareawith prey,they rarelywere the nest was empty.
The rolesof group membersin attackingand
allowedto deliverprey directlyto the nest.Prey
was taken from them by a dominant member harassing Great Horned Owls in the Cholla
who took it to the nest. A sample of prey de- Grove group (1986) were typical of mostlarge
liveries to the nest area (within 50 m) was taken
groups(group size = 5) (Table 6). The number
from 4 breeding groups of identical sizesand of hawks that harassedan owl ranged from 2
sex, age, and hierarchy compositions(Fig. 3). to 5 (œ= 2.9 hawks). The alpha male was the
Betamalesaccountedfor the highestpercentage most aggressivemember followed by the betaof total deliveriesof prey, followed by alpha male, the gamma male, and the gamma female.
males,and then by gamma males.Gammafe- The alpha female left the nest and actively hamalesrarelydeliveredprey to the nearestarea. rassedowls only when shewasaloneat the nest
Mean prey delivery ratesto nestsduring the or when the owl was within 100 m. Both theperiod when nestlingswere actively fed (1-18 alpha and beta males were initially aggressive
days of age) ranged from 2 to 18 per day (œ= toward owls, but strikes (i.e. physical contact
4.9 per day; n= 74 days, 29 groups,64 nests). between hawk and owl) were made primarily
The alpha femalefed nestlingsfrom a prey item by the alpha male. All membersparticipatedto
until they were satiatedor the prey was con- somedegree in chases,but only the alpha andsumed.Alpha femalesoften ate part of the prey beta malesalwayschasedowls.
item beforeor after feedingthe nestlings.Largeprey items (e.g. the hindquartersof a cottontail
DISCUSSION
rabbit) that were not entirely consumedwere
left in the nest and were fed to the brood several
Breeding
statusanddominance.--Thesocialorhours later.
ganization of Harris' Hawks in Arizona inDetecting
andharassing
potential
predators.--The cludespairs,groupsconsistingof an alpha pairrole of helpersin harassingpredatorsthat came and helpers,and groupsconsistingof 2 alphainto the nest area was conspicuous.Harris' females and an alpha male plus helpers. Our
Hawks became excited and used the alarm-callobservations of copulatory behavior did not(Mader 1975a) repeatedly when a potential support multiple paternity in groups and are
predator camewithin 150 m of the nest. Alarm- consistentwith the electrophoreticevidence of
calling drew other membersinto the area, and monogamy in 2 groups in New Mexico (Bedthey usuallyperchedon saguarosnear the pred- narz 1987).However,polygamyin Harris'Hawk
ator and continued to vocalize until the pred- groupswas recently confirmedby DNA fingerator left.
printing (R. Sheehy, J. Dawson, and K. Oishi
We found evidence of predation of a fledg- unpubl. data). Polyandry, polygyny, and moling by a coyote (Canislatrans),and we once nogamy were documented in groups thatobserved a Common Raven (Corvis corax) takecontaineda Harris' Hawk egg. Interspecificconflict wastherefore, described as a communal breeding(Emlen 1984) or mate-sharing species(Brown
1987, Koenig and Mumme 1987). Genetic evidence also revealed that observationsof copu-

greatest between Harris' Hawks and Great

Horned Owls (Bubovirginianus).The nesting
densityof Great Horned Owls in the studyarea>2adults. Harris'Hawks

can be,656

DAWSONAND MANNAN[Auk, Vol. 108

divided equally betweenalpha and betamales,
parentalcarealsoshouldbe equallysharedby

40-

males at some nests. In a trio of Harris' Hawks,

Mader (1979) reported that participation in parental duties at the nest and copulations was
similar for both males. His conclusion that the

?

M

Alpha

M

F

Beta

trio was polyandrouswas probably correct.In
contrast,we found that parental behavior ofbeta males, if present at all, was always well
below that provided by alpha males.
WoolfendenandFitzpatrick(1977)foundthat
hierarchiesin groupsof ScrubJays(AphelocomaMcoeruiescens)were male-dominated to the extent

Gamma

Fig. 3. Deliveriesof prey (n = 136)to the nestarea
by Harris'Hawks.Datatakenfrom 4 groupsthatcontained identicalcompositions.

•hat male helpersdominated female breeders.
Althoughfemaledominanceat the alpha level
was the rule in groupsof Harris' Hawks, we
did not observe absolute dominanceof one sex

over the other. Also, we did not observe in-latory behavior provided a poor way to assess dependent hierarchiesamong the sexes(i.e.
mating systemsin groups(R. Sheehy,J. Daw- separatemale and female hierarchies).Domison, and K. Oishi unpubl. data). The alpha-2 nance within a statuslevel usually conformed
females and beta males that we studied may to sex and age conventions,but relationships
have been breeders

between hawks in different

or nonbreeders.

levels often did not.

tf parentalinvestmentispositivelycorrelated For example,adultmales(alphaandbeta)oftenwith geneticinvestment(Dow 1978,Vehren- were dominant over extra adult females(gamcamp 1977,Josteet al. 1982), our observations ma) in groups. Other fac,•orsmust thereforeof parentalbehaviorssuggestthat monogamy overlay age- and sex-relateddominance. Seis the most common mating systemin Harris' niority, breeding experience,establishedassoHawk groups.The nestingrolesof alpha-2fe- ciations with dominants, and relatedness tomales (i.e. levels of parental care nearly equal dominantsalso may affect position within ato thatof alphafemales)stronglysuggested
that group hierarchy.
Our division of heirarchiesinto alpha, beta,
they were breeding,but groupswith alpha-2femaleswere relatively rare.The mostcommon and gammalevelswassupportedby differencesgroupstructurecontaineda betaadultmale,but in nestingrolesamonglevels.Theselevelsrepparentalbehaviorby mostbetamalesdid not resentat least2 stagesof helping. The gamma
suggestgenetic investmentin broods.Assum- level is composedof immaturesand adults ofing that paternityof youngin a broodcouldbe both sexes,mostof which are offspringhelping
TABLE
6. Responses
of Harris'Hawksin theChollaGroveGroup,1986,to GreatHornedOwlsin the nestarea(n = 34incidentsof trespassing
by GreatHornedOwls,151hoursof observation).
Abbreviations:
Adult= A, immature = I, male = M, and female = F.)

Percentageof nActiveDominance
status Age/sex

Initialna

responseb

attack
•

Chase
d

Alpha

A/F

34

26.5

14.7

26.5

11.8

Beta
Gamma

r\/M
A/M
A/M
I/F

27
24
9
6

100.0
100.0
77.7
66.6

55.5
50.0
22.2
0.0

100.0
100.0
55.5
50.0

25.9
8.3
0.0
0.0

Number of observationsof owl harassmentin which a member was present.
Movement from a perch toward an owl.
The first closingflight toward an owl.

Chaseswere often performedby > 1 hawk per observation.
Any physicalcontactbetweena hawk and an owl.

Strike'July1991]

Dominance
andHelping
in a Hawk

their parents.All offspringapparentlydispersefrom their natalrangesby their fourth year,andyoung Harris' Hawks do not appear to use inheritanceof the natalterritoryasthe usualrouteto breeding.
We suggestthat gamma helpers, many ofwhich are offspringof alpha members,do not
actively pursue a breeding position in natal

657

groups or territories. Gamma hawks showed acomplete lack of sexualbehavior and immature

pation in nestingactivities(i.e. exposureto eggs
and beggingyoung) maybe necessaryto induce
breeding condition in helpers. This idea has
been proposed in various forms to account for
copulationsby female breederswith male helpers in several species(Dow 1978; Craig 1980;
Stacey1979;Emlen 1982, 1984). PerhapsHarris'
Hawk breedersmust allow helpersto occasionally enter the nest to encouragehelper participation in providing food to the nest and inhelpers,all of which were gammahawks,have

territorial

defense.

Partialexclusion

from

the

levels of breeding hormoneslower than those nest may prevent helpers from attaining fullof adult helpers,manyof which werebetamales breeding condition. A helper in full breeding(Mays et al. 1991).Intoleranceby alpha males conditioncouldhavenegativeeffectson reproof gammahawksnear the nestfurther suggests duction through increasedintragroup conflictthat breedingis unlikely at the gammalevel. It or emigration from the group (Vehrencamp
is possiblethat gamma hawks remain on natal. 1977).
Differential
tolerance in the nest and differterritoriesuntil a positionin an unrelatedgroup
canbe found. Dispersalfrom their natal groups encesin levels of participation in parental bemay representthe shortestroute to breeding haviorsby betaand gammahawkslend support
for youngHarris'Hawks,a relativelylong-lived to breeder manipulation of helpers. Betamales
specieswith stablebreedinggroupsover time. are allowed greateraccessto the nestthan gamAlternatively, dispersalpatterns in the Harris' ma hawks,and are toleratedby alpha malesand
Hawk may be due to selectionto avoid inbreed- females during attempted copulations. Beta
ing (Mays et al. 1991).
malesalsoshow higher participationin coopIn contrast,alpha-2 females, beta males, and erativehunting, foodtransport,and harassment
somegamma hawks are probably unrelated to of predators. In monogamousgroups, alpha
the alpha pair. From nestingroles,sexualbe- males may sacrificelittle in terms of exclusive
havior, and instancesin which helpersascend- paternity by allowing beta males to engage in
ed in hierarchies (Dawson unpubl. data), we copulatorybehavior. Despite the lack of matesuggestthat extra adults at the alpha and beta guarding by males, confidenceof paternity bylevels activelypursuea breeding positionwith- alpha males may be maintained via noncoopin groups.
eration by alpha femalesduring copulatoryatDominanceandhelpercontributions.--We
found tempts. As in mate-sharing speciesin which

thatHarris'Hawk helpersactivelycapturedand

male dominance is the rule, the outcome of ex-

transportedprey to the nest,harassedpredators, trapair copulatorybehavior in Harris' Hawks isand defended the territory from conspecific controlledby the most dominant membersof
trespassers(Dawson and Mannan 1991). Whengroups--alpha females.
helpersattemptedto participatein parental-type
Betamalesmay increasetheir statusby formbehaviorsat the nest,they werepreventedfrom ing the beginnings of pair bonds with alphadoing so by the dominant breeders.
females.Rudimentary pair bonds may allow a
These observations are consistent with conbeta male to more quickly establisha breeding
ceptsof socialmanipulationof subordinatesby relationship with the alpha female when thebreedersor parents.Suppressionof breedingin alphamaleis lostfrom the group.Thismayoffer
helpershasbeen a prominent conceptin mod- advantagesin a speciesthat can breed yearels of the evolution of sociality(Michener and round, often producesseveralbroodsper year,
Brothers 1974, Alexander 1974, Emlen 1982,

can initiate

anotherbroodless thana monthVehrencamp1983)and hasbeen suggestedin after the first has fledged, and has a sex ratio
severalspeciesthat breedcooperatively(Reyer skewedtoward adult males.Also, the presence
et al. 1986, Vehrencamp 1977, Carrick 1972).
of cooperativepolyandry in somegroupssugCopulatory behavior by Harris' Hawk beta geststhat the costsof lost reproductive oppormalesimplies that they are physiologicallyca- tunities to a male serving in a beta position may
pable of breeding,a suggestionsupportedby be mitigated by mate sharing.
hormonalevidence(Mays et al. 1991).ParticiExclusionof helpersfrom mostparental-type

658

DAWSONANDMANNAN[Auk, Vol. 108

behaviorswould appearto reducepotentialfit- Purple Gallinule (Porphyrulamartinica),and thenessbenefitsto breedersthat could be gained ScrubJay (Rabenold 1984, Hunter 1985, Woolthroughincreasedcareof the offspringor "load- fenden 1978). Mader (1979) proposedthat thesharing" of nest duties among all membersof load sharingof hunting dutiesby a third adulta group (Brown 1987).In particular,allofeeding Harris' Hawk may increasenest attentiveness(feeding of nestlingsby a nonparent) is com- and defenseagainstpredators,although he did
mon among cooperativebreeders(Brown 1987, not identify predationas a significantfactorof
Skutch 1987)yet it is conspicuouslylacking in nestling mortality. We found that Great Horned
Harris'Hawks.

Owls posed the greatestthreat of predation to

Allofeedingmaynot be importantin a species youngHarris' Hawks in our studyarea.We sugthat relies on relatively large prey that are dif- gestthat hawks were effective in driving owls
ficult to capture. Harris' Hawks in Arizona use from the nest area during the day but wereprey speciesthat range widely in size, but rely unable to prevent owls from taking young at
heavily on 2 relatively large species--thedesert night. We alsofound evidencethat mostyoung
cottontail and Gambel's Quail (Callipeplagam- hawkswere killed during the late nestlingand
belii). Insectivorousbirds that are cooperative early fledgling stages.We suspectthat young
breedersrely on numerous small prey items to hawks were most vulnerable to predation by
feed nestlings.Stallcupand Woolfenden(1978) owlsduring this periodbecausehawksstopped
found that feeding rates in the Scrub Jay av- broodingnestlingsat night about 1 week before
eraged8 per hour. Fooddelivery ratesfor other fledging. Likewise, recently fledged hawks
speciesrange from 3 per hour in the Kooka- roosted alone in small trees near the nest andburra(Dacelogigas)to > 20 per hour in the Noisy were not yet skilled enough at flight to evade
Miner (Manorina melanocephala)and Green owls.
Woodhoopoe(Phoeniculus
purpureus)(Parry 1973,
We speculatethat harassmentby groups ofDow 1970, Grimes 1975). In thesespecies,the hawks may influence the choiceof a nest siteenergy coststo breedersof entering the nest by owls in relation to an activehawk territory.
and placingfood in the mouth of a nestlingon The aggressivenature of harassmentsuggests
each delivery must be high. Helpers usually that owls could be injured or killed by hawks,
make significant contributions to this chore although we did not seethis during our study.
(Stallcup and Woolfenden 1978, Brown 1972, Thesefactorsmight reducethe threat posedby
Rabenold 1984).
owlsin the territorybeforehawk nestlingsreach
In contrast, Harris' Hawks make fewer delivthe vulnerablestageof development.The preseries(ca.5 per day) but deliver a large amount ence of helpers probably increasedthe likeliof food each time. Often an entire brood can behood that predatorswere detected in the tersatiated with food contained in one food delivritory and increasedthe durationand intensity
ery. Becausenestlings can be fed a number of of harassment (Woolfenden and Fitzpatricktimes from a single prey item and only a few 1984, Brown 1987).
deliveriesare madeeachday, the taskof tearing
We proposedelsewherethat group living infood into small piecesand placing it into the the Harris' Hawk is related to an ecologicalconmouth of a nestling can be accomplishedfor straint (Koenig and Mumme 1987) imposedby
the entire broodby a single adult. Furthermore, the scarcityof open water sourcesin the desert
nestlings can eat unaided by an adult at 1.5-2.5(Dawson and Mannan 1991). The behavior ofweeksof age (nestling period = ca. 6.4 weeks) hawks at the nest suggeststhe helpers may inand food is often placed in the nest without fluencereproductivesuccessin at least 3 ways.
being fed to the nestlings.Consequently,al- First, the survival of nestling and fledgling
lofeeding would probably have little effect on hawks may be increasedthrough predator dethe amountof food or the frequencyof nestling terrence. Second,helpers also defend the termeals. Both factors may be influenced by the ritory from conspecifics(Dawsonand Mannan
effortsof helpersin capturingand transporting 1991). Third, the efforts of helpers to procure
prey to the nest (Mader 1975a,Brown 1987).
and deliver food to the nest area may increase
Predator detection and deterrence.--Increased
the amount and frequencyof deliveriesto theprotectionfrom predatorshasbeen proposedas nest area (Mader 1975a,Brown 1987). Coopera benefitof cooperativebreedingin the Striped- ative hunting, firstreportedfor the Harris' Hawk
backed Wren (Campylorhynchus
nuchalis),the by Mader (1975a),is generally more successful

July1991]

Dominance
andHelping
ina Hawk

659

than hunting alone (Bednarz 1988, Dawson BROWN,J. L. 1972. Communal feeding of nestlingsin the Mexican Jay (Aphelocoma
ultamarina):in1988). Whether or not this activity results in
terflockcomparisons.
Anim. Behav.18: 366-378.
increasedfood suppliesat the nest has yet to1978. Avian communal breeding systems.
be established.It is possiblethat the benefitsof
Annu. Rev. Ecol. Syst.9: 123-155.
cooperativehunting are accruedprimarily by
1987. Helping and communalbreeding in
the individual hawks in a successfulhunting

birds. Princeton, Princeton Univ. Press.

party (Bednarz1988,Bednarzand Ligon 1989). CARI•ICK,
R. 1972. Populationecologyof the AusIn thissituation,reproductiveenhancementmay

tralianBlack-backed
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occurafter offspringleavethe nestand become
41-99.
proficientenoughat flight to follow andeventually join hunting parties.Offspringsurvival CR•IG,J.L. 1980. Pairandgroupbreedingbehaviourof a communalgallinule,the Pukeko,Porphyrio
may be higher for young that hunt (i.e. share
p. melanotus.
Anim. Behav.28: 593-603.
kills) with oldergroupmembersduring the peDAWSON,
J.W. 1988. The cooperativebreeding sysriod after activeprovisioninghas stoppedandtem of the Harris' Hawk in Arizona. M.S. thesis,
before young have developedadequatehuntTucson, Univ. Arizona.
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two methods of estimatingbreeding group size
ACKNOWLEDGMENTS

in Harris'--,&

Hawks.

Auk 106: 480-483.

. 1991. The role of territoriality inthe socialorganizationof Harris' Hawks. Auk
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Smith, and L. SowIs generouslyallowed the use of Dow, D. D. 1970. Communal behavior of nesting
Noisy Miners. Emu 70: 131-134.
facilitiesand equipmentof the Arizona Cooperative
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vided logisticalsupport in the field. We thank N.
Mays and D. Lett for their individual effortsto color- EMrEN,S.T. 1982. The evolution of helping. II. Theband hawksand V. Meretskyfor help with dataanalysis.T. HueIs, N. Smith, N. Snyder,C. Vleck, and two
anonymous reviewers commented critically on the

manuscript.Funding was provided by grantsfrom
the Nongame Branchof the Arizona Game and Fish
Department, Raptor EducationFoundation, Arizona
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tion, and National Science Foundation grant BNS8606548

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