Effects of Spacing and Synchrony on Breeding Success in the Great Frigatebird (Fregata Minor)

EFFECTS OF SPACING AND SYNCHRONY ON BREEDING SUCCESS IN THE GREAT FRIGATEBIRD (FREGATA MINOR)

BARRYJ. REVILLE Departmentof Zoology,University of Aberdeen,Aberdeen, Scotland

ABSTRACT.--Nearest-neighbormethods of spatial analysisconfirmed that coloniesof Great Frigatebirds(Fregata minor) on Aldabra Atoll were subdividedinto clustersof up to 22 nests/ cluster. Each cluster arose from a nucleus of advertising, unpaired males. Within a cluster, siteswere spacedregularly at intervals of 0.5-1 m. Clustersdiffered in degree of synchrony of female settlement.Hatching successcorrelated positively with synchrony of female settlement. The interval between female settlement and laying date decreasedas the season progressed.Hatching or fledging successwas not significantly affected by date of laying. Breeding success(fledglings/egg) exceeded50% in both seasons. Great Frigatebird malesformed display clustersone month before femalessettled, competed for displaysites within clusters,and preferredclusters that containedeight or moredisplaying males.Females settled more readily by males in the larger colony and, early in the season, preferred clustersthat already contained nests. I suggestthat usurpationof nestsby unpaired maleswas a principal causeof nestingfailure, and that synchronousfemale settlementwithin a cluster made the cluster lessattractive to prospectingmales than one that contained mostly advertising males. Selectivemate choice by femalesappears to be the principal agent inducing spatial clusteringby males.Received 4 May 1987, accepted23 October1987.

LARGEcolonies of nesting seabirdsare fre- (1984) commentedon the apparent paradox of quently composedof distinct subgroupsthat nestingin densecolonies when conspecificsare can be demarcatedby spatial analysis and by the chief predators.In the Gal•pagosunpaired socialinteractions within each subgroup(Nel- males seeking a display site usurped nesting son 1970, 1980; Parsons 1976; Veen 1977; Goch- pairs and destroyed75% of eggs and chicks feld 1979). In Herring Gulls (Larusargentatus), (Nelson 1975).Because predation frequently has for example,synchrony of egglaying increased beenof fundamentalimportance in shapingthe with colony size up to about 200 pairs, but de- spatialpattern of colonialspecies (Burger 1982, creasedin larger units (Burger1979). Subgroups Wittenberger and Hunt 1985), I suspectedthat were alsospatially distinct (Burgerand Shisler the advantage of greater synchrony would be 1980). Synchrony within, but not between, to minimize conspecificpredation. subgroupshas now been reported for many I attemptedto confirm that Great Frigatebird species(references in Gochfeld 1980). colonieswere composedof distinct subgroups Nelson (1967, 1975) described colonies of of up to 30 nests,that subgroupsdiffered in Great Frigatebirds(Fregata minor) as subdivided synchrony of breeding activities,and that dif- into numerous distinct nest clusters. Each clus- ferencesin synchronywere associatedwith dif- ter consistedof 10-30 closelyspaced nests and ferencesin breeding success.I proposea mech- arosefrom a cluster of displaying males.As in anism that links spatial pattern and synchrony most Pelecaniformes (van Tets 1965), the nest with breeding successthrough minimal con- is built on the display site after the male has specificpredation. attracted a female. Small clusterswith high nest densityshould STUDY AREA AND METHODS provide excellent opportunites for synchro- The birds studied nested on Aldabra Atoll, Sey- nized breeding activities. Nelson (1975: 122) cheiles. Aidabra (9ø24'S,46ø20'E) is an elevated, cor- proposedthat synchronizationoccurs within alline limestone atoll ca. 420 km northwest of Mad- clustersand is presumablyadaptive. The nature agascar(see Reville 1983: fig. 1). Mangroves fringe of the adaptiveness,however, has not beende- the lagoon and many lagoon islets (Macnae 1971). termined. Both Nelson (1975, 1985) and de Vries Most of the land mass is covered with dense thicket

252 The Auk 105: 252-259. April 1988 April1988] FrigatebirdSpacing and Synchrony 253 dominatedby Pernphisacidula, a broad-leavedmicro- photographsof the frontalview of MC:RM takenwith phyllous,evergreen sclerophyll (Fosberg 1971). Great an 85-mm lens at 25 m, I made a map of scaleca. 1:50 Frigatebirds nested only in mangrove, principally by tracing the outline and internal relief of vegeta- Rhizophorarnucronata (44% of cases)and Bruguieragyrn- tion. On each subsequentvisit I noted on the map norhiza(22%). Nesting colonies were confinedto three the position of each bird, its species,sex, age, and locationsalong the southernshore of Ile Malabar.The breedingstatus. When there was no longer a risk of majorityof nests(ca. 80%) were built on west-to north- my intrusioncausing chick loss, I measuredthe length, facingshores, presumably to obtain shelter from the height, and depth of the habitat patch and photo- prevailingsoutheast tradewinds and to assistlanding graphed the vegetation profile. (Diamond 1975). Nests were placed on, rather than Becausethe initial map was created from frontal- under or within, the canopy. view photographs,it indicatedonly the vertical dis- The studylasted from January1976 to January1978. tance between nestsof different depth. This did not Observations(560 h) from a permanentblind were accountfor the horizontalcomponent caused by the initiated at 8-day intervals, and each stint lasted up front-to-rearcurvature of the vegetationcanopy (lat- to 4 days.Results are for birdsnever handled or closely eral curvatureof the habitatpatch was minimal). The approachedbecause Aldabran frigatebirds were very actual distancebetween sites was obtained by mea- sensitive to human disturbance. This limited accurate suring the distancebetween them on the vegetation estimationof laying dates,so settlementdates of lay- profile rather than the difference in their heights. ing femaleswere used to calculatemeasures of syn- Similarly, the surface area of vegetation was ex- chrony. The settlementdate is the day a female first pressedas a flat surfacewith dimensions:length of landed by the advertisingmale with which she even- vegetationprofile x length of habitatpatch. A surface tually bonded.Detailed descriptions of the studysites area of 217.5 m 2 was estimated for MC:RM. andof the observationschedule were givenby Reville A 1-m2 grid was then superimposedon the new (1983). map and the position of each active site specifiedby Terrninology.--Iapplied the term "colony" to ag- a 6-figure Cartesiancoordinate. I encoded these co- gregationsof nestsfound in widely separatelocalities ordinates onto computer file (Honeywell 66, Univ. on the atoll, i.e. Camp Frigate, Middle Camp, and AberdeenComputing Center) and usedthe program Gionnet. Within colonies the distance between ad- "CLUSTER ANALYSIS OF CASES, BMDP-2M" (En- jacentgroups of nestsrarely exceeded100 m, whereas gelmann 1977; prepared by Univ. KansasComputa- several kilometersseparated colonies. Decisions on tion Center) to calculatethe distancebetween all pos- colony boundaries are typically arbitrary (Nelson sible pairs of sites.Simple Euclidean distanceswere 1980).My usageagrees with previousAldabran stud- usedas the measureof separationbetween each pair ies (Diamond 1975) and does not conflict with other of sites.Distances among 15 advertising male Great authorswho havestressed social integration (Coulson Frigatebirdsin MC:RM measuredfrom the map for 5 and Dixon 1979)or centralizedlocation (Wittenberger August 1977 were compared with visual estimates and Hunt 1985).Birds entering or leaving the colony (bird lengths) made on that day: No significant dif- were exposedto the sightand soundof hundredsof ferences was detectable (df = 10, T = 38.5, P > 0.05) neighbors.There was no evidenceof frequent visits with the Wilcoxon matched-pairssigned-ranks test between colonies. (Siegel 1956). The term "habitatpatch" describesa patchof con- Detectingnonrandomness.--I used the R index (Clark tinuousvegetation in which nestswere built. "Clus- and Evans1954) to testfor significantnonrandomness ter" describesan aggregationof activesites within a (R > 1 indicatesregularity of spacing,R < 1 indicates habitatpatch asestablished by the computerprogram clumping, ra = mean nearest-neighbordistance) and CLUSTERANALYSIS OF CASES(see below). The term Thompson'stest to the nth nearestneighbor (Thomp- "activesite" was applied to sitesoccupied by any bird son 1956) to identify averagecluster size. In both of adult plumage involved in breeding activitiesini- analysesthe nest siteswere treated as points rather tiated in the current season(including advertising than circles.Nest diameter approachedhalf the ex- males,courting pairs, and sitting birds) or by a nest- pected mean nearest-neighbordistance for points ling hatchedin the currentseason. Standard devia- (Simberloff 1979); however, the slope of the canopy tion was adoptedas the measureof synchrony.To allowed accesswithout disturbingneighbors. calculatevalues for synchronyof settlementand egg Identifyingclusters.--I used the CLUSTER ANAL- laying, only pairsknown to havebeen the first ten- YSISOF CASESprogram to produceclusters of neigh- ants of a nest were considered. boring sitesin MC:RM for dateson which there was Mapping of sites.--To obtain detailed information statisticalevidence of contagion.The clusteranalysis on sitespacing within a habitatpatch, I chosean islet, cannot tell whether the contagionwas due to, for MC:RM at Middle Camp, with continuousmangrove example,10 small or 5 larger clusters.Therefore, av- and open to scrutinyfrom a permanentblind. The erage cluster size was determined statisticallyby as- area of habitat appearedadequate for multiple clus- suming that the level of nearestneighbor at which ters of a size anticipatedfrom the literature. From contagionwas last detectedby Thompson'stest cot- 254 BARRYJ. REVILLE [Auk, VoL 105

TABLE1. Nearest-neighboranalyses of Great Frig- atebird site distribution in habitat patch MC:RM (R index).

No. of ra Date sites (cm) Ra P All active sites 23 Oct. 1976 100 60 0.82 <0.001 5 Aug. 1977 72 64 0.74 <0.0002 24Oct. 1977 110 56 0.79 <0.0002 10 Dec. 1977 68 76 0.85 <0.02 Advertisingmales only 5 Aug. 1977 57 78 0.8 <0.001 24 Oct. 1977 19 132 0.78 <0.08 Nestsonly 5 Aug. 1977 11 138 0.62 <0.01 24 Oct. 1977 84 63 0.79 <0.0002

indicatesclumping of sites.

responded to the average cluster size in the distribution.The cluster-analysispattern that producedthis same average cluster size was then taken to define cluster boundaries. The complete sequenceof steps involved was: (1) 0 5 lO •5 20 25 selecthabitat patch large enough to allow multiple LENGTH (m) clusters;(2) prepare two-dimensional map of sitesin Fig. 1. Distribution of active sites within habitat the habitat patch, preserving real distancespresent patchMC:RM with clusterboundaries. (a) 23 October in the three-dimensional field situation; (3) test for 1976, (b) 5 August 1977, (c) 24 October 1977. ß = statistical evidence of clumping using R index; (4) displaying male, ß = courting pair, ß = nest. producea variety of possiblecluster configurations using the cluster-analysisprogram; and (5) selectcor- rect clusterconfiguration by comparingmean cluster I examined the arrangement of sites within size with that obtained from Thompson's test. a cluster for two clusters of different date and composition.The first contained 16 advertising RESULTS males and 3 nests, the second 3 advertising males,2 courting pairs,and 19 nests.Areas (14.1 Nature of spatialpattern.--Great Frigatebirds m2 and 18.3m 2, respectively)were estimatedby in habitat patch MC:RM exhibited a consistent the convex-polygonmethod (Flowerdew 1976). and significant tendency to cluster (Table 1). In eachcase sites were regularly spacedwithin This was so for advertisingmales before many the cluster (ra = 58 cm, R = 1.37, P < 0.01; ra = femaleshad settled(e.g. 5 August1977), for nest 59 cm, R = 1.35, P < 0.001). owners at low (e.g. 5 August 1977) and high Temporalpatterns.--In 1976 clusters differed (e.g. 24 October1977) nest densities, and for all significantlyin the varianceof settlementdates active sites considered together. of females that eventually laid eggs (Bartlett's Thompson'stest suggestedthat the average homogeneityof variance,Bc = 56.96,df = 6, P < number of occupied sites per cluster differed 0.01). In 1977 differencesapproached but did between seasons and increased as the season not reach significance(Be = 9.52, df = 5, P < progressed,e.g. 14-15 sites(23 October1976, 5 0.1; Table 2). In 1976 the time between female August 1977)and 22-23 sites(24 October1977). settlementand egg laying declinedfrom 26 days Becausethe number of active sites present in in early August to 5 days in mid-October (n = MC:RM on 23 October 1976 (100 sites) did not 51, Spearmanrs = -0.603, P < 0.001). In 1977 differ significantlyfrom that on 24 October1977 the interval declined from 20 to 5 daysbetween (110 sites; X2 = 0.48, df = 1, P > 0.25), the dis- early Augustand October(n = 36, rs= -0.508, tribution of structurally suitable sites in MC: P < 0.002). The 5 females that settled after 19 RM did not dictateaverage cluster size (Fig. 1). October reverted to longer intervals, i.e. 17-30 April 1988] FrigatebirdSpacing and Synchrony 255

TABLE2. Settlement dates of female Great Frigate- birds in clusterswithin habitat patch MC:RM. •1oo

No. of Cluster females Mean date SD (days) 1976 •so I 2 14 Aug. 10 2 10 27 Sept. 36 3 7 12 Sept. 38

4 19 13 Sept. 31 o 5 17 6 Sept. 33 o 40 20 40 6 22 2 Sept. 25 S D DATES OF FEMALE SETTLEMENT 7 5 11 Sept. 48 ( days ) MC:RM 82 10 Sept. 32 Fig. 2. Hatching successas a function of syn- 1977 chrony(SD) of female settlementdates in clustersin 1 10 4 Sept. 29 MC:RM. (a) 1976:rs = -0.893, P = 0.01; (b) 1977:r, = 2 19 28 Aug. 21 -0.841, P < 0.05. 3 8 30 Sept. 23 4 20 31 Aug. 33 5 21 3 Sept. 23 laying seasoncontained a new displayingmale 6 2 16 Aug. 26 or courting pair when first seen after failure, MC:RM 80 3 Sept. 27 indicating that potential usurperswere available. Third, during an apparent food shortage from mid-Augustto mid-September1976, when days. Despite the potential for postsettlement many unpaired malestemporarily desertedthe synchronization, synchrony of subsequent nesting colonies,no incubating pairs lost their breeding events was highly correlated with sitesdespite exceptionally long incubationstints synchrony of settlement, e.g. SD of hatching (13.5 days cf. 4.3-6.3 days otherwise). Further, dates compared with SD of settlement dates in both yearsthere was a steep drop from early among clusters in 1976 (n = 7, rs = 0.929, P < October in numbers of unpaired males attend- 0.01).Clusters with higher synchronyof settle- ing the colonies.The first chicksbegan hatch- ment of first-tenant females had significantly ing about mid-October. Such timing of hatch- higher hatchingsuccess (1976: n = 7, rs= -0.893, ing would be adaptive if young chickswere, as P = 0.01; 1977:n = 6, rs= -0.841, P < 0.05;Fig. in other populations,targets for site usurpation 2). by unpaired males. This would not be a strat- There was no significant correlation among agem against predation of chicks for food be- clusters between mean date of female settle- causenesting males still attended the colonies. ment and any index of breeding success.Hatch- If prospectingmales were attractedto clusters ing or fledging successwas not significantly of displaying malesrather than to clusterscon- affectedby laying date, although the few fe- taining mostly nests,then synchronousfemale males that settled after mid-October had lower settlementwould rapidly changethe character breeding success(1976: 33% vs. 58%; 1977: 33% of a displaycluster and discourageinvasion by vs. 59%). further prospectingmales. Clusters with great- Nest usurpation.--Relativelyhigh breeding er synchronyof female settlementwould suffer successin 1976 and 1977 (Table 3) reduced the opportunity for viewing nesting failure, and I did not observe the moment of loss of a Great TABLE3. Componentsof breeding successof Great Frigatebirds in habitat patch MC:RM. Frigatebird nest. The possibility of nest usurpation by malesis suggestedby the following 1976 1977 a observations.First, unpaired male Great Frig- Eggs 111 100 atebirdsfrequently attemptedto obtain females Hatched 60 55 by displacing the males of courting Great Fledged 57 * Frigatebird pairs. They also forced incubating Chicks/egg (%) 54 55 LesserFrigatebirds (Fregata ariel) off nests;hence, Fledglings/chick (%) 95 * some usurpation occurred. Second, more than Fledglings/egg (%) 51 * 80% of nests that failed in the first half of the all chicks alive when observations were terminated. 256 BARRYJ. RœVlI,I•E [Auk, Vol. 105

4C 1976• 50

• 20

50 5 10 MEAN NO. ADVERTISING MALES

Fig. 4. Relative variability in numbers of advertising males attending clustersthat containeddiffer- ent numbers of males. Mean values obtained from counts each 5 min over 4 h.

0 J J A S 0 N D of clustersize over a wide range of sizes (2-14 DATE males/cluster, 14 clusters, rs = 0.23). Fig. 3. Attendanceof advertisingmales (O) at and Females appeared to discriminate in their recruitment of nest (0) to MC:RM. choiceof male or cluster.They consistentlypre- ferred the larger of the two colonies. At the larger colony (Middle Camp) in 1976, 54 days less usurpation. The following factorssuggest of advertising were invested.male-•-nest-• re- that males were attracted by other displaying cruited, compared with 78 days per male at males.First, clusterswere establishedby inter- smallerCamp Frigate(calculated by measuring actions among advertising males more than a the area between the curves for number of males month before females began settling (Fig. 3). and number of nestsvs. date, then dividing by Second, there was considerable competition the number of nestsrecruited). In 1977 the fig- amongmales for siteswithin theseclusters even ureswere 55 and 91 days,respectively. Further, when alternative sites, later to receive nests, femalesdid not begin settling and pair bonding were unoccupied.Supplanting, threatening, and until many males were advertising in clusters fighting among advertising males were com- (Fig. 3). Finally, when prospecting,the female mon, in contrast to the situation in the Gal•t- did not hover over a particular male, but swept pagoswhere such events happened only after low over a seriesof clusters,setting off a wave the acquisition of a female (Nelson 1967, 1975). of displayin her wake. In 0 of 5 intervalsduring Third, the numbers of advertising malesat larg- the nesting seasondid the recruitment of nests er display clusterswere less variable than at correlatesignificantly with the distribution of smaller clusters,suggesting that clusterswhere displaying males among clusters (7 clusters, more malesdisplayed were preferred (n = 14, maximumrs = 0.56). In the early nesting season rs= -0.937, P < 0.01). Variability changedlittle the recruitment of new nestswas significantly once cluster size exceeded8 displaying males correlatedwith the distributionof existingnests (Fig. 4). Larger clusters did not facilitate the (n = 7, rs = 0.79, P = 0.05). This suggeststhat displayof individual malesbecause the amount females settling early in the seasonpreferred of gular presentationper male was independent clustersalready containing nests.The mean in- April 1988] FrigatebirdSpacing and Synchrony 257 terval between recruitment of nests declined by breaking off the attack if the resident, who linearly from 26 days for a 2-nest cluster to 4 cannot move from the nest, resistsstrongly. In days for clustersof more than 11 nests(n = 7, addition, the usurper acquiresa ready-built nest rs= -0.954, P < 0.01). Femalesprobably visited platform, an important energy consideration several males in different clusters before form- becausenest material typically is gathered by ing a stable pair for the season.Copulation of the male. During poor conditions the usurper one male with several females was confirmed is likely to be in a better nutritional state than by paint-marking, but the effectivenessof the a nesting bird becausehe is not prevented from copulations could not be determined. In 11% feeding by long incubation stints. Also, resis- (10/92) of hatchingsthe female settled by her tance from the resident bird is less probable if eventual mate less than 5 days before the egg it is already stressedby shortageof food. Con- was laid. versely, good feeding conditionswill promote stronger resistanceand discourageattempts at DISCUSSION usurpation. Consistently poor feeding conditions will also discourageusurpation because The spacingpattern of Great Frigatebirdson the breeding attempt of the usurperis alsolike- Aidabra resembled that on Tower Island in the ly to fail. Thus, usurpation is likely to be most Gal/•pagos(Nelson 1967, 1975).The similarity frequent in populations experiencing unpre- between two distant populations in different dictable feeding conditions.Conditions may be habitats suggeststhat subdivision of a colony poor at the time of usurpation, but the usurper in discreteclusters is typical of the species(sensu gambles,at little cost,that an improvementmay Veen 1977). Differences between the popula- SOOn Occur. tions included lower breeding success,more On Tower Island incubation stints averaged frequent site usurpationby conspecificmales, 10 days and extended to 18 days (Nelson 1975: and greater synchronizationof breeding activ- 153). Nelson (1967) frequently describedfood ities on Tower Island. Identifying the causesof availability as "unpredictable." In contrast,in- thesedifferences helps clarify the origin of the cubationstints on Aidabrausually averaged 4.3- clustering phenomenon. 6.3 days and extended to 13.5 days in one ex- Frigatebirdsare not the only group that il- ceptional period. This difference may account lustratesintraspecific interference (e.g. gulls: for the higher incidence of site usurpation on Parsons 1971, 1976; Ewald et al. 1980; Schoen Tower Island. Becausesynchrony of female set- and Morris 1984; corvids: Yom-Tov 1975; re- tlement within the cluster discouragesthe at- viewed by Wittenberger and Hunt 1985); how- traction of potential usurpersto the cluster, it ever, the extent of sabotageof nestingattempts will be most apparent in populationswhere en- by unpaired males has been considered re- vironmental conditionsfavor usurpation. markable (e.g. Bent 1922). Recently, de Vries The aggregation of advertising Great Frig- (1984), in an argument reminiscent of group atebird males in clustersresembles the spatial selection,suggested that this behavior contrib- pattern of males in many lekking species(re- utes to population regulation. Nelson (1985) view by Bradbury and Gibson 1983). Further emphasizedthe primary role of individual se- parallelsare the conspicuouscommunal adver- lection and pointed out that "by disrupting tisementby males, competitionbetween males breedingpairs, non-breeding males increase the for positionsin display clusters(Diamond 1975; supplyof availableand experiencedfemales and contraNelson 1967, 1975), and development of thereby their own chancesof acquiring such a male epigamicstructures (e.g. inflated gular sac) mate." and behavior (e.g. gular presentation;Nelson The argument can be extended to establish 1975) used both in male-male threats and in when it is advantageousfor an unpaired male advertisement to females. to attempt usurping the site of a nesting pair. Selectivemate choiceby the females,as in Clearly, there mustbe somechance for the usur- some lekking species (Davies 1978, Halliday per to acquire a female, and there must be suf- 1983), may have forced the males to compete ficient time remaining in the laying seasonto for arbitrary display positions, that is, sites initiate a successfulbreeding attempt. The cost within clusters.(In Great Frigatebirdsthe dis- to the usurperis relatively small. There is a risk play site must alsohave suitablephysical char- of physical injury, but this can be minimized acteristicsfor supporting the nest.) Becausefe- 258 BARRYJ. REVILLE [Auk, Vol. 105 maleGreat Frigatebirds visit a numberof clusters populationsin different environmental condi- and males before pair bonding, characteristics tions. of the males probably are assessedas well as ACKNOWLEDGMENTS compositionof the cluster.Female Great Frig- atebirdson Aldabrawere well placedto exercise The study was supportedby an Aberdeen Univer- such discrimination.In many speciesthere is sity PostgraduateStudentship, the Royal Society of an advantagein mating early in the season;oth- London, and the Frank Chapman Memorial Foun- erwise, the best mates, the best sites, or the best dation. I am grateful to J. M. Cullen, A. W. Diamond, seasonalconditions may be missed(Perrins 1970, N. Dunlop, J. B. Nelson, A. Stokes,G. F. van Tets, R. Hunt and Hunt 1976, Hunt 1980). These con- Woollet, and an anonymousreviewer for comments straints did not apply to female Great Frig- on drafts of the manuscript. M. S. Gould and C. J. Peet gave invaluable assistancein the field. atebirds on Aldabra. There was no significant differencein breedingsuccess for birds laying LITERATURE CITED at differentstages of the season,and the quality of nest siteswas not physicallylimited but de- BENT, A. C. 1922. Life histories of North American pended on a repetitive combination of social petrels and pelicansand their allies. Bull. U.S. Natl. Mus. 121. factors.Most importantly, there was a surplus BRADBURY,J. W., & R. M. GIBSON. 1983. Leks and of males available each season (Reville 1983), mate choice. Pp. 109-138 in Mate choice (P. Bate- sofemales could delay settlementyet be certain son,Ed.). Cambridge,England, Cambridge Univ. of finding a mate.In speciesexhibiting a surplus Press. of males,sexual selection is likely to be strong BURGER,J. 1979. Colony size: a test for breeding (Halliday 1978). synchronyin Herring Gull (Larusargentatus) col- I suggestthat clusteringis likely where it can onies. Auk 96: 694-703. be enforcedby femaleselectivity, as when more 1981. Sexual differencesin parental activi- males than females are available each laying ties of breeding Black Skimmers.Am. Nat. 117: season.A surplusof malesat courtshiphas been 975-984. demonstrated only for the Great Frigatebird ß 1982. An overview of proximate factors af- fecting reproductive successin colonial birds: (Diamond 1975,Reville 1983)and the Magnif- concludingremarks and summaryof panel dis- icentFrigatebird (F. magnificens;Diamond 1972). cussion. Colon. Waterbirds 5: 58-65. In the latter, females outnumber males overall, , & J. SHISLER.1980. The processof colony but males breed annually and females bienni- formationamong Herring Gulls Larusargentatus ally (Diamond 1972, 1973; Coello et al. 1977; nesting in New Jersey.Ibis 122: 15-26. Trivelpiece and Ferraris 1987); hence, fewer fe- CLARK,P. J., •1;F. C. EVANS. 1954. Distance to nearest malesthan males are available for courtshipin neighboras a measureof spatialrelationships in mid-September (Diamond 1972). In F. ariel on populations.Ecology 35: 445-453. Aldabrathere seemto be equalnumbers of males COELLO,F., C. HERNANDEZ, g. L. ORTEGA,g. L. VRIES, and females available (Diamond 1975, Reville & T. J. VRIES. 1977. Reproducciony frecuencia 1983). Insufficient information on sex ratio is alimenticiade Fregataminor en Genovesay Fre- gatamagnificens en Seymour,Galapagos. Rev. Univ. available for F. aquilaor F. andrewsi.Whenever Catolico Quito 5: 71-110. females can copulate with a number of males COULSON,J. C., & F. DIXON. 1979. Colonialbreeding before deciding on an eventual mate, male in- in sea-birds.Pp. 445-458 in Biologyand system- vestmentin the chick is likely to be lower than aticsof colonial organisms(G. Larwood and B. that of the female (Trivers 1972, Burger 1981). R. Rosen, Eds.). Syst. Assoc.Spec. Vol. No. 11. Where feeding conditionspermit the female to London, Academic Press. raise the chick alone, it is to the male's advan- DAVIES,1N[. B. 1978. Ecologicalquestions about ter- tage to abandon rearing and attempt to breed ritorial behaviour. Pp. 317-350 in Behavioural annually.Consequently, feeding conditionswill ecology:an evolutionary approach(J. R. Krebs and N. B. Davies, Eds.). Oxford, Blackwell Sci- affectthe nature of the spatialpattern through entific Publ. the ratio of males and females available for DE VRIES,T. 1984. Why are frigate-birds colonial? courtship as well as through the likelihood of Not. Galapagos40: 19-22. site usurpation. Differences in spatial pattern DIAMOND,A.W. 1972. Sexualdimorphism in breed- may be expected,not only between different ing cyclesand unequalsex ratio in Magnificent frigatebird speciesbut alsobetween conspecific Frigate-birds.Ibis 114:395-398. April1988] FrigatebirdSpacing and Synchrony 259

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