ANNUAL REPRODUCTIVE YIELD IN THE COOPERATIVE PYGMY (SITTA PYGMAEA)

WILLIAM J. SYDEMAN,1 MARCEL GONTERT,2 AND RUSSELLP. BALDA Departmentof BiologicalSciences, Northern Arizona University, Flagstaff, Arizona 86011 USA

ABSTRACT.--Westudied cooperative-breedingPygmy (Sitta pygmaea)for 4 yr in northern Arizona. Breeding units contained2-5 . Helpers were found at about 30% of all nests. All helpers that later bred on the study area were male. Helpers were mostly yearlings, and offspring or siblings of the birds that they aided, but often aided at least one unrelatedbreeder. Breeding units with helpersproduced significantly more young than those without helpers. Breeding units in habitats with the greatest floral diversity and structural maturity fledged significantly more young than those in other habitats. Habitat did not influencethe effectof helpers.Year effectsincreased the strengthof the relationshipbetween helpers and annual reproductive output. Previous breeding experience and pair-bond du- ration were not related to reproductive success.Total brood loss,although rare, was respon- sible for the difference in reproductive output among pairs with and without helpers and between habitats. Breeding birds with helpers benefit by an increasein direct fitness. The advantage to the helpersis not clear but may be an increasein indirect fitnessassociated with aiding relatives. Helpersmay benefit directly, however,by sharingroosting cavities on a group territoW thereby enhancing overwinter survival. Received26 September1986, accepted 15 September1987.

COOPERATIVE-BREEDINGPygmy Nuthatches cavities.Communal roosting is a means of re- (Sittapygmaea) were first documented in central ducing nightly energy expenditureby sleeping California. Helper Pygmy Nuthatches aid the in cavities, huddling, and hypothermia (Hay breeding birds by feeding the incubating fe- 1983, Gtintert in press). male, feeding the nestlings and fledglings, and Avian cooperative breeding is a model ver- defending the nesting territory and cavity tebrate system for testing hypothesesrelating (Norris 1958, Sydeman 1985).Norris (1958: 177) inclusive fitness (Hamilton 1964, Brown and attributed the presence of nonbreeders to a Brown 1981) to the evolution of helping be- biased sex ratio in favor of males. Unlike most havior. Understanding the adaptive signifi- cooperativebirds (Fry 1972, Brown 1978), Pyg- cance and selective pressuresleading to coop- my Nuthatches inhabit cold-temperate envi- eration, such as helping at the nest in birds, ronments. Pygmy Nuthatches are social requiresexamination of the costsand benefits throughout the year. Winter groups average 5- of this behavior to the recipient breeders and 15 birds (Norris 1958, Sydeman 1985, Gtintert donor helpers. Studies on the demography of in press),but larger assemblageshave been re- birds that breed with and without helpers are corded (Knorr 1957, Sydeman and Gtintert, a crucial step to understandingthe benefits of 1983). Winter groups forage as a flock and roost avian cooperation. communally in tree cavities located within a We examined several hypotheses regarding group territory. Individual membershipin win- variation in annual reproductive yield and in- ter groups is consistent within and between clusive fitness. Inclusive fitness has two com- years(unpubl. data). Group size may be limited ponents:direct and indirect fitness(Brown and by the size and thermodynamics of roosting Brown 1981). Breeders with helpers that raise more young than breederswithout helpersben- efit by an increasein direct fitness.The poten- •Present address: Point Reyes Observatory, tial cost of allowing competitors on a territory 4990 Shoreline Highway, Stinson Beach, California may then be reduced.Helpers may benefit in- 94970 USA. directly if they enhance the reproductive suc- 2 Present address: Naturhistorisches Museum, 3005 cessof relatives (Rowley 1965, 1981;Parry 1973; Bern, Switzerland. Stacey1979; Brown and Brown 1981;Emlen 1981;

70 The Auk 105: 70-77. January1988 January1988] Reproductionin Pygmy Nuthatches 71

Koenig 1981; Woolfenden 1981; Kinnaird and T^BLE1. Tree density and diversity in Walnut Can- Grant 1982; Rabenold 1984; Austad and Raben- yon National Monument and Coconino National Forest, Arizona. old 1985;Hunter 1985).However, reproductive enhancement has not been observed consis- Ponder- Juniper Gambel tently (Gaston1973, Zahavi 1974,Vehrencamp osa spp. oak Snags 1978,Craig 1980, Ligon 1981),helpers have been (no./ha) (no./ha) (no./ha) (no.) unrelatedto the breeders(Reyer 1980,1984; Em- WCNM 54.1 36.1 27.9 112 len 1981;Ligon 1981), and complicatinginflu- CNF 60.9 28.6 18.6 24 ences have prohibited conclusion of a direct causeand effectrelationship between helpers and reproductive output (but see Brown et al. 1982). June,and July, when Pygmy Nuthatchesbreed, the Ecologicaland demographicfeatures inter- mean monthly low was 5.4øC,with a high of 26.0øC. correlatedwith helpersmay createspurious re- Precipitationalso varied seasonally,with snowfallor rain showers in winter and thunderstorms in sum- lationshipsamong the criticalvariables. Repro- ruer. ductive successmay be affected by territory The forest of WCNM has been undisturbed for about quality or size (Lack 1968, Zahavi 1974, Brown 70 yr. In comparison,CNF was logged selectivelyin andBalda 1977, Gaston 1978, Vehrencamp 1978, 1967 and has been cut continually for fuelwood. We Brown and Brown 1981, Koenig 1981, Lewis sampledfeatures of the vegetationusing a modified 1981),yearly variation in resources(Stacey 1979, point-quartermethod (Cottam and Curtis 1956).Points Koenig1981, Woolfenden and Fitzpatrick1984), were selectedat 100-m intervals along 5 east-west and age or experience of the breeders (Lack transects,spaced 150 m apartthrough the entire study 1968,Maynard-Smith and Ridpath 1972,Wool- site. Becauseof the irregular dimensionsof the site, fenden 1975,Koenig 1981, Rowley 1981).These 3 1,500-m transects were established in CNF and 2 alternativehypotheses are not mutually exclu- 1,700-m transectswere located in WCNM. Using a SpiegelRelaskop at eachpoint we measuredthe dis- sive and may interact with helper effects.Con- tanceto and height of the closestponderosa pine with sequently, we have focused on the interrela- a diameterat breastheight (DBH) greaterthan 10 cm tionshipsamong these factors. in 4 quadrants.We calculatedbasal area by counting We compared productivity of birds repro- trees from each point that showed a DBH > 10 cm in ducing with and without helpers.Our objec- the Relaskop.Each point served as the center of a tives were to describe cooperative-breeding 20-m circularplot in which we tallied the number of Pygmy Nuthatches in northern Arizona, ex- oak patchesand junipers.We countedthe total num- aminethe interactingeffects of helpers,habitat, ber of snagsin the study area and included fallen year, and breeding experience on annual re- snagsif they stoodfor at least2 of the 4 yr of study. productive yield, and evaluate the role of re- Marking and censusing.--From November 1980 through August 1984 we capturedadult and young productive yield as a selectiveforce in the evo- nuthatches w!th mist nets at artificial and natural water lutionof cooperativebreeding. Survivorship and holes during dry periods,and at roost and nest cav- nestling care were not consideredhere. ities asthey tried to escape.More than 500 nuthatches were individually color-markedusing combinations METHODS of 4 bands (a U.S. Fish and Wildlife Service aluminum band and 3 color bands).Two bandswere placed on Studyarea and vegetation measurements.--We studied each leg, and color bandswere sealedwith acetone. nuthatchesin an extensive (approx. 1,000,000ha) Individual nuthatcheswere sexedonly during the ponderosa pine ()forest. The site, breedingseason by broodpatch (femalesonly; Norris roughly 250 ha, is located15 km east of Flagstaff, 1958)and cloacalprotuberance (males only). Young Arizona (elevation2,195 m) and is bisectedby the of theyear were identified up to 3 monthsafter fledg- northern border of Walnut Canyon National Monu- ing by the color of the baseof the mandible (pink to ment (WCNM) and Coconino National Forest (CNF). pale off-white vs. gray to black in adults). Other dominant vegetation includes intermittent We systematicallycensused the population by or- standsof gambeloak (Quercusgambelii), junipers (Ju- deredvisits to groupterritories. Individual color-band niperusspp.), and pinyon pine (Pinusedulis). combinationswere read every month for 4 yr. The The environment is cold-temperatewith a seasonal transitionfrom winter to the breedingcycle was char- climate. Air temperaturesat WCNM varied season- acterizedwhen winter groupssplit into breedingunits. ally, with an averagemonthly low of -4.9øC in the At this time we monitoredthe statusof eachbreeding winter and high of 27.1øCin the summer. In May, unit at 3-dayintervals. We locatednest cavities during 72 $YDEMAN,GONTERT, AND BALDA [Auk,Vol. 105

TABLE2. Characteristicsof ponderosapine in Walnut CanyonNational Monument and CoconinoNational Forest, Arizona.

WCNM CNF

n Mean _+ SD n Mean _+ SD t-test a Basal area (m2/ha) 34 13.97 -+ 8.69 45 10.46 + 6.04 2.13' DBH (cm) 136 50.65 -+ 23.69 180 40.37 _+ 19.96 4.17'* Height (m) 136 18.82 _+7.99 180 15.36 _+7.23 4.01'*

• * = P < 0.05, ** = P < 0.001. excavation.We determined the number of fledglings and 7 servedas helpers for 2 yr. The latter group producedat each nest at fledging. usually aided the samebreeders each year (n = The number of young fledged per breeding unit 5). Genetic relationshipsbetween most helpers each year and failed nesting attemptsfor units with and nestlingswere unknown. For helperswhose and without helpers were recorded.Successful breed- parentage was known (n = 11), 2 aided both ing units were defined as those that fledged at least 1 young/yr. We alsocompared the proportionof 1983 parents,2 helped their mother and presumably fledglings (banded as nestlings) that were observed their father (the father was unbanded), 6 aided in the 1984 breeding seasonfor units with and with- fathers and an unrelated female, and 1 helped out helpers.Factorial analysis of variance(ANOVA) a brother and an unrelated individual. wasused to examinethe effectsof year,helpers, and Twelve helpers that later bred on the study habitats. area were males. Most helpers were yearlings. In 1984 we monitored the reproductive activi- RESULTS ties of 33 yearlings;7 were breeding females. Of the remaining 26 individuals, 4 were breed- Habitat.--The halves of the study area dif- ing males, 13 helped, and 9 neither helped nor fered in tree density and diversity (Table 1). We bred (i.e. floated). This last group was larger presumethat the diversity of potential foraging than observedin earlier years,and we have no and nestingsubstrates was considerablygreater data on the fate of these individuals. If we as- in WCNM than CNF. sumethat all 1984 helperswere malesand omit The halves of the study area also differed in breeding females and floaters, then 76% of the pine structure.The ponderosapines in WCNM yearling maleshelped. Only 2 birds helped for coveredmore ground,were significantlytaller, a second time in 1984, but these birds were and had larger diametersthan treesin CNF (Ta- banded asadults and their ageswere unknown. ble 2). If pine age is correlated with tree size, Annual reproductive yield.--We recorded then WCNM may be consideredan older, more fledgling production of 141 first nesting at- mature ponderosa-pinecommunity. tempts during 4 breeding seasons.First at- Socialorganization.--Breeding units consisted tempts of 16 breeding units failed to produce of a mated pair or a pair with a complement of young. Secondnesting attemptsafter nest fail- 1-3 helpers. The averageunit size was 2.5 birds. ure were unusual: only 2 occurred in 1981, 1 in Three observationssuggest that helpers did not 1982, and 1 in 1983; 3 of these were successful. contribute genetically to the production of A total of 13 breedingunits failed to fledgeany young. First, only the breedersparticipated in young. Renests after successfulfirst nests oc- cavity excavation and nest building (but see curred twice in 1982; both units successfully Norris 1958: 209). Second,many helpers were producedfledglings. We added the resultsof initially followed as "singletons" each year. first and secondbreeding attemptsto quantify Singletonswere seen calling and roaming in the total reproductive output for each breeding territory unoccupiedby other nuthatches.Third, unit per year. potential helpersoccasionally were driven from Breedingunits in WCNM (n = 64) fledged an nest cavities by the breeders before the eggs averageof 5.5 young, but units in CNF (n = 77) hatched. Some of these birds were singletons. producedan averageof 4.4 fledglings(Fig. 1). Forty-four color-bandedbirds servedas help- Pairs with helpers (n = 56) fledged an average ers for 51 breeding pairs during 4 breeding sea- of 5.2 young/yr, and pairs without helpers (n = sons;37 individualswere helpersfor only 1 yr, 85) averaged 4.3 young/yr. Yearly variation January1988] Reproductionin Pygmy Nuthatches 73

ersin CNF, closedboxes show units without helpers 2 3 4 5 in WCNM, right hatchedboxes show units with help- Unit Size ers in WCNM. Sample sizes are given above error bars. Fig. 2. Annual reproductiveyield in relation to breeding-unitsize, showing the mean + 2 SEand the samplesize for eachunit. rangedfrom 4.2 young producedper breeding unit in 1981 (n = 37) to 5.0 in 1982 (n = 34) and 1984 (n = 39). In 1983 (n = 31) the average frequencyof failed nestswas higher for breed- number of young fledgedper breeding unit was ing units without helpers (13 of 17; X2 = 3.46, 4.9. df = 1, 0.05 < P < 0.10). Annual reproductive yield increasedsignif- Breeding units that failed to produce young icantly,albeit weakly, with breeding-unitsize were responsiblefor the differencein fledgling (Fig. 2). No differencewas found betweenunits yield between units with and without helpers with 1, 2, or 3 helpers (F2,s3= 0.03, P > 0.05). and between habitats. Ten of 13 units that failed Two factors,helpers and habitat (either WCNM in CNF were without helpers. The remaining or CNF), had significant effects on annual re- unit failures, in WCNM, had helpers. The im- productiveyield (Table 3). No year effectwas portanceof complete unit failure was demon- found, but a significant interaction between strated when only successfulbreeding units helpers and year was documented.This inter- were entered in the ANOVA (Table 3). The ef- action illustrates the reproductive yield of fects of helpers and habitat were lost, and a breedingunits with helpersin 1983.Only in highly significantyear effect was established. this year did units with helpers fledge signifi- We alsofound significantinteractions between cantly more young than units without helpers yearand helpersand year and habitat.The year (Table 4). effectand its associatedinteractions may be ar- Variation in reproductive output may be tificial, the result of increasing the average causedby a differential in completebrood loss, fledglingproductivity in 1983and 1984.Eleven even though only 17 of 147 nestswere unsuc- of the 13breeding-unit failuresoccurred in these cessful.Nest failure was causedby predation years. by chipmunks(Eutamias spp., n = 2 known, 2 Neither previous experience of the breeding suspected),Acorn (Melanerpesformicivorus) or pair nor pair-bond duration was related to an- Hairy (Picoidesvillosus) woodpeckers (n = 3 sus- nual reproductiveyield (Table 5). We compared pected), and nest abandonment (n = 2 known); novice pairs (both membersyearlings with no for 8 failures the cause was unknown. More breeding experience),pairs with mixed levels nestsfailed in CNF than WCNM (13 of 17; X2 = of experience(one membera yearling,the other 3.46, df = 1, 0.05 < P < 0.10). Similarly, the with breeding experience), and experienced 74 $YDEMAN,GONTERT, AND BALDA [Auk, Vol. 105

TABLE3. Analysis of variance on factorsaffecting TABLE4. Analysisof varianceon the influenceof annual reproductive yield in Pygmy Nuthatches helpersand habitat on annual reproductiveyield for all breedingunits and successfulbreeding units in PygmyNuthatches for eachyear of study.See only (thosethat fledgedat least 1 young/yr). Fig. 1 for data.

All Successful F ratio a

breeding units breeding units Effect 1981 1982 1983 1984 F F Habitat 0.15 3.63 0.39 2.93 Effect df ratio a df ratio a Helpers 1.35 2.37 7.13' 0.15 Habitat 1 6.20* 1 1.60 Helpers x habitat 1.23 1.49 0.05 1.40

Helpers 1 4.07* 1 1.46 •* •P < 0.0I. Year 3 0.55 3 7.60* Habitat x helpers 1 0.00 1 0.09 Habitat x year 3 0.54 3 2.72* Our data on annual reproductive yield raise Helpers x year 3 2.83* 3 4.15'* Habitat x helpers x a number of points regarding the effectof help- year 3 1.25 3 1.06 ers in varied habitats and different years. The Residual 125 112 weak correlation between the number of help-

• * = P < 0.05, ** P < 0.01. ersand annual reproductiveyield indicatesthat helpers contribute substantially in only a few pairs (both members with breeding experi- casesand years. The 1983 breeding seasonhad ence). The influence of pair-bond duration was a strong effect on the relationship between testedby comparisonsof new pairs (members helpersand annual reproductiveyield. Without the 1983 data, significant variation in annual of the pair breedingtogether for the first time) and old pairs(members of the pair havingbred productivity among breeding units with and without helpers would not have been found. togetherin the previousyear). Annual variation in reproductive successin Survivalof youngto one year.--A long-term measurement of the benefit of helping is sur- some cooperativebirds has affected breeding vivorship to the first breedingseason of young units with and without helpers similarly (Wool- reared with and without helpers. In 1983 we fenden and Fitzpatrick 1984). We found that the marked48 nestlings,27 from unitswith helpers reproductive successof pairs without helpers (n = 5) and 21 from units without helpers (n = did not increaseconcurrently with helped pairs in 1983. 6). Thesenestlings represented 75% of the total number of young fledged by the 11 breeding A relationship may exist between nesting units. Forty-eight percentof the young reared phenologyand the effectof helpers.Units with with helperswere observedin the 1984breed- helpersshowed the highestreproductive yield ing season,while only 38%of thoseraised with- in 1983,when breeding occurredat a later date out helperswere seen.The differencein these than any other year of study (Sydeman 1985). proportions,however, was not significant(P > 0.05), and the high survivorship of the 1983 cohortmay have been due to a relatively mild TABLE5. Annual fledgling yield as related to pre- vious reproductiveexperience and pair-bond du- winter (Sydeman 1985). ration of breeding pairs.

DISCUSSION Units with Units without helpersa helpersa The annual fledgling yield in Pygmy Nut- n Mean ñ SD n Mean _+ SD hatchesis influenced by helpers, habitat, and the year of observation.We found no relation- Novice pairs 0 3 5.0 _+4.58 Mixed pairs 3 4.0 _+3.46 9 4.7 ñ 2.35 shipbetween breeding experience or pair-bond Experienced durationand reproductiveoutput. The possible pairs 12 6.8 + 1.40 9 5.1 + 3.22 interaction between breeding experience and New pairsb 20 5.5 _+2.24 36 4.0 _+2.68 helpersat the nest was not investigated.Other Old pairs 4 7.3 ñ 1.50 6 5.3 _+3.08 studiesalso have failed to find significanteffects • All comparisonsbetween experience levels while controllingfor the effectsof helperswere nonsignificantby Mann-WhitneyU-tests; from breedingexperience (Rowley 1965,Brown P > 0.05. and Brown 1981, Lewis 1981, Brown et al. 1982, bThere was a significant difference between new pairs with and Rabenold 1984). without helpersby Mann-WhitneyU-test; P < 0.025. January1988] Reproductionin Pygmy Nuthatches 75

Helpers contributedmore to feeding nestlings responsiblefor the differencein reproductive in 1983 than 1984,the latestaftd earliestyears yield among units with and without helpers of study,respectively (Sydeman 1985). The tim- and between disturbed and mature habitats. The ing of breedingmay be influencedby the vari- contribution of unit failure was very strong, able climate in the pine forestsof northern Ar- even though complete brood loss occurred at izona. Early spring snowstorms and high less than 12% of the nestswe monitored (< 10% precipitationoccurred in 1983.Rainfall hasbeen of the breeding birds). correlatedwith clutchsize and productivityin The advantageof cooperativerearing of off- other cooperativebirds (Emlen 1981, Woolfen- spring to the breeding birds was obvious. On den and Fitzpatrick1984). Winter precipitation average,breeders with helpers increasedan- mayincrease food availability during the breed- nual fledgling productivityby almost1 young/ ing season.If so,then helper Pygmy Nuthatch- yr. Our long-term measurementof the signifi- eswere mosteffective under favorablebreeding cance of helping, survivorship of fledgling to conditions. one year of age, is difficult to interpret because We found variation in annual productivity mortality after fledging may be confoundedby among habitats. The habitat effectswere inde- autumn dispersal(pers. obs.). pendent of the contributionof helpers,how- The benefits of helping behavior to helper ever (see also Brown and Brown 1981, Lewis Pygmy Nuthatcheswere not as obvious.Help- 1981, Brown et al. 1982). We examined the ef- ers delay reproductionand aid breeding birds. fects of territory characteristicsindirectly by An increase in the reproductive output of comparing annual productivity in habitats that breedersthat are kin may compensatethe po- differed extensively in vegetation structure. tential lossof fitnessdue to delayed breeding; Whetherthis contrast reflects breeding territory our data show that helpers and breeders are "quality," or resourceavailability, is open to related.Helper nuthatchesalways aided breed- speculation.The smallerterritory sizein WCNM ers from their winter group (unpubl. data), (Sydeman 1985) supportsour contention that thereby increasingthe chanceof aiding a ge- this habitat was of higher quality. netic relative. Consequently,the benefits of in- Habitat effectsmay be related to cavity char- direct fitness (Brown 1974, Brown and Brown acteristics(Hay and G•intert 1983)or the quan- 1981)cannot be immediatelydiscarded as a fac- tity or quality of nesting sites in WCNM. The tor in the evolution of helping behavior in Pyg- age and size of ponderosapines and snagden- my Nuthatches. sity may increasethe availability of high-qual- Somejuvenile Pygmy Nuthatchesdisperse in ity nest sites. Pygmy Nuthatches can excavate the autumn of each year, while others remain a cavity in the soft wood of snags or dead on natal group territories throughout the win- branchesand lightning scarsof largelive trees, ter and breed, help, or float in the following thereby creating a quality nest site if none are spring. Floaters are unusual in the social or- available (pers. obs.). ganization of cooperativebirds (Brown 1978, Habitat effectsalso may be due to variation Koenigand Pitelka 1981),and we have no sound in the vegetation on breeding territories. Re- explanationregarding their obviouspresence productive successof Gray-crownedBabblers in 1984.The factthat certainyoung remain with (Pomatostomustemporalis) was determined in part their parentsin a winter group arguesfor ex- by the densityof a single tree species(Brown tended parental care and sharing of resources and Brown 1981). The importanceof gambel as a selectiveforce in the evolution of helping oak for PygmyNuthatches was suggested by a behavior (Ligon and Ligon 1978, Woolfenden high percentageof foraging observationson and Fitzpatrick1978, Emlen 1981,Koenig and this species(unpubl. data). The larger pinesof Pitelka 1981, Ligon 1981, Woolfenden 1981). WCNM may also provide the birds with more Communalroosting and group foraging may foliageon which to forage,and greatertree di- enhancethe probability of overwinter survival versity in WCNM could increase the availabil- (G'fintertin press).If dispersalis costly(Brown ity and diversityof prey. 1978) and membership in communal roostsis We believethat factorsaffecting nest defense limited (i.e. entranceprohibited by other birds), or abandonmentmay be the most important then Pygmy Nuthatch young that remain in determinantsof fledglingproduction. Breeding associationwith their parents may benefit by unitsthat failedto produceyoung were directly membershipin a winter group.For a small res- 76 $YDEMAN,GONTERT, AND BALDA [Auk,Vol. 105 ident speciesinhabiting an energeticallystress- 1978. Demographyof the Jungle Babbler, ful environment,this may be criticalto survival Turdoidesstriatus. J. Anim. Ecol. 47: 845-879. and, hence, any measurementof fitness. G'ONTERT,M. In press. Communalroosting in Pygmy Nuthatches:a winter survivalstrategy. Proc. 19th Int. Ornithol. Congr., Ottawa, Canada. ACKNOWLEDGMENTS HAMILTON,W. D. 1964. The geneticalevolution of social behavior. J. Theor. Biol. 7: 1-52. We are indebted to Dwayne Collier and the staffof HAY, D. B. 1983. Physiologicaland behavioralecol- Walnut Canyon National Monument for permission ogy of communallyroosting Pygmy Nuthatches. to work in the park and for collectingweather data. Ph.D. dissertation,Flagstaff, Northern Arizona S. D. Emslie,D. B. Hay, C. M. Madonia, H. Reiser,C. Univ. Sanford, S. Schwann, and P. G. Smithers contributed --, & M. GONTERT. 1983. Seasonal selection of in the field. We thank G. Bell and W. J. Boecklen for tree cavitiesby PygmyNuthatches based on cav- statistical advice. G. C. Bateman, S. D. Emslie, J. D. ity characteristics.U.S. For. Serv.Gen. Tech. Rep. Ligon, J. M. Marzluff, C. M. Madonia, T. M. Penni- RM-99. man, and two anonymousreviewers provided helpful HUNTERßL. g. 1985. The effectsof helpers on co- commentson draftsof this manuscript.Financial sup- operatively breeding Purple Gallinules. Behav. port for this study was provided by Sigma Xi Grants Ecol. Sociobiol. 18.' 147-153. in Aid of Research(WJS), the Chapman Fund of The KINNAIRD,M. F., & P. R. GRANT. 1982. Cooperative American Museum of Natural History (WJS),and the breedingby the GalapagosMockingbird, Neso- Swiss National Science Foundation (MG). rnirnusparulus. Behav. Ecol. Sociobiol.10: 65-73. KNORRßO. g. 1957. Communalroosting in the Pyg- LITERATURE CITED my Nuthatch. Condor 59: 398. KOENIG,W. D. 1981. Reproductivesuccessß group AUSTAD,$. N., & K. RABENOLD.1985. Reproductive size and the evolution of cooperativebreeding enhancementby helpersand an experimentalin- in the Acorn Woodpecker. Am. Nat. 117: 421- quiry into its mechanismin the BicoloredWren. 443. Behav. Ecol. Sociobiol. 17: 19-27. --, & F. A. PITELKA.1981. Ecologicalfactors and BROWNßJ.L. 1974. Alternative routesto socialityin kin selection in the evolution of cooperative jays--with a theory for the evolutionof altruism breedingin birds. Pp. 261-280 in Natural selec- and communalbreeding. Am. Zool. 14: 63-80. tion and social behavior: recent research and new 1978. Avian communal breeding systems. theory (R. D. Alexanderand D. TinkleßEds.). New Annu. Rev. Ecol.Syst. 9: 123-156. Yorkß Chiron Press. --, & R. P. BALDA. 1977. The relationshipof LACK,D. 1968. Ecologicaladaptations for breeding habitat quality to group size in Hall's Babbler, in birds. London, Methuen. Pornatostornus halli. Condor 79: 312-320. LEWIS,D. M. 1981. Determinants of reproductive ß & E. R. BROWN. 1981. Kin selection and in- successin the White-browed Sparrow Weaver. dividual selectionin babblers.Pp. 244-256 in Nat- Behav. Ecol. Sociobiol. 9: 83-93. ural selection and social behavior: recent re- LIGON,J. D. 1981. Demographicpatterns and com- searchand new theory (R. D. 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