PLASMA LUTEINIZING BEHAVIORAL COOPERATIVELY HORMONE, STEROID HORMONES, ROLE, AND NEST STAGE IN BREEDING HARRIS' HAWKS (PARABUTEO UNICINCTUS) NORA A. MAYS,• CAROL MASTERSVLECK,1 AND JAMESDAWSON2 •Departmentof EcologyandEvolutionary Biology,Universityof Arizona, Tucson, Arizona 85721 USA, and 2School of Renewable NaturalResources, Universityof Arizona,Tucson, Arizona85721USA ABSTRACT.--We measuredplasmalevels of testosterone(T), estradiol 17fl(E), corticosterone, and luteinizing hormone(LH) in adult breeders,adult-plumagedhelpers,and juvenal-plumagedhelpersof the cooperativelybreedingHarris' Hawk (Parabuteo unicinctus) in southeastern Arizona. In the males,both adult breedersand mostadult-plumagedhelpershad elevatedT and LH during February and March, when nest building occurs,whereasmost juvenalplumagedhelpersshowedno changein LH, T, or E throughouttheyear.Thusadult-plumaged helpers,usuallynot relatedto the breeders,are hormonallyreadyto breedin the spring.We suggestthat thesehelpersattainbreedingreadinessbecause potentialbenefitsthey canderive from either occasionalsuccessful copulationsor possibleattainmentof the breedingposition make reproductivereadinessadvantageous. In contrast,mostjuvenal-plumagedhelpersare aiding their own parentson their natal territory and are not hormonallyreadyto breed,and thus they physiologicallyavoid the potential for inbreeding. An exceptionto this was a juvenal-plumagedmale helper with elevatedtestosteronelevels who was not helping his parents.The female breederswe examinedhad elevatedT, LH, and E during the nest-building period, whereasthe helpers(both adult- and juvenal-plumaged)did not show elevationsat any time of the year. During the early part of the nestingcycle,the adult-plumagedfemale helpersweighed significantlyless(ca. 10%)than the breeders.It is possiblethat poor body condition,subordinatestatusto the breedingfemales,or both were related to their lack of breedingreadiness.In all birds,corticosterone levelsincreasedwith handling time, andwere higherduring the nestingperiodsthan during non-nestingperiods,but showedno consistent relationwith helper or breederstatus.As in other speciesthat are relatively social,maintain long-term pair bonds, or are tropical in distribution, plasmaconcentrationsof LH and sex steroidsin Harris' Hawks, even when elevated,were relatively low. Received 10 May 1990, accepted13 January1991. COOPERATIVE breeding, in which more than breeding(Ligon et al. 1988).In this study we two adultsassistin raisingyoung, occursin at present endocrine evidence for reproductive least222 speciesof birds and is taxonomically readinessin breedersand helpersin the Harris' widespread among altricial species (Brown Hawk (Parabuteo unicinctus) and discusspossible 1987).Although the evolutionaryoriginsof co- proximate and ultimate causesfor lack of breedoperativebreedingsystemsremain unclear (see ing in helpers.In an accompanying paper(Vleck Brown 1983, Emlen 1984, Jamiesonand Craig et al. 1991),we discussthe physiologicalbasis 1987, Jamieson1989) many authors have dis- of parental or helping behavior in the Harris' Hawk. cussedthe ecologicalfactorsthat promote the developmentof cooperativebreedingin differHarris'Hawk breeding biology.--Harris'Hawks ent species(e.g. Emlen and Vehrencamp 1983) breed cooperativelyin New Mexico, Texas,and and the potentialadvantagesaccruedby non- Arizona (Mader 1975a, Griffin 1976, Brannon breeding helpers (e.g. Ligon and Ligon 1978; 1980,Whaley 1986,Bednarz1987a,Dawsonand Woolfenden and Fitzpatrick 1978, 1984;Brown Mannan 1989). The mean number of hawks asand Brown 1981). Few studies, however, have addressedeither the physiologicalbasisof this type of behavior (Reyer et al. 1986, Wingfield et al. in press,Schoechet al. 1991)or physiological constraints involved in cooperative 619 sociatedwith a nest in the population we studied was 3.8 (+1.3 SD) and ranged from 2 to 7 birds.Harris' Hawks may be polyandrous(Mader 1979), but behavioral and electrophoretic evidencesuggeststhat groupsgenerallyconsist The Auk 108:619-637. July 1991 620 M^¾s,VLECK, ,•'qDD^WSON of 1 breeding pair and 1-5 nonreproducingbirds (Bednarz 1987a; Dawson and Mannan 1991a, b) [Auk, Vol. 108 son and Mannan 1991a). Approximately three fourths of observedjuvenal-plumagedgamma that may or may not be relatedto the breeders. DNA analysisof all membersof severalHarris' Hawk groupsindicatesthat polygynyand polyandry, as well as monogamy,occur in the Arizona population (Sheehyet al. MS), but the frequency of each of these mating systemsin the population is unknown. helperswere bandedoffspringof the breeders the groups also have another alpha-level fe- persedgammahelpers.In Arizona <6% of female offspring remain on natal territories by they were helping, and about one fourth were immigrants. Furthermore, no birds banded in the nest were observed to attain alpha or beta statuswithin their natal group or territory, although three males were observed to do so within unrelated groups. Adult male helpers The behavioral roles of different members of can be older than 4 yr; three adult beta helpers the group have been describedin detail else- on the study area had been banded more than where (Dawson and Mannan 1991a) for this Ar8 yr previously by earlier workers (Dawson unizona population. The dominant pair in each publ. data). We did not divide our helpers into beta and group (termedthe alpha male and female)progamma types becausewe did not always have vides direct care of the eggs and young, including nest building, egg laying, incubation enough information to do so. Rather, we clasand brooding, and shading and feeding the sified them as either in adult plumage or in nestlings.The dominant pair also participates juvenal plumage (Table 1). Basedon the above in group hunting although the alpha female information, however, the probability that our rarely leavesthe nest area during the breeding adult-plumaged male helpers were of the unseasonand is usually supplied with food by the related, beta type is ca. 80%; the other 20% of alpha male or helpers. (Approximately 9% of adult-plumaged helpers were probably nondismale, an alpha-2 bird, but none of the females we capturedin this study were of this type.) In Harris' Hawks, alpha breeders remain on the sameterritory year after year; thus helpers that remain on natal territoriesare likely to be help- ing their parentsand those that disperseare likely to be helping birdsthat are not their parents. The helpers' major contributionsto the breedingeffortare to captureand transportprey to the nest area, detect and harass predators, and help with defenseof the nesting territory. Bednarz (1988) and Bednarz and Ligon (1988) suggestedthat cooperativehunting may be the basisfor the social organization of the Harris' Hawk and participationin hunting may be one of the largestcontributionsthat helpersmake to the breeding effort. Based on behavior, Dawson and Mannan (1991a)distinguishtwo typesof helpersin Harris' Hawks: beta malesand gammamalesor females. They find that approximately 80% of adult-plumaged male helpers are beta birds their second year (after molting into adult plumage),and none remain by their third year (Dawson and Mannan 1991a). Thus, the adult femalehelperswe caughtwere even lesslikely to havebeen offspringof the breedersthey were helping. All of our juvenal-plumagedbirdswere gammahelpers (no juvenal-plumagedbeta birds have been observed). Approximately 75% of them were probably helping their parents,and 25% were not. Endocrinemechanisms and lack of breedingin helpers.--Elevationsin reproductive hormones are commonly interpreted as an indication of readinessto breed. In all seasonallybreeding birds in which annual cycles in reproductive hormones have been studied, there are seasonal increasesin plasmalevels of gonadotropinand sexsteroidsassociated with gonadalmaturation or recrudescence(Wingfield and Farner 1980, Wingfield and Moore 1987). The extent of the elevation varies between species,however, and hormone levels often decline after the initia- (Dawson and Mannan 1991a: table 3) and are not related to the breeders.Gamma helpers may be in adult plumage (•22%) but are more frequently in juvenal plumage (• 78%).They may tion of breeding cycles.If gonadotropin and sex steroid levels in a speciesare not elevated in helpers at the same time that they are elevated be either male (•63%) in breeders, then it seems reasonable that lack or female (•37%) and of breeding in helpersmust be causallyrelated, at least in part, to this hormonal difference. We use analysesof hormonesknown to be of helpers comesfrom 4 yr of group-composition analysis,banding,and dispersaldata(Daw- involved in avian breeding cyclesand acom- are usuallyoffspringof the breeding pair. The evidence for the relatedness of these two types July1991] Hormones inCooperative Breeders 621 TABLE1. Categoriesand sample size for each sex of the compositevariable called nestrole used in the statisticalanalysisof hormone levels in Harris' Hawks. The 12 categoriesof nest role for each sex are determinedby the individual'sbehavioralrole and stageof the nestwhen the bloodsamplewas taken. Behavioral Adult breeders Nest stage M F Nonbreeding Nest building -5 Adult-plumaged helpers role Juvenal-plumaged helpers M F M F 4 10 3 7 I 5 3 4 Total M F 1 9 6 16 6 24 Incubation 9 7 3 1 1 1 13 9 Feeding young 9 12 6 1 2 1 17 14 19 8 10 12 Total 23 33 capturedbirds with a heparinized syringe and 25gauge %-in needle. Handling time was then calculated as maximum possible time on trap plus processingtime. The mean maximum handling time was 70 +- 61 min. Bloodwas stored on ice in heparinized test tubesuntil the end of the field day when it was tradiol-17•3 (E), and corticosterone vary with centrifuged.The plasmawasthen removedand stored season, behavior, social status and relatedness at -20øC until analyzed. amongindividualHarris'Hawks.We measured We banded each bird with a unique combination parisonof levelsbetweenhelpersand breeders to investigatewhether or not Harris' Hawk helpersare physiologicallyreadyto breed.We describethe way that plasmaconcentrationsof luteinizing hormone(LH), testosterone (T), es- corticosterone levels as an indicator of stress (e.g. Wingfield, Smith, and Farner 1982, Deviche 1983,Harvey et al. 1984,Wingfield 1985, Wingfield and Silverin 1986),in order to explore the possibilitythat lack of breedingby helpers is due to high stresslevels associated with their subordinate social status. METHODS Studyareas.--Thisstudy was conductedfrom May 1985throughAugust1987on an approx.200-km2site north of Tucson, Arizona, and south of Florence, Ar- izona. Approx. 25% of the site overlapsthat used by Dawson and Mannan (1991a, b). Harris' Hawks com- monlynestin saguarocactusandpaloverdeandmesquite treesin this habitat. Fieldmeasurements.--To capture 157 Harris' Hawks (n = 157),we set a balchatri trap (Bergerand Mueller 1959)near a perchedbird and watchedfrom 300-400 m until the bird wascaught,or we stakedtrapswithin 200 m of active nestsand checkedtraps every 1-2 h. Birdsusuallydonot hunt when closelyobservedand will not approacha trap if humansare within sight. We were thus forced to remain away from the traps for long periodsof time to increasethe probability of capturinga bird. The maximum possibletime a bird could have been ensnaredon a trap was recorded as time elapsedsincethe trap was set until the bird was removed from the trap. Mean (+SD) maximum time birds spenton the trap was55 +58 min (median time = 30 min). We withdrew 3 ml of blood from a wing vein of of three colored bands and a numbered aluminum USFWSserviceband (permit no. 09335).Morphological measurements were taken, and each bird was weighed. A 2,000-g pesolascalewas used to weigh eachbird to +_25g. To assessthe breeding condition of males, we examined the cloacalcontentsof 26 males for the presence or absenceof sperm. The cloacalcontentswere extruded onto a slide (Bird et al. 1976), the slide was air-dried and later examinedfor the presenceof sperm under a phase contrast light microscope.We found spermin 4 of 8 breedingmalesand no spermin the cloacalcontentsof 7 adult helpers, 4 juvenal-plumagedhelpers,and 7 maleswhoserole wasunknown. We believe, however, that detectionof cloacalsperm in Harris' Hawks by the "stripping" techniquemay reflectonly the remnantsof a recentsperm ejaculation, rather than evidence of spermatogenesis.For instance, we found no sperm in the cloaca of one alpha breeder5 daysbeforethe first egg. This male was the only male at a nest lacking helpers,and the eggsin the nest hatched.When sperm were present, we found only a few per microliter.Othershave also noted low levelsof spermin cloacalsamplescollected from breeding raptors(Corten 1973). Age andsexof individuals.--Based on their appearance, we identified banded hawks as belonging to one of two agecategories:adult or juvenal-plumaged. The juvenal plumage is retained through one breeding seasonafter fledging (Brown and Amadon 1968). The sexesare monomorphic in plumage coloration. We assigneda sexto eachindividual in the field (after separationby age) on the basisof mass(Hamerstrom and Hamerstrom 1978). Mass of adult males (range: 622 MAYS,VI•EClC, ANI• DAWSON 610-900g;• = 726g) overlappedthat of adult females (range:900-1,225g; • = 1,046g) in only one male. The next largestmale weighed 825 g, and the sexof the one unusuallylarge male was supportedusing principalcomponents analysisof sevenmorphological measurements(Mays 1989).The massof juvenal- plumagedmales(range:650-800g; œ= 706 g) did not overlapthat of juvenal-plumagedfemales(range: 850-1,086 g; • = 942 g). Field observations.--We identified a hawk as a mem- ber of a groupif it wastrappedor seenwithin 300 m of an active nest. Harris' Hawks actively expel trespassersfrom within at least 500 m of an active nest (Dawson and Mannan 1991b);therefore trapping a nonmember within this zone is unlikely. Most behavioral information on color-marked birds [Auk, Vol. 108 45 dayspost-hatch(Mader 1975a),and group memberscontinueto feed the young for severalmonths after fledging (pets.obs.). Hormone assays.--Testosterone (T) and Estradiol-17/• (E) in 200 •1 plasmawere assayedafter extractionin 2.5 ml diethyl etherand chromatographic separation on celite columnsby radioimmunoassay (RIA) fol- lowingthe methodsof Abraham(1974)with modificationsdescribedby Wingfield and Farner (1975) for usewith avianplasma.For the corticosterone RIA, the hormonewasextractedfrom 30 •1 plasmain 3 ml dichloromethane,and it was then assayedin duplicate. Least detectableconcentrationswere 15 pg/ml for T, 1 ng/ml for corticosterone, and 12 pg/ml for E.Interassay variabilityandintraassay variability(SD/ mean x 100) for T were 10.8%and 7.0%; for cortico- was gatheredfrom fully enclosed,elevatedblinds, sterone, 7.0% and 10.9%;and for E, 23.2% and 8.7%, respectively. We assayedluteinizing hormone (LH) in the laboratoryof JohnWingfield (Univ. Washington,Seattle, Washington)with a double-antibody,postprecipita9 x binocularsto identify and observethe behavior tion RIA for avian LH developedby Follett et al. of marked birds. We recordedbehavioral activity of (1972) and modified by Follett et al. (1975). Luteinmarked individuals for a minimum of 10 h at each izing antiserum(anti 3/ 3) andLH standard(PRC-AEInest (see Dawson and Mannan 1989, 1991a, b). At I) were kindly provided by Peter Sharp (Poultry Resomenests,we useda fully enclosedcloth blind erect- searchCentre, Roslin Edinburgh). The least detected on the groundwithin 10 m of active nestsand able concentrationwas approx. 0.1 ng/mL Statistical analysis ofeffectofseason.--Allvalueswere recordedbehavioralactivityof markedindividualsat the nestfor 3-16 h per nest.Behavioralobservations first transformedwith natural logs to minimize the at 47 different nests allowed us to identify the be- effects of outliers and normalize the data. Statistical havioral role (breeder or helper) of 105 of the 157 tests were performed using general linear models trappedindividualsfor whomwe hadbloodsamples. contained in SAS statisticalpackage(SAS 1984). To Neststage.--Weclassifiedthe nest stagefor each elucidateseasonalpatternsin hormonelevels,we did hawk for the time the blood sample was taken by three-wayANOVAs for eachhormoneafter separatcountingforward or backwardfrom the estimated ing samplesby sex.To testfor the effectof ageof the dateof the first-laidegg.The dateof the first-laidegg bird, month of year, and maximumpossiblehandling was either observeddirectly, calculatedfrom obser- time on hormone levels, we used samplesfrom all vations of the first-hatched chick, or calculated from yearscombined(Table2A). All 157samplescouldbe estimatesof the ageof the nestlingsbasedon plumage used for this analysis.Two-way ANOVAs were perdevelopment(Bednarz1987a).Nest building begins formed (after separationof the data by sexand age) approx.36daysbeforeegglaying(Mader1975b).De- to determinewhether valueschangedsignificantlyin fense of a nest site from human intruders (circling a given month from the previousmonth. Handling and screaming)beginsapproximatelythe sametime time was included in all two- and three-way ANOVAs as nestbuilding, and we observedit a maximumof becausehandling times were long. A pilot study 66 daysbeforethe first-laidegg(œ= 32 + 16 days,n showed that testosterone and corticosterone levels in = 14 nests).We used the mean time of nest-sitede- serial samplestend to increasewith handling time, fenseplusoneSD to indicatethe earlybeginningof althoughestradioland luteinizinghormonelevelsdo the nestingcycle.Thus samplescollected>50 days not change(Maysand Vleck 1987;unpubl. data).We beforethe first-laidegg were consideredto be from excludedhandling time from one-wayANOVAs only the nonbreedingstageof the annualcycles(approx- after it showedno significanteffectin two- or threeimatelyAugustthroughearlyJanuary).Samplescol- way ANOVAs. Statistical analysis ofeffectofnestrole.--Harris'Hawks lected50 daysbeforethe first egg up to the dateof the first egg were consideredto be from the nest- do not breed synchronously;first-eggdatesranged buildingand gamete-production stage.Samplescol- from 21 Februarythrough5 May. Hormonelevelsare with neststagethan lectedfrom the date of the first-laidegg to 35 days probablymorecloselycorrelated later were from the incubationstage(Mader 1975b), with day of the year. Thus,we analyzedseparately and thosecollectedfrom 36 to 100daysafter the first- the data from the 105 hawks for which we knew the laid eggwerefrom the stagewhenyoungwerebeing behavioralrole and the stageof its associatednest. fed. The chicksremain in or near the nestfor approx. Due to the incompletedesigninherent in our study, which were erected3-10 m from the nest gradually over a period of severaldays.Blindswere entered either before dawn or by two people,one of whom later left the area.We useda 20 x spottingscopeand July1991] Hormones in Cooperative Breeders we designateda compositevariable that we called nest role for use in the ANOVA. variable based on the combined We defined behavioral this role of the individual at the nest, the stageof the nest,and the ageof the bird. Nest role separatesthe valuesfor each sexinto 12 possiblecategories;samplesizesfor each nest role are shown in Table 1. For the 105 hawks for whom we knew nest role, we examined the effects of sex, nest role, and han- dling time on the levels of eachof the hormoneswith a three-way ANOVA (statisticsnot shown).After separation by sex,we testedthe 105 samplesfor the combined effectsof nest role and handling time with a two-way ANOVA (Table 2B). For casesin which handling time showedno effect,we did one-wayANO- VAs for eachrole category,to test whether means differedbetweenneststagesfor eachbehavioralrole (Table 2C). One-way ANOVAs were also done for eachnest stage,to test whether hormone levelsdifferedbetweenrolesduring the sameneststage(Table 2D). Eachvariablethat showeda significanteffect(P < 0.05) wasfurther examinedusingBonferronimultiple-comparisontests(BON) to indicatewhich means differed (SAS 1984). RESULTS TESTOSTERONE Effectof season in males.--MeanplasmaT concentrationsvaried significantly by month in maleHarris Hawks(Fig. la), but handlingtime did not significantlyaffectT levels (Table 2A). In adult-plumagedmales exclusively,mean T levels differed significantlybetween months (two-way ANOVA, F = 4.94, P = 0.007, n = 33). Levelsrose during Februaryand remained elevatedduring March (nest-buildingperiod for mostbirds).Levelsof T declinedin April, which coincidedwith incubationat mostnests.In juvenal-plumagedmales,T levels did not vary amongmonths.Adult levels were higher than those in juvenal-plumagedbirds, but the differencesin T betweenagegroupsdid not quite 623 egg date for both study sites and all 3 yr was 30 March (+ 17 days,n = 113nests)and previous studies(Balthazart 1983;Wingfield and Farner 1978a,b; Fivizzani and Oring 1986; Wingfield and Moore 1987;Wingfield 1984a)have shown that thesetwo hormonesare highestin the period before egg laying in most species.During March, T levels were significantly higher in adult males than in juvenal-plumaged males (one-way ANOVA, F = 5.68, P = 0.038, n = 12). Effectof nestrolein males.--AverageplasmaT levelsdifferedsignificantlydependingon nest roles (Fig. lb, Table 2B). In adult breeders,T was significantlyhigher during the nest-building stagethan during incubation or feeding of young (BON, t = 2.63, P < 0.05, n = 22). In the adult helpers, T was also significantlyhigher during the nest-building stagethan during incubation (BON, t = 3.07, P < 0.05, n = 18). There were no statisticallysignificant changesin T betweenneststagesin juvenal-plumagedhelpers (Table 2C). Sample sizeswere small for juvenal-plumaged helpers (total n = 10), but T levels were low in all but the individual who was not on his natal territory (Fig. lb). We also examinedthe effect of role during each nest stage (Table 2D). During the nestbuilding stage, T levels in breeder and adult helper were similar to each other and higher thanjuvenal-plumaged helpers,but this differencewas not significant.Excludingthe T value for the one male helper that was not on his parents' territory decreasedthe mean T value for juvenal-plumaged helpers during nest building, and the differencesin breedersand adult helpersfrom the threeremainingjuvenalplumagedhelpersapproachedsignificance(F = 2.24, P = 0.08, n = 14). Nest role did have a significanteffectin the two-wayANOVA (Table 2B), so the inability of the one-way ANOVA to detect significant differences between the groupsduring the nest-buildingstagewaspresumablydue to smallsamplesizesthat resulted from partitioning the databy neststages. Effectof season in females.--Levelsof T in the 82 femalesdiffered significantlybetween age groupsand months,and due to handling time (Table2A); averageT levelswere lower than in males(three-wayANOVA, F = 10.66,P = 0.002, reach significance(Table 2A). One juvenalplumagedmale plotted individually (Fig. la) hadan elevatedT level latein February,30 days beforethe first eggwaslaid in the nestat which he helped.This individualwas the only juvenal-plumagedmale sampledduring the nestbuilding stagethat wasknown to be helpingat a nest not belonging to his parents.The other n = 101). In adult females, T levels differed three had been bandedin the previousyears' significantlyamongmonths(two-wayANOVA, nestsof the birdsthey were currentlyhelping. F = 2.94,P = 0.005,n = 67),rising during March We examined the male T and LH data from (Fig. lc), but there were no significantchanges March in more detail becausethe averagefirst- betweenmonthsin juvenal-plumagedfemales. 624 MAYS,VLœClC, ANDDAWSON [Auk,Vol. 108 v v v ddddddd July1991] Hormones inCooperative Breeders 625 Levels ofT in adultS females weresignificantly higherthan thosein juvenal-plumaged females (Table 2A). Effectof nestrole in females.In femalesthere were also significantdifferencesin plasma T levels between nest roles and due to handling time (Table2B).Breedingadult femaleshad the highestT levels during the nest-building stage (Fig. ld). We think that the higher levels of T in breedingfemaleswhen comparedwith helper femalesduring nestbuilding are unlikely to be solelydue to handlingtime becausethe mean handling time for the adult breeding females during this nest stagewas less (œ= 62 min, n = 10) than that for the adult helper females (œ = 90 min, n = 5), and similar to that for the juvenal-plumagedhelper females(œ= 57 min, n = 9). Handling time (maximum time on trap plus processingtime) significantlyaffectedT levels in females (Table 2: A and C), but not in males. Previously, we reported that T levels increase with handling time in serially sampledHarris' Hawk males(Mays and Vleck 1987). Stressdue to handling time may causean adrenalcortical release of T as well as corticosterone. The max- imum T valuesresultingfrom this stress-related releasearean orderof magnitudelessthan those in males during nest building and territorial defense,so it is not surprisingthat there were no statisticaleffectsof handling time on male T, but there were effects on female T. ESTRADIOL Effectofseason infemales.--MeanplasmaE lev- els variedsignificantlyover the year in female Harris Hawks (Fig. 2a) and were significantly higher in adultsthan in juvenal-plumaged birds (Table 2A). Plasma E levels in adults rose in ¸ o March just before egg laying, but not significantly so, whereas in juvenal-plumagedbirds, there were no changes in levels during the months when birds were sampled. Effectof nestroleinfemales.--Nestrole, but not handling time, had a significanteffecton E levelsin females(Table2B,Fig. 2b).In adult breeders, E levels were significantlyhigher during the nest-buildingstagethan during incubation and feeding of young and were lower during feeding of the young than during the nonbreeding stage(BON, t = 2.83, P < 0.05, n = 626 MAYS,VLECK, ANDDAWSON [Auk, Vol. 108 Males C & r• Adult Juv 800 4oo O N Non- Breedin O J F M Nest A Building d .... J J A O N D J F M A M' J J Feeding bating , .i6o' ' ' •s'o.... M Incu- Young Breeding Buildin bating Young , 6'0' ' ' i'o Time relative to first •99, da•s .•oo .so o so •oo Time relative to first •99• Fig. 1. Meanplasmatestosterone levelsin Harris'Hawk males(a andb) and females(c andd). The top panelsindicatethemeanlevelforall adult-plumaged andjuvenal-plumaged birdsin eachmonth.Thebottom panelsplotthe meanvaluesd•ing eachneststagefor thoseindividuals forwhomnest-role is known.The nonbreeding se•on extends&omapproximately AugustthroughearlyJanua•.Ad Br = adultbreeder,Ad H = adult-plumaged helper,andJuvH = juvenal-plumaged helper.Errorbarsindicateß 1 SE.Lackof error barsindicatesno variance,sometimes becausen = 1 (seeTable1).The open•iangle (in a) andclosed•iangle (in b) showthe testosterone v•ue for the one juvenal-plmagedmalehelperwho wasnot on his parent's te•ito•; thisvaluewasnot includedin the meanfor juvenal-plmageda•iliafies for thatinte•al. Within eachneststagethe meanhormonevalueis plottedon the meandayon whichsamples werecollected. 34). Estradiol-17•Slevels did not differ signifi- cantly between nest stagesin either adult or juvenal-plumagedhelpers(Table2C). During the nest-buildingstage,levelsof E in breedingfemaleswere significantlyhigherthan in either adult helpers or juvenal-plumaged helpers(Table 2D, BON, t = 2.60, P < 0.05, n = 24). Adult- and juvenal-plumagedhelper fe- LUTEINIZING HORMONE Effectof season in males.--PlasmaLH levelsin males differed from month to month (Fig. 3a, Table 2A). In adult-plumagedmales,LH rosein January and fell in May (two-way ANOVA, F = 8.54, P = 0.0001, n = 52). In contrast to the stages.Malesexhibitedno variation in E with timeof yearor nestrole,andvalueswerealways seasonalpattern in T, LH levels remained high throughout April (during incubation). Plasma LH did not vary significantly with month in juvenal-plumaged birds, where levels were lower overall than in adults, but not significantly so (Table 2A). In March, during nest building and mateguarding,LH levelsin adults were significantlyhigher than in juvenal-plumaged males (one-way ANOVA, F = 8.26, P = low (Table 2A). 0.017, n = 12). male levelsdid not differ significantlyfrom each other. This is in contrastto the pattern of T in malesin which both breeder and helper adults had higher levelsthan juvenal-plumaged helpers. There were no significant changesin E in femalesof different roles during the other nest July1991] Hormones inCooperative Breeders Effectof nestrolein males.--Plasma LH levels differed significantly between nest roles (Fig. 3b, Table 2B). In adult breeders,LH was significantly higher during nestbuilding and incubation than during feeding of young (Table 2D; BON, t = 2.64, P < 0.05, n = 22). For adult helpers,LH was highest during nest building but not significantly so. There were no significant changesin LH levels of juvenal-plumaged helpers between nest stages.Luteinizing hormone levels in breedersand adult helperswere similar during eachneststageand higher than thosein juvenal-plumagedhelpers(exceptduring feedingof young),but thesedifferenceswere not statisticallysignificant(Table 2D). Overall, LH levels were lower 100' o ß 8O (three-way ANOVA, F = 5.26, P = 0.024, n = Effectof season in females.--Inplasmasamples of females, LH levels differed significantly among months (Fig. 3c), but there were no statistically demonstrabledifferencesdue to age or handling time (Table2A). In adult-plumaged females,LH levelsdifferedamongmonths(twoway ANOVA, F = 2.16, P = 0.036, n = 62); they rose in January and remained relatively high throughApril. The one femalesampledduring November had a high level of LH (1.68 ng/ml), which may indicate an undiscovered autumn nest (Radke and Klimosewski 1977, Bednarz Adult Juv 60' 40' 2O O I N 'D •J ' F ' M 'A ' M'J in males than in females 98). 627 120 Non- Nest Breeding Building T ! b 100 Incubating Feeding Young • • [] Ad Br ' AdH / : 80 • • 40 • • 'J ' A ß auvH 20 0 -' 00 , -50 I 0 - - -'o - 5'0 - 1987b). There was no statisticallysignificant Time relative tr, first egg, days seasonalpattern detectedin the plasmaLH valFig. 2. Meanplasmaestradiollevelsin female ues from juvenal-plumagedfemales,and their Harris'Hawks.The(a)panelindicates themeanlevel levels did not differ significantlyfrom thoseof for all adultsand juvenal-plumaged birds in each adults. month,whereas(b)plotsthemeanvalueduringeach Effectof nestrolein females.--Therewere no neststagefor thoseindividualsfor whomnestrole significantdifferencesin female plasmaLH lev- is known.Symbolsand statistics asin Figure1. els between nest roles or due to handling time (Fig. 3d, Table 2B). In breeding females, LH levelswere significantlyhigher during the nest- there were also significanteffectsof age and building stagethan during feeding of young monthof yearin males(Fig.4a, Table2A). Cor(Table 2C; BON, t = 2.83, P < 0.05, n = 34). ticosteronelevels were generally lower in juAdult breeders had higher levels of LH than venal-plumagedmalesthan in adultsand were either adult or juvenal-plumagedhelpers dur- higher in adults during the breeding season ing all nest stages,but these differenceswere than during the nonbreeding season.In fenot significant(Table 2D). The adult helper fe- males, corticosteronelevels differed signifimales had lower levels of LH than either adult breederor juvenal-plumagedfemalesduring all stagesexceptnonbreeding(Fig. 3d). CORTICOSTERONE cantlyfrommonthto monthandwith handling time (Fig. 4b); however,there were no significant differences between adult- and juvenal- plumagedbirds (Table2A). As with the males, corticosteronelevels in females were higher during the breedingseasonthan during the Effectof season.--Although mostof the variancein corticosteronewas due to handling time, nonbreeding season. Effectof nestrole.--Averageplasmacortico- 628 MAYS,VLECK,ANDDAWSON Males [Auk, Vol. 108 Females 2.0' 2.5 c i• Adult ;] Adult AJuv 2.0 e o o 0,5 e 0.5 o.g O;N•D•J •F •M•A Non2.5 Breeding •M•J Neet Innu- Building batlnu •J •A Neet Incu- BreedingBuildln•lbating Young b Feeding Young 2.0 =AdBr, A Juv H Non- Feeding I : , T {).5' 0.5 0.0 -50 0 50 Time relative to first egg, days Fig.3. Meanplasmaluteinizinghormonelevelsin Harris'Hawk males(a andb) andfemales(c andd) in eachmonth(top)or duringeachneststagefor thoseindividualsfor whom nestrole is known(bottom). Symbolsand statisticsas in Figure 1. sterone levels did not differ significantly between nest roles.The only statisticallysignificant variation in corticosterone levels we could accountfor in this data setwas due to handling time (Table 2B), although the effectof nest role approachedsignificancein males (P = 0.052). Corticosteroneis the major adrenocorticoidin birds associated with stress, and an effect of handling time has been demonstratedin several other species(reviewed in Harvey et al. 1984). BODY MASS affectedbody mass.Breedingadults were significantly heavierthan adult helpers(BON, t = 2.03, P < 0.05). During the nonbreedingand nest-buildingstages,breeding femaleshad a mean massof 1,100 g (n = 10, range:975-1,225 g), but the helper femalesaveragedonly 990 g (n = 7, range:925-1,050g). Aswith males,body massdecreasedthough the breeding seasonin the adult breeding females(Fig. 5b). DISCUSSION INTERSPECIFIC COMPARISONS PlasmaLH levels in breeding male and female Harris' Hawks are generally lower than levels in breeding membersof the samesex in nificant effect (Table 2E). Males were heaviest most other speciesstudied (e.g. Lincoln et al. during the nonbreeding seasonand massde- 1980, Silverin and Wingfield 1982, Dufty and creasedthroughout the nesting seasonin both Wingfield 1986,Hiatt et al. 1987). In addition, helpersand breeders(Fig. 5a).In adult females, T levels in breeding male Harris' Hawks were both behavioralrole and neststagestatistically lower than thosein breeding malesduring the In adult male Harris' Hawks, there were no differencesbetween the body massesof the helpers and breeders,but nest stagehad a sig- July1991] Hormones in Cooperative Breeders 100 a 629 Non- Melee Nest Breeding {3 Adult ß 80 Incu- Building Feeding bating Young I Males Juv Ad Br 800 Ad H Juv H 60' 40' 700. 20' 0 O N D J F M A M J J A 600 -100 -50 0 50 100 Femelee Adult NonBreeding Nest Building Incubating Feeding Young b t2oo. Females o Ad Br i Ad H 30ttO0. i ! I 10' tOO0 O N D J F M A M J J A Fig. 4. Mean plasma corticosteronelevels in male (a) and female (b) Harris' Hawks in each month. Val- ues for adult-plumagedand juvenal-plumagedbirds are plotted separately.Symbols and statisticsas in Figure 1. nesting seasonin most monogamousand polygamousspecies(e.g. Wingfield and Farner 900 ,. -100 -50 . ß .... 0 , .... 50 100 Time relative to first egg, days Fig. 5. Mean bodymassin male (a) and female(b) Harris' Hawks during eachnest stagefor thoseindividualsfor whom nestrole is known.Symbolsand statisticsas in Figure 1. 1978b, 1980; Lincoln et al. 1980; Silverin and Wingfield 1982;Wingfield 1984a,b; Fivizzani et al. 1986;Fivizzani and Oring 1986;Hegner and Wingfield 1986a, b; Ball and Wingfield 1987). Low LH levels, similar to those in Harris' Hawks,arefoundin breedingpolyandrous male Spotted Sandpipers (Actitis macularia;Rissman Relatively low lvels of LH and sex steroids seemto be found in those speciesthat are not strongly territorial such as the Spotted Sandpiper and Mallard or those that pair for life or return to the sameterritory year after year, such as the Western Gull. For instance,Wingfield et and Wingfield 1984)and in Pied Flycatcherfemales(Ficedulahypoleuca; Silverin and Wing- al. (1987) correlated the low T levels found in field 1982). Levels of T in Harris' Hawks are only slightly lower than concentrationsin the Spotted Sandpiper and are similar to levels re- tition portedin the Mallard (Anasplatyrhynchos; Donham 1979) and male Western Gulls (Larusoccidentalis wymani;Wingfield,Newman, Hunt, and Farner 1982). Plasma T and E levels are lower male Western Gulls with low levels of compebetween males for mates and nest sites. Relatively low levels of LH and sexsteroidsare alsofound in sometropicalspecies(e.g. Dittami and Gwinner 1985, Dittami 1986, Reyer et al. 1986).Severalof thesecharacteristics apply also to the Harris' Hawk, such as their generally tropical distribution (southwestern United in breedingHarris' Hawks than in the captive- Statesto Argentina and Chile), year-round resbreeding American Kestrel (Falcosparverius; idency,and long-term pair bondsbetweenbirds. Rehder et al. 1986, 1988). Bednarz (1987a) described the Harris' Hawk 630 MAYS, VLECK, ANDDAWSON in New Mexico as not obviouslyterritorial. In contrast,Dawsonand Mannan (1991b)reported that groupsin Arizona resideon nestingterritoriesall yearand continuouslydefendat least a portion of their territory, and that all group [Auk,Vol. 108 ENDOCRINE DIFFERENCES BETWEEN HELPERS AND BREEDERS Understandingwhy (both mechanistically andfunctionally)helpersdo not breediscentral members participate in this defense to some to understandingthe evolution of cooperative degree.Aggressiveencounters(attacking,chas- breedingsystems, becausethe lackof breeding ing, and foot grabbing)occurbetweenmembers would seemto producea heavy costin terms of differentgroupsduring the winter (Dawson of direct fitness(Woolfendenand Fitzpatrick and Mannan 1991b)but to a lesserdegreethan 1977, Brown 1978, Emlen 1984). We first exin intraspecificterritorial disputesin other rap- amineseveralalternative,but not mutuallyextors (de Vries 1975, Newton 1979). Members clusive,hypothesesthat could accountfor the from morethan onegroupmay hunt, feed,and lack of breeding in helper Harris' Hawks and perchtogetherduring the winter (Mader 1975b, then discussour interpretationof our hormonal Whaley 1986,Dawsonand Mannan 1991b).The datafrom breedingand helping Harris' Hawks relatively low amplitude of T cyclesin male in the light of these hypotheses.We propose breeding Harris' Hawks comparedwith other that the proximate and ultimate causesfor the speciesmaypartly reflectthe relativelylow lev- lack of breeding in Harris' Hawks are not the el of conflictpresentin territorialdisputes.The samefor all helpersin the group. rise in T during the nest-buildingstageis preReproductive maturity.--Absenceof breeding sumablyassociated primarilywith nestdefense, by helpers could be due to incompletematusperm production and copulatorybehavior. ration and not a particular socialfactor.Lack of Among female Harris' Hawks, T levels are breedingand low reproductive-hormonelevels elevatedonlyin alphafemalesandonly during found in mostjuvenal-plumagedhelperscould the nest-building stage. In other species,fe- be simply due to their age.Both sexesof Harris' malesshowelevatedT levelsonly when they Hawks, however,have been reportedto breed participatein territorialdefense(Wingfieldand while still in juvenal plumage (Mader 1975a, Farher 1978b,Rissmanand Wingfield 1984). Whaley 1986,Dawsonand Mannan 1991a,Lett Harris' Hawk breeding females participate in pets. comm.), and one juvenal-plumagedmale territorial defenseand are especiallylikely to we sampledhad a T level higher than the mean exhibit aggressive behavior toward other value for breedingmales(Fig. 1). Clearly posbreedingfemalesor nestpredators(Dawsonand sessionof juvenalplumagedoesnot necessarily Mannan 1991b). Unlike Western Gulls, in which mean that a bird is sexuallyimmature. femaleshavelevelsof T equivalentto thoseof Reyeret al. (1986)rejectedthe hypothesisthat the males, T levels in Harris' Hawk females, the low T levelsin (usuallyyounger)primary althoughelevated,are significantlylower than helpers of the cooperativelybreeding Pied thoseof the males.Levelsof T in female helpers Kingfisher(Cerylerudis)were due to incomplete are not elevatedduring nest building, and fe- maturation. They found that T levels did not male helpersare muchlesslikely to defendthe necessarilycorrelatewith age but with helper nest (Dawson and Mannan 1991b). status.In addition, some primary helpers are As in mostmonogamousspeciesstudied,ste- failed breeders that have returned to their natal roid levels fell in breeding Harris' Hawks of territories. In other cooperatively breeding both sexes at the onset of incubation. Elevated speciesthat previouslywere thought to exhibit T levels are thought to be incompatiblewith delayed maturation, it is now known that firstparentalcarein males(Silverin and Wingfield year individuals can and do breed under ap1982; Hegner and Wingfield 1986a, 1987a; propriate circumstances(Stallcup and WoolfenWingfield and Moore 1987). Estradiol-17fllev- den 1978, Koenig and Mumme 1987). els decreasefollowing egg laying in the AmerBehavioral suppression.--Helpers may not breed ican Kestrel(Rehderet al. 1986),as they do in because of behavioral interactions with the breedingfemaleHarris'Hawks,althoughE does breeders(e.g.mateguarding)eventhoughthey not necessarilydecreaseat the onset of incu- are physiologicallyreadyto do so.The ultimate bation in all species(Donham 1979,Schwablet reasonfor this may be that dominant breeders al. 1980,Wingfield 1984a). have an interestin protectingtheir geneticin- July1991] Hormones in Cooperative Breeders 631 vestmentby interfering with, or not cooperating in, mating attemptsby helpers. Most, but not all, adult-plumagedmale Harris' Hawks-whetherbreedersor helpers--haveelevatedLH and T during the nest-buildingstage.Thus,most adultmalehelpersappearreproductivelyready, basedon LH and T levelsindistinguishable from vary with the relatednessof the helper to the breedersbecauserelatednessaffectsthe prob- those in breeders at the same time. In fact, adult A helper relatedto the breederof the opposite sex should not ascendto breeding statusbecausedoing so would risk inbreeding and po- helpermalesoftenattemptto copulatewith the alphafemale,and their apparentlackof success is due mainly to her lack of cooperation.In 46 observationsof mounting of the alpha female by the alpha male, copulationwas completed in 44, whereasonly 1 in 22 observedmountings by beta malesappearedto be successful(Daw- ability that a helper can becomea breeder in a nonconsanguineousmating. In monogamous groups,a helper unrelatedto the breederof the oppositesexcouldascendto breedingstatuson the departureof the alpha bird of the samesex. tentially reduce the inclusive fitness of both birds (although the adverseeffectsof inbreeding in birds are controversial;cf. Greenwood et al. 1978,Craig and Jamieson1988). If inbreeding is avoided, ascent of a related helper to sonand Mannan 1991a,pers.obs.).Femaleswill breeding statuswould require the lossof both occasionallycopulatewith more than one male, breedersfrom the group: lossof one to free a which results in some polyandrousmating breeding slot and loss of the other to avoid groups(Mader 1979,Whaley 1986,Sheehyet inbreeding.Thus, while the energeticcostsof al. MS). becomingphysiologicallyready to reproduce Physiologicalsuppression.--Alternatively, helpersmay be sexuallymaturebut not physiologicallyreadyto breed(i.e.with inactivegonadsand low levelsof reproductivehormones). In Harris' Hawks, all helper femalesand most juvenal-plumagedmale helpers appear to be physiologically unpreparedfor breeding,based would on very low levelsof LH, T, and E. At the same time, these hormones were elevated in breeders and adult-plumagedmale helpers.Physiological readinessto breedcouldbe constrainedby factorssuchaspoorbodycondition,or it could be the result of social interactions between birds. For instance,physiologicalinhibition could re- suitfromstressdueto dominationby the breeder of the samesexto assureits geneticparentage (Reyeret al. 1986).Forexample,an alphafemale might dominate a potentially competingfemale, causing hormonal suppressionin the helper and preventing breeding.On the other hand, physiologicalinhibition couldoccurdue to the presenceof the parentof the opposite sex, be the same for both related and unre- latedhelpers,the probabilityof receivingany benefit from this readiness would be much low- er for a helper related to the breeder of the oppositesex.This leadsto a prediction:if we assumethat opportunitiesto breed outsidethe breedingunit are few, helpersthat are related to the breederof the oppositesexshouldbe less likely to invest in reproductivereadiness(gonad growth, gamete production,costsof carrying and maintainingreproductivetissues,etc.) than helpersthat are unrelatedto the breeders. (This energeticargument is analogousto that which wasbeenusedto explainthe regression of gonadsin seasonal breedersduring the nonbreeding seasonwhen chancesof successfulreproduction are slim). The pattern of LH and sex steroid levels we observedin Harris' Hawk helpersis consistent with the hypothesisthat inbreedingavoidance is an important ultimate selectivefactor determining their reproductivereadiness(exceptin presumablyas a result of selectionto avoid in- adult femalehelpers,seebelow).Mostjuvenalbreeding.Under this hypothesis,physiological plumagedHarris' Hawks (•75%) are helping readinessto breed would dependprimarily on theirownparentsandarenothormonallyready the relatedness betweenhelpersand breeders, to breed with the opposite-sex parent even if rather than on age or statuswithin the domi- the same-sexbreederdisappeared.In contrast, nancehierarchy.Of course,physiological sup- mostadult-plumagedmalehelpersarenot helppressionof reproductioncouldbe due to a com- ing their own parents(• 80%).We suggestthat, bination of these factors. for thesebirds,the costof becomingreproduc- Behavioralvs. physiological suppression.--We tively readyduring the breedingseasonis more suggestthat whether behavioralor physiolog- than offsetby the potentialbenefitsto be deical suppression of breedingoccursis likely to rived from either occasionalsuccessfulcopu- 632 MAYS,VLECK, ANDDAWSON lations with the breeding female or attainment of alpha status.Thoseadult helperswhoseLH and T levels were not elevated during the nestbuilding stagewere presumablynot reproductively ready, and they were possibly related gammahelpersrather than unrelatedbetahelpers.For instance,one of the six adult-plumaged male helperswe sampledduring the nest-building stagehad a T value in the samerange(<15 pg/ml) as all the juvenal-plumagedmale help- [Auk, Vol. 108 er than that for the male helpers in Harris' Hawks. The costof recrudescencein a 1,000 g female bird (growth of oviduct and functional but nonovulating ovary) is probably ca. 100150%of a single day'sbasalenergyexpenditure, whereas estimates of the costs of testicular re- crudescence in male birds range from only 7% to 40%(basedon data in Walsberg 1983). Helpersin other cooperativelybreedingspeciesmay be constrainedto help rather than breed beers on their natal territories. The other adultcausethey are energeticallyincapableof attainplumaged males,whether helpers or breeders, ing breeding condition (Brown 1983). Unsuchad T values 2-70 times higher than related cessfulbreeding has been attributed to a less juvenal-plumagedhelpers.We do not know the than adequateweight gain in femalesfor many relatedness of thisadult-plumagedhelperwhose wild and captive species including raptors T value was low. We do know that the only (Newton 1979, Hardy et al. 1981, Rehder et al. juvenal-plumagedhelper who was known not 1986), and reduced food intake has dramatic to be helping on its parents'territory was the antigonadal effects(see Wingfield 1983 for reonly such bird with high sex steroid levels (T view) or results in lower reproductive output > 900 pg/ml). (Drent and Daan 1980) in all avian speciesinvestigated.For instance,poor body condition (i.e. low fat depositsand body mass)resultsin MECHANISMS FOR PHYSIOLOGICAL low circulating levels of T in male Song Sparrows (Melospizamelodia;Wingfield 1985). Poorbodycondition.--Adultfemale helpersdo Dominancesuppression.--Onthe other hand, not appearto be reproductivelyready to breed dominancebehaviorby alphafemalescouldreeven though they are unlikely to be on their sult in physiologicalsuppressionof reproducnatal territory. These birds also weigh signifi- tion in adult female helpers either directly or cantly less than breeding females. In contrast, indirectly. For instance, subordinate females male adult helpers and breedersdo not differ may have low accessto resourcesdue to their in mass. In fact, these female helpers weigh low rank in the dominancehierarchy.An adult • 10%lessthan the breeding femalesduring the female helper that is capable of reproducing 6-7 weeks before the first egg, when gonadal may pose a greater threat to the alpha female developmentis probablyoccurringin thosefe- or to groupstabilitythan a reproductivelyready males that becomebreeders.The greater mass adultmalehelper,possiblybecausetwo or more in the female breeders comparedwith female laying femaleswould increasethe number of helpersis not likely to be simply due to having chicksto be raised.One normal-sizedbroodmay eggs in the oviduct. Body massesof laying be all a Harris' Hawk group can care for (FaaAmerican Kestrelsexhibited distinct peaksrel- borg and Bednarz 1990). In the Acorn Woodcompetition ative to nonlaying females, but only for ap- pecker (Melanerpesformicivorous), proximately 1 week before and 1 week after egg among communally nesting females is greater laying (Rehder et aL 1986). The low hormone than that among males and decreasesthe relevels in the adult female helpers during the production of groups significantly more than early nesting stagesmay be due to poor body doesmale-malecompetitionin the group (Koeconditionrather than to any socialsuppression. ning et al. 1983). Dominant females in a group Adult-plumaged female helpers do not usually can suppressreproductiveoutput in birds (e.g. remain with a group for > 1 yr, and have never Jamiesonand Craig 1987), and a similar phebeen observedto replacean alpha female(Daw- nomenon is seen in some mammals (Wasser and sonand Mannan 1991a).It maybe that they join Barash 1983 and references therein). a group primarily to benefit from group huntMechanisms by which breeding females ing (Bednarz and Ligon 1988) rather than to might suppressreproduction in adult female actively participate in reproduction. helpers are not clear. One might predict that The cost/benefitratio of becomingreproduc- the mechanism of (or a correlate of) dominancetively readyin femalehelpersis probablygreat- induced hormonal suppressionwould involve SUPPRESSIONOF REPRODUCTION July1991] Hormones in Cooperative Breeders 633 the adrenocorticoids,which are normally ele- peckers(Koenig and Pitelka 1979,Koenig et al. 1983) and Florida Scrub Jays (Aphelocoma coerone levels in females did not differ between rulescens; Woolfenden and Fitzpatrick 1978) breedersand helpers,however, and thus levels show well-developed behavioral patterns to were not correlated with the subordinate status avoid closeconsanguineous matings,but nothand reproductivehormonesuppression of adult ing is known of the hormonalstatusof the nonfemalehelpers.If reproductive-hormone levels breeders in these species.The likelihood that are suppressedin helpersbecauseof their sub- othermaleswill cuckoldthe alphamale,or othordinate status it must work via some mechaer females will parasitize the alpha female, nism other than adrenocorticoid-induced inshould be greater when the individuals inhibition. volvedareunrelated(Koenig1981,Faaborgand vated with stress (Deviche 1983). Corticoste- In juvenal-plumaged helpers,we suggestthat the breeding birds do not directly causehormonal suppressionvia dominance.Rather, we Bednarz 1990). Whether this results from a hor- monal suppressionin relatedbirdsis unknown. Hormone data for Pied Kingfisherhelpersare believe that relatedjuvenal-plumagedhelpers alsoconsistentwith inbreedingavoidance.The remain reproductively inactive to avoid in- Pied Kingfisherhas two types of helpers:pribreeding.Lackof evidencefor dominancesup- mary and secondary (Reyer 1980). Primary pressionis of two types. First, corticosterone helpersare offspringof the breeding pair from levelsin thejuvenal-plumaged maleswerelow- a previous year, whereassecondaryhelpers are er, not higher, than those in the adults, which not related to the breeders(Reyer 1986). Priagain does not support their involvement in mary helpers are associatedwith the breeders hormonalsuppression. Second,moststudiesthat throughoutthe entire nestingcycle.In contrast, correlatehormonelevels with degree of dom- secondary helpersaretoleratedby the breeders inance have indicated that subordinate individonly after the younghatchand then only when uals do not differ from dominant individuals in the parents' feeding capacitiesare exceeded sex-steroidlevels once socialrelationshipsbe- (Reyer1984,Reyerand Westerterp1985).Reyer tween birds have become well established, alet al. (1986) found that before egg laying, secthough they can differ in corticosteroidlevels ondary helpers have T levels similar to the (Balthazart et al. 1979, Tsutsui and Ishii 1981, breeders,whereasprimary helpers have T levRohwer and Wingfield 1981). Similar condi- els significantlylower. They attributed the low tionshavebeenfoundin free-rangingoliveba- levelsof T in the primary helpersto dominance boons(Papioanubis;Sapolsky1987). Levels of suppressionby the breeding males.Their hortestosteronecan play an important role in de- monedata are alsoconsistentwith the hypothtermining statusin the first place(Ramenofsky esisthat reproductive hormonesare suppressed 1984,Hegner and Wingfield 1987b)and T levels in the primary helpers becausethe potential increasein male birds following repeatedin- mate is also a parent. Primary helpers are, of tense aggressiveinteractions with other males course,alsosubordinateto their parents,sothese (Searcy and Wingfield 1980, Wingfield and two hypothesescannotbe distinguishedin this Moore 1987). Harris' Hawk group hierarchies, case. Reyer's (1986) behavioral data provide addihowever,arewell established andstableby the nestingseasonin March with little aggression tional support for inbreeding avoidance.In 7 displayed between members (Dawson and casessecondaryhelpershad an opportunityto Mannan 1991b). breedin a slotvacatedby the malebreederthey Inbreeding avoidance.--Iflow T levels in birds helped. In each of the 7 casesthe secondary helping on their natal territories are related to male helper moved into the vacated slot and inbreeding avoidance,it shouldbe possibleto bred with the widowed female that was not his identify a link betweenkin recognitionand re- mother. In 6 casesprimary helpers had an opproductiveinhibition, but little is known about portunityto moveinto a slotvacatedby a breedhow birdsrecognizerelativesor how this affects er. In 4 of the 6 casesthe primary helper disphysiologicalprocesses.In Harris' Hawks, LH persedandultimatelybredwith anotherfemale, concentrations in differenttypesof helperspar- and in 2 caseshe moved into the vacated poallel concentrations of T, thus the mechanism sition. In the 4 casesof dispersal,if the helper is presumablycentral and actsabove the ante- had assumed the vacated slot, he would have rior pituitary/hypothalamic axis.Acorn Wood- mated with his mother; in the 2 cases where 634 MAYSß VLECK, ANDDAWSON the primary helper bred with the widowed female, she was not his mother. The hormone levels of these individuals are not known, but it may be that primary helpers have high T levels,similarto the secondary helpers,if they are not helping their own mothers. [Auk,Vol. 108 steroid hormonesin relation to multiple-broodedness and nest-site density in male starlings. Physiol. Zool. 60(2): 191-199. BALTHAZART, J. 1983. Hormonal correlates of behav- ior. Pp. 221-365 in Avian biologyßvol. 7. (D. S. Farner, J. R. King, and K. C. Parkes,Eds.).New Yorkß Academic Press. In Harris' Hawks,physiologicalsuppression --, R. MASSA,& P. NEGRI-CEsI.1979. Photopeof reproductivereadinessin helperson their riodic controlof testosteronemetabolismß plasma natal territoriesand the lack of suchsuppresgonadotrophins,cloacal gland growth and reproductivebehaviourin the JapaneseQuail. Gen. sionin helpersnot on their natal territoriesare Comp. Endocrinol.39: 222-235. consistentwith the hypothesisof inbreeding avoidance.Physiological readiness is necessary BEDNm•Z,J. C. 1987a. Pair and group reproductive success, polyandry,and cooperativebreedingin but not sufficientfor a helper to breed.ReproHarris' Hawks. Auk 104: 393-404. ductively ready male helpersare presumably 1987b. Successivenesting and autumnal not often able to breed because of behavioral interactions with the breeders (Dawson and Mannan 1991a).In our study,the one juvenalplumaged male with high T did not breed after leaving his natal territory, presumablybecause of his low socialstatusin his new group. Inbreeding avoidance also does not account for breeding in HarrisßHawks. Auk 104: 85-96. 1988. Cooperativehunting in Harris' Hawks (Parabuteounicinctus).Science 239: 1525-1527. ß& J.D. LIGON. 1988. A studyof the ecological basesof cooperative breedingin the Harris'Hawk. Ecology69: 1176-1187. BERGER, D. D., & H. C. MUELLER. 1959. The bal-chatri: the low hormonelevelsin adult femalehelpers a trap for the birds of prey. Bird-banding30: 18- becausethey are unlikely to be related to the breedersin the group. Either poor body con- BIRD, D. M., P. C. LAQUE, & R. B. BUCKLAND. 1976. dition, behavioral interactions with other birds 26. Artificial inseminationvs. natural mating in captive American Kestrels. Can. J. Zool. 54: 1183- in the group,or both couldbe proximatecauses 1191. for their hormonal suppression. BRANNON, J. D. 1980. The reproductiveecologyof a Texas Harris ßHawk (Parabuteounicinctusharrisi) ACKNOWLEDGMENTS We thank all thoseindividualswho helpedwith the fieldworkßincluding Rick Bowers,Ellen Weintraub, JeffWhite, Lynn Oliphant, BobScheibe,and other membersof the TucsonAudubonSociety.We acknowledgethe loan of equipmentfrom OscarWard, Holly Hobart,Mable Mays,and StephenRussell.Bill Mannan, Dave Vleck, StephenRussell,John B. Dun- population.M.S. thesisßAustinßUniv. Texas. BROWN,J. L. 1978. Avian communalbreeding systems.Annu. Rev. Ecol.& Syst.9: 123-156. 1983. Cooperation--a biologistßsdilemma. Adv. Stud. Avian Biol. 13: 1-37. ß 1987. Helping and communalbreeding in birds. Princeton, Princeton Univ. Press. ., & E. R. BROWN. 1981. 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