Group Adherence in the Common Tern •

BIRD -BAND ING A JOURNALOF ORNITHOLOGICALINVESTIGATION

VOL. XXII JANUARY,1951 No. ].

GROUP ADHERENCE IN THE COMMON TERN •

BY OLIVER L. AUSTIN, M.D.

In a precedingcontribution (Austin, 1940; 153) it waspostulated that coloniesof Common Terns (Sterna hirundo Linnaeus) breeding on CapeCod, Massachusetts,form a distinct,concrete group of individuals which is self-sustainingand relatively free from association with other groupsduring the nestingseason. The very small interchangeof membershipbetween this and othergroups never occurs on the breedinggrounds, but only duringmigration or on the wintering grounds. An addi.tionaldecade of fieldwork and studyin this areahas yielded datawhich not only further substantiatethe conceptof the Cape,Cod groupof .terns,but alsoshow some of the reasonsfor the persistence of this phenomenon.I,t suggeststhat while kinshipis an important causativefactor, as in similar associationsof other forms, it is su.pple- mentedby long-continuedalliance of the componentindividuals, and increasesin effect with age. Prerequisiteto ,thecohesion of •theCape's tern coloniesinto a discrete groupis the habit of colonialnesting, which in turn is actuatedmainly by two major behaviortraits, site tenacity and groupadherence. The two functionconcurrently, each enhancing the accomplishmentof the other. Site tenacityis basedon a tern's attachmen*to specificterrain. Group adherenceis the outcomeof the attachmentof terns to one another. Site tenacityhas alreadybeen discussedin detail (Austin, 1946) but its bearingon the establishmentand persistenceof the ,Cape aggregationwas not stressed. The purposeof this contributionis to reviseand delineatethe concept of the CapeCod groupof tern coloniesin the light of more recently acquireddata, to evaluatethe phenomenonof group adherence,and to show its role in the formation and maintenanceof the Cape society. The extensivebandings and returns on which these conclusionsare basedwere obtainedduring 20 consecutiveyears of work in the Cape Cod colonies,1929 through1948. Printing costsforbid their publica- tion in detail. The files of this stationshow the followingtotals: Adults banded ...... 47,409 Chicks banded ...... 165,610 Returns& recoveries,breeding grounds ...... 31,867 Repeats,breeding grounds ...... 4,800 Recoveries,wintering groun.ds ...... 292

Total ...... 249,686

•Contribution number 45 from the Austin Ornithological Research Station. AUSTIN,Group Adherence in the Common Tern Bird-BandingJanuary

THE CAPE COD GROUP OF TERNS The CapeCod tern coloniesare locatedwithin a circleabout 30 miles in diameter. Its centeris on the south shore of Massachus•tsBay midwaybetween the Cape,Cod ,Canal and the AtlanticOcean. At the four cornersof a squareinside this circleare the Tern Island,Jeremy's Point, Plymouth,and Ram Island rookeries. That Ram Island is an integralpart of •the.Cape group is con,trary to an opinionexpressed in 1940. Subsequentfield work has demonstratedregular and extensive interchangeof its individualswith thoseof otherCape colonies. Indeed, when the Tern Island colonywas disruptedin 1944, one-fourthof its populationrenested temporarily at Ram Island. Within thisperimeter 13 othersmaller nesting sites have been used for varyingperiods by coloniesof from 50 to 3000 pairs of terns. Always about1000 birdsnest in smallflocks of a few pairs in widelyscattered places. For the last two decades,despite constant shifting of popula- tionsbetween the siteswithin .the circle, the totalpopulation of the .Cape group has remainedapproxima,tely 25,000 terns. The lows in the populationhave alwaysoccurred the third and fourth yearsfollowing seasonsof abnormallylow chickyields. Thisindicates that the population is maintainedby its own reproductionrather than by accretions from other colonies. The bestproof of the postulaiedisolation of ,the,Cape group of terns is suppliedby an analysisof the 31,867returns taken from 1933 through 1948. An adequatesample of ten percentof the group'spopulation was trappedfor the first time in 1933, and larger sampleshave been handled every successiveseason since exceptin 1934. The average annualsampling is 20 percent,one fifth of the population. These takes reveal that a small number of terns raised in colonies elsewhereappear regularly in the Capeterritory, and becomeintegral membersof the group. Theseso-called "foreign recoveries" are ,listed in the fol,lowingtable: RECOVERIES IN THE CAPE COD COLONYES OF TERNS BANDED ELSEWHERE, THROUGH 1948. Banded as 1933 '35 '36 '37 '38 '39 '40 '41 '42 '43 '44 '45 '46 '47 '48 Adt. Juv. T1. PenikeseIsland 1 2 3 9 19 23 20 22 18 21 10 8 4 12 31 141 172 Weepecket Island 5 2 9 11 11 12 15 23 17 13 5 5 5 3 42 94 136 IViachias Seal Is., Me. 1 1 2 2 Petit Monan Is.,]•e. 1 1 Stratton Is., Me. 2 2 2 Bumkin Is., Me I I 1 IV[oriches Inlet, N. ¾ I I 5 4 2 I I 13 14 Cartwright Is., N.Y. 1 1 2 2 Tuckers Is., N. ,I. 1 Brant Beach, N.J. 1 1 2 2 Charity Is., iVfich. 1 1 2 2 Black River Is., iVfich. 1 5 1 7 7 West Pipe Is., Mich. 1 1 1 Oak Is., Mich 1 1 1 Scarescrow Is., 1•ich. 1 1 2 2 Leach Lake, Minn. I 1 1 Ah Gwah Ching, Minn. 1 Total I 5 4 12 21 30 41 39 55 41 39 16 15 12 17 74 274 348 These348 foreign recoveriesare 1.1 percentof the returnstaken. The remaining98.9 percen,t were all bandedoriginally within ,the ,Cape territory. The averageannual capture of theseforeign birds is 23.5 individualsfor the 15-yearperiod. As the annual sampleis roughly one-fifth of •the total population,this indicatesan averageof 117 foreign-raisedbirds presentat any one time amongthe 25,000 Cape ¾o•.1951xx• AUSTIN,Group .ddherence in the Common Tern [3

terns. But 77 of the 348 foreignreturns were made by 48 individual birds retrappedfrom a secondto a seventhseason after their first appearance.Thus the foreign recoveries represent only 271 individuals, an averageof 18 annually,reducing to 90 the probablenumber of foreignbirds present at any onetime, or 0.036percent of the group population. This totalis sosmall that, whileit may be of importancegenetically, it haslittle distributionalsignificance, and the circumstancecannot be considereda major behavior trait. Neverthelessthe presenceof these foreignbirds requires explanation. How andwhy theyhave overcome the impellingurge of site-tenacitywill be suggestedlater. A breakdownof the totalsinto regionalsources as follows: Vineyard region: Penikese Island 172 Weepecket Island 136

308 88.5 percent From the north: Maine 6 1.7 percent

From the south: New York 16 New Jersey 3

-- 19 5.5 percent From the •vest: Michigan 13 Minnesota 2

-- 15 4.3 percent showsthat an overwhelmingproportion of •heseforeigners comes from thenearest adjoining colonies to the south,which is onlyto be expected, and will be discussedin detail later. But distance,is evidentlynot a factor of moment,for the next nearestsource of terns,•he New Hamp- shire and the Maine colonies to the north, whose birds should theo- reticallypass by ,Capeterritory in migration,show the lowestpercentage of incidence,and are surpassednot only by the more distantsouthern colonieson the AtlanticCoast, but by thosein the GreatLakes region. So far as is known, the Great Lakes birds follow a flyway up and down the Mississippivalley, •and do not come in contactwith the Atlanticcoast individuals until both reachthe vicinity of their common winteringgrounds in the ,Caribbeanand southward.Latitudinal migration betweenthe Atlanticand GreatLakes regions ,has been limited to a singleoccurrence (Lincoln, 1927; 27) whichmay well be regardedas accidental.The logicalexplanation for the appearanceof thesewestern birds is that it is the result of their pairing with ,Capebirds on the winteringgrounds, and accompanyingthe new mate backto the breeding grounds. When rematingoccurs in the Capecolonies the nesting siteused is the onemost frequently occupied by one of the pair. Thus sitetenacity is overcomein theseinstances by faithfulnessto a newmate. Whenthe 'Cape's total tern populationfor the year hasbeen completed by the arrival of the last migrantsfrom the winteringgrounds, appar- 4• AUSTIN,Grottp Adherence in the Common Tern Bird-BandingJanuary entlyno further changeoccurs in it during,the remainder of the breeding season. Althoughterns frequentlyshift from one colonyto another within the area beforelaying, and later for renesting,no tern trapped breedingin the Cape group'sterritory has ever been taken nesting outside it the same season,and vice versa.

THE VINEYARD GROUP OF TERNS It 'is postulatedthat a groupof tern coloniesduplicating the Cape Cod group occupiesthe area south of the •Capefrom the Elizabeth Islandsto Nantucket,with its geographicalcenter at Martha'sVineyard. This group had ,threelarge terneries,on Muskeget,Penikese, and WeepeckerIslands, and the usualquota of smallerrookeries where terns still breedscattered along the shoresof Martha'sVineyard, Nantucket, andthe 'Elizabeth chain. Theseterneries are soclose to the•Cape group geographica}lythat they might seemto form part of it. Weepecketand Penikese are nearer to Ram Island ,than the latter is to Tern Island and Plymouth. Yet the bandingevidence shows .no such close affinity of Weepecketand Penikeseterns to Ram Island as the birds from the Cape rookeriesdo. The nearestVineyard colonyto ,the Cape'sterritory was, until its final dispersalin 1940 ('Crowell;1946), on the WeepecketIslands, off NaushonIsland, seven miles southeast of Ram Island. Next in proxim- ity, five miles farther south,is .Penikese.Then 22 miles to the eastward are the MuskegetIslands, formerly occupied by one of New England's lar.gesttern colonies,which in recentyears has been largely displaced by gulls. Ten miles east of 'Muskeget,and 25 miles south of Tern Island, are the few scattered rookeries on Nantucket Island. The .ternson Muskegetwere never bandedin quantity.nor has any attentionever beenpaid to the smallsatellite colonies on the Nantucket and Martha'sVineyard beaches. The Weepecketand Penikeseterns, however,have been banded in somenumbers. Terns nestedfor at least 40 yearson the Weepecketsuntil .theywere dispossessedby Herring Gullsten yearsago (Crowell; idem}. The averagepopulation through 1934 was 3500 birds. Thereafter it diminishedrapidly until the final nestingin 1940. In 1934 the colonyyielded 1135 chicks,586 in 1939, and in 1940 only eight. Altogether1057 adultsand 6005 chickswere bandedat Weepecketbefore its destruction. PenikeseIsland has domici|eda colonyaveraging 5000 individuals for many years. It hasexperienced the seasonalvariations in size,and the usualmishaps which occur from ,timeto time in all large rookeries. Someyears it has maturedno chicks. Variousworkers have banded a total of 991 adults and 17,539 chicks there. The numbers of thesebirds taken in the Cape ternerieshas never increasedwhen the Penikese colonywas knownto be sms•llerthan usual. In 1938, when the •Cape group'spopulation was at an unprecedentedlow, two dayswere spent trappingadults at Penikesein a ,searchfor the missing,Cape birds. Not oneof the 616 adultstaken had .been ,banded in a •Capecolony. Casual trapping the followingyear by the residentwarden yielded similarly insignificantresults. However, in early June 1950, 531 adults were trappedthere, of which 12 were returns: sevenfrom Penikeseitself, two from Ram Island,and one each from Tern,Bird, andWeepecker Islands. Vol. 1951XXI! AUSTIN,Group Adherence in the Common Tern [5

The mostcompelling reason for consideringthe Vineyardand Cape coloniesa singlerather than separategroups is their geographical closeness.It is difficult.to accept the fact that proximityof nestingsites has but smallinfluence on an interchangeof individuals. An examina- tion of thebanding evidence, however, shows that distancesup to at least 30 miles have a negligibleinfluence on tern .behavior. The air-line, tern-flightdistances between Weepecket, ,Penikese, and the major ,Cape terneriesare shownin the followingtable:

DISTANCES IN MILES BETWEEN COLONIES Weepecket Penikese Ram Bird Tern Plymouth Weepecker 9 7 91/:, 44 28 Penikese 9 11 ¾._, 16 53 35 Ram 7 Ill/._, 5 38 23 Bird 9•a 16 5 35 19 Tern 44 53 38 35 36 Plymoutk 28 35 23 19 36 Weepecketand Penikesebirds would .be expectedon geographical groundsto showtheir closestaffinities to Ram and Bird Islands,which are four times as close to their customarynesting sites as the next nearest Cape colony. However, an analysis of the Penikeseand Weepeckerrecoveries as follows: l•anded at: Recovered at: Ram Is. Bird Is. Other Cape ,Sites Total Penikese I•,. 28 18 96 (67.5%) 142 Weepecker Is. 22 11 60 (64.9%) 93 shows•that only one-thirdwere taken at the nearestsites. These recoveriessuggest that the Weepecketcolony, although closer to Cape territory than Penikese,has a slightly,less affinity with the 'Capegroup. The banding evidenceis unfortunatelyone-sided. Al.though both adultsand chickswere bandedrather extensivelyat Weepecketbefore its destruction,and chickswere banded in wholesalequantities some yearsat Penikese,.the combined totals are only a smallfraction of the totalsbanded in the ,Capecolonies. Likewisethe samplingof the adult populationa.t neitherof thosesites has approachedthe thoroughness and adequacyof the Capesampling. The percentageof bandedbirds presentin the Capecolonies is manytimes higher than at Weepeckerand Penikese. Theseimportant variables can be minimizedby comparing the returnsas percentages of the totalbanded birds available from each site. The 142 individualrecoveries of Penikesebirds are 2.84 percentof its estimated5000 population,and 0.008 percentof the total banded. The 93 Weepecketterns recovered are 2.66 percentof the peakpopula- tion therein 1935, and 0.01 percentof the total banded. This givesa truerpicture, showing that aspercentages of the bandedbirds available, moreWeepecket than Penikese birds have appeared in the,Cape colonies. However,the Weepecketbirds weredispersed in 1940, whilethe occu- pancy of Penikesehas continuedover the years. Hencea higher percentageof Weepeckerbirds should appear elswhere. A further breakdownof the Weepecketand •Penikeserecoveries in 6] AUSTIN,Group •tdherence in the Common Tern Bird-BandingJanuary

the Capeterritory is givenin .thefollowing table, with a setof similar Tern Island recoveryfigures for comparison.These figuresare all percentages,based on the numbersof bandedbirds available from each sourceof origin:

RECOVERY PERCENTAGESOF TERNS BANDED AT THE WEEPECKET, PENIKESE, AND TERN ISLAND COLONIES,THROUGH 1948

WEEPECKET RECOVERIES Banded as: Ram IsL Bird Isl. Plymou.th Entire •Cape Adults .014 .0057 .0038 Chicks .0023 .0023 .0022 Total .004 .0028 .0024 .008

PENIKESE RECOVERIES Banded as: Ram Isl., Bird Is]. Plymou,th Entire ,Cape Adults .004 .003 .004 Chicks .001 .001 .0012 Tot'a] .0013 .001 .0019 .009

TERN ISLAND RECOVERIES Banded as: Ram Is], Bird Is]. Plymouth Entire Gape Adults .0043 .0093 .014 Chicks .004 .0008 .0026 Total .0012 .0026 .005 .164 This tableshows even more clearly the irrelevancyof geographical proximityto choiceof site. While twice as many Weepecketbirds wentto Ram Islandas to Bird Island,a mere2« milesfarther away, almostequal percentagesof them relocatedat Bird Island, 9« miles away,and at Plymouth,28 milesdistant. The percentages.of Penikese birds taken at Ram Island, 11« miles, and at Bird Island, 16 miles distant,are smallerthan at .Plymouth,35 miles away. Also, almost exactlythe samepercentages of birds cameto Ram from Tern Island, 38 milesdistant, as from nearbyPenikese, while a far higherpercentage of Tern Islandemigres went 36 milesto Plymouth. The dissociationis moreapparent when data undistorted by variables are selected.Two-thirds of the .chicksbanded at Weepeckerwere markedin the 1930, 1931, 1933, and 1934 seasons(Crowell; idem), whenthe colony was at itspeak production. The recoveriesin theCape are from this selectedgroup, in percentsof the total banded,are as follows: Ram Isl. Bird Isl. Plymouth Entire Cape .0017 .00024 .001 .014 Theserecoveries show beyond question that geographicalproximity hasno effecton the selectionof newbreeding sites. The308 recoveries of Weepeckerand Penikese birds in ,Capeterritory compriseless than onepercent of the 31,178returns and recoveriestaken in the Cape coloniesfrom 1933 through 1948. As Penikeseterns furnish0.5 percent,Weepecker birds 0.4 percent,the relationshipof bothcolonies to theCape group is almostequal. This contrasts markedly withthe seasonal interchange of membershipsbetween the Capecolonies themselves,which is rarelyless than fivepercent (Austin; 1949). The 76 membersof the dispersedWeepecket colony recovered in the •rol.1951xxII AUSTIN,Group •4dherence inthe Common Tern [7

Capecolonies since 1938 representsso smalla fractionof the probable survivors that the remainder must have relocated elsewhere in the Vineyardvicinity whereno adult trappinghas yet been done to dis- cover them. This is what terns have been found to do under similar circumstancesin the Capegroup. The principalreason for assigningthe Weepeckerand Penikeseterns to the postulatedVineyard group in the absenceof positivebanding evidenceis the negativeevidence against their holdingclose membership in the Capegroup. Thereexists between them and the Capecolonies no suchbond as unitesthe latter into a unit. The small interchangeof membershipswhich has occurredbetween them is simplythe logical resultof environmentalchanges rather than of a tie betweenthe birds themselves. Other corrobora.torydata are available, but the foregoing seem sufficientto indicatestrongly that the Weepecketand Penikesecolonies arenot an integralpart of the,Cape group, but ratherare part of another similar associationadjoining it ,to .thesouth. The evidence,however, is not conclusive,and additional banding data from the Vineyard coloniesmay modify the postulate.

GROUP ADHERENCE A predominanttrend in all colonialnestings is the annualreturn of the samegroups of individualsto the siteseach had occupiedbefore. The implicationis that site tenacityis whollyresponsible, which is not entirelytrue amongthe Cape,Cod terns. Althoughthe group always nestswithin the sameloosely defined perimeters, the degreeto which site tenacityis effectiveis in direct ratio to the size of the associationor unit concerned,and to the comparativeamount of territory each nnit occupies. Each colonyuses only a part of the group'sterritory, and eachpair of birds holds only a few squarefeet within its colony'sboundaries. The Cape group as a whole, it has been shownabove, consists 98.9 percentof birds raised or bandedhere. In its componentcolonies, which are smallerunits, the correspondingpercentage drops to 75.4. Of individual birds, only 56.8 percent return to the territory ea& occupiedthe previousseason (Austin; 1949, 12). If site tenacityalone determinedwhere the birds nested,the size of the unit would not matter, and all thesepercentages would be aboutequal. Hencethere existsa secondinfluence which increases in potencyas the sizeof the unitsgrows larger. This is group adherence. Apparentlythere exist within tern colonies,especially large ones, associations or dans of from a few to even 100 or more birds. Such societiesmay persistfor two or more seasons.The organizationsare rather loose,yet someof them appearto be maintainedquite as consistentlyas are the coloniesthemselves. They follow the generalbe- haviorpattern as consistentlyas doesthe colonyas a whole,but some acts,performed in unisonby the membersof suchan association,are not duplicatedsimultaneously by the remainderof the colony. Atten- tion was first drawn to this during adult trappingat Tern Island. It was observedrepeatedly that the serial band numbersof the returnstaken on a given .day were too nearly consecutiveto be a 8] AUS?IN,Group •4dherence in the Common Tern Bird-BandingJanuary coincidence.Reference to the sta,tion'srecords which show the part of eachsite trapped each day, revealedthat the seriesof bandsin question had been appliedone or more years previouslyin the samerestricted locality. At first it was thoughtthat slatetenacity was responsiblefor this grouping. But later, whenit was foundthat smallgroups of Tern Island birds, likewi'sewearing consecutivenumbers, were trapped renestingcompactly at •Bird Island and Plymouth,it becameevident some additional factor existed. Every seasonsome unknown condition prevails temporarily over both site tenacity and group adherenceto cause a moderateamount of impermanentshifting of breedingsites. This is shownby data obtained from Tern Islandbirds duringthree seasons when the colonywas 1000 birds smaller than usual, and one when it was 3000 birds smaller. Of the Tern Island birds recoveredat other sitesthose years, more than one-halfof thoserecovered su,bsequently had returned again to Tern Island. From 1932 through1943, coincidentwith maximumpopulations at Tern Island,there were large nestingson North Point, one mile away. This colonywas always started each year by an overflowof Tern Island birds,which was occasionally augmented later eachseason by renesting individuals from other colonies. .No terns have nested on North Point since1943. Of the birds which formerly nestedthere, 10 at most have been taken since then elsewhere than at Tern Island. There have beenseasons when an entire colonyhas failed to occupy its customarysite. Therewas no nestingat Plymouthin 1943 and 1944. The larger part of that colonyrepaired to Tern Island and raised its occupancyto a peak of 18,000birds. In 1945, however,the usual4000 birds relocatedat Plymouth,and only 16 of their fellowmembers were retaken at Tern Island. The behaviorpattern of renestingshows that when only a few birds are involved,they tend to use a different part of the same site. In a marshbordering Tern Island is a small elevatedislet 1200 squarefeet in extent,100 feet away from the main colony. Almostall the 50-odd pairs nestingon this islet were trapped and color-bandedfor three consecutiveyears. Shortlybefore the colonyreturned the fourth year, a severestorm buried the islet under a massof wreckage. The evicted birds, easilyidentified by their colored,bands, did not scatterover the large island,,but nested closely together on a small,nearby section .of it. At Plymouthin 1947 rats destroyedall the eggsand chicksin the outer part of thepoint where nesting, as always, had beenearliest and heaviest. The two dunesat the extremetip were trapped before their clutches were destroyed.When thesebirds renested,most of themlocated in a body at the oppositeend of the colony. When •largenumbers are frustrated,they usuallyemigrate in a body to nesttogether elsewhere (Austin; 1940, 165: and 1942, 17: and 1946, 25). Rarely havefewer than 25 percentof theserenesting individuals recapturedfollowing seasons failed to rejoin their original coloniesthe ensmng year. ,Conspicuousexamples of this followed the wanton wreckageof the Billingsgatecolony by picnickersin 1929,and the com- pletedispersal of the Tern Island colonyby rat predationlate in June 1933. The only notableexception is the failure of one-halfof .theTern Vol. XXII 1951 AUSTIN,Group/ldherence in the Common Tern [9

Island colony.to return after its debaclein 1944 (Austin; 1946). More indicative.of the existenceof group adherenceand its influence in determiningthe membershipof a colony are the involvedfindings at Little Sipson,a small island about four acresin extentin Pleasant Bay, threemiles from Tern Island. Occupiedfor the precedingdecade by only a few pairs of terns,500 birds nestedthere in 1935, and 400 in 1936. , Since then only a few pairs have usedthe site. It was well sampledthe last two yearsof its peak occupancyas follows: Adults banded Returns taken 1935 ...... 93 14 1936 ...... 163 49 Of the 14 returnstaken in 1935, 11 or 78.6 percenthad beenbanded at Tern Island. Ten of them, or 71.3 percent,had been banded as chicksin 1932 and 1933, and, returningfor their first or secondnesting in 1935, accompaniedTern Island adultsto a site with which they had no previouscontact. Three of these 14 birds were retaken at Little Sipsonin 1936, one at Tern Island. Since 1936, four of the 14 have been trapped at Tern Island, and none elsewhere. Of the 93 adults bandedat Little Sipsonin 1935, 21 were retrappedthere in 1936, none elsewhere. All but two of the 22 of these 93 birds retaken since 1936 wererenesting on Tern Island. Of the 49 returnstaken at Little Sipson in 1936, 24 or 48.8 percent had been taken there in 1935. Of the remaining25 Little Sipson1936 returnsnot trappedthere in 1935, 17 or 68 percenthad •beenbanded at Tern Island. Of all the 212 adults takenat Little Sipsonin 1936, 54 weretaken in subsequentyears. Of these45, or 83.4 percent,were .breeding at Tern Island. Sinceall the foregoingpercentages meet the requirementsof a major behaviortrait, the incident representsfar more than the erratic doings of a few individuals. Obviouslythis temporary colony at Little Sipson.consisted essentially of a portion of the Tern Island colonywhich returnedto its original site in 1937. The transientseparation is accountedfor by the peak populationsat Tern Islandin 1935 and 1936,with a resukingreduction in ,he amountof nestingterritory available. A duplicatingoverflow of Tern Island birds went to North ,Point at the same time. The presenceat Little Sipsonin 1935 of the 10 Tern Islandchicks nesting for the firsttime suggests that groupadherence may begin to function very early in the }ire of the CommonTern. Eversince a back-logof bandedadults varying from 20 to 60 percent wasestablished in the Capecolonies 15 yearsago, the ratio of returns has averagedfour of birds bandedas adultsto one bandedas chicks. The ratio is invariably[highest when Tern Islandbirds are inv.olved, becauseaduk bandinghas beenfar more comprehensivethere than elsewhere,the only markedexception is the recoveryof Weepecker birds at Ram Island, sevenbanded as adultsfor each one as a chick. Thisis evidencethat the bond which united .the Weepecket birds into a persistentcolony tended to keepgroups of themtogether when they werecompelled to usea newnesting site. Alsoit showsthat group adherenceis at timesof morevalue than site tenacity in preservingthe species. AUSTIN,Group Adherence in the Common Tern Bird-BandingJanuary

In all probability group adherenceis strongestduring the nesting seasonbecause then is the only time ,CommonTerns consortclosely together in large numbers. Several other behavior traits function activelyonly for limitedperiods. A betterevaluation of thetrait results from knowingas far as possiblehow dormantit becomesduring the winter months,and to what extent, if at all, it modifiesthe species' northwardand southwardmigrations. It may be possibleat least to learn someof the end resultsof its activity during the breedingmonths.

GROUP ADHERENCE ON THE WINTERING GROUNDS The literaturecontains comparatively few observationson the wintering grounddistribution and behaviorof the many thousandsof Common Terns that breed in North America. They only reliable data available are the recoveries of banded birds:

WINTERING GROUND RECOVERIES OF CAPE COD TERNS Locality Adults Juvenals Totals WEST INDIES Bahamas 0 1 1 Cuba 2 1 3 Jamaica 0 1 1 Haiti 8 33 41 Porto Rico 9 51 60 Virgin Islands 1 1 2 Barbadoes 0 2 2 Guadeloupe 0 2 2 Martinique 5 6 11 Grenada 1 3 4 Trinidad 4 56 60

CENTRAL AMERICA San Salvador 0 1 1 Panama 0 2 2 SOUTH AMERICA, WEST COAST Peru 1 2 3 SOUTH AMERI. CA, NORTH COAST Aruba 0 1 1 Colom'bia 0 3 3 Venezuela 3 33 36 SOUTH AMERICA, EAST COAST British Guiana 5 14 19 Dutch Guiana 1 0 1 French Guiana 5 8 13 Brazil 13 13 26

TOTALS 58 (19.9%) 234 (80.1%) 292 The winteringrange of the CommonTern is extensive. It includes the WestIndies, .Central America, the entire'Caribbean coast line, the eastcoast of SouthAmerica south to Patagonia,and the westcoast at leastto Peru. With an evendispersal of the speciesthere would be no largeconcentrations in any one locality, such as the banding recoveries show. Most of the peoplelikely to comein contactwith ternsin their Vol. XXII 1951 •{USTIN,Group Adherence in the Common Tern [11 winteringarea probablyknow nothingof the import of a bird band, and are unable to comprehend.the directions on the band concerning reporting. With few exceptionsall the recoveriesare reportedas found dead,or killed for someunspecified reason. So the relative number of recoveriesin each countrydoes not necessarilymirror the actual dis- persalof the species. Of greatestimportance is .thegross inadequacy of the sampling,which is too small and insufficientlydiversified to eliminate even minor variables. That 80.1 percentof the recoverieswere bandedas chicks, of which74 percentwere recoveredthe first winter after banding,does not indicatethe relative number of young and old birds present,but reflectsinstead the higher susceptibilityof inexperiencedjuvenals to accidentand capture. Thesedeficiencies suggest that, at most, all the recoveriescan do is to indicatetrends. They cannotmake the existence of a behaviortrait factualas do the adequatesamplings obtained during the nestingseason. The disproportionaterecovery of adultsand young raises the question of howthe latter maketheir way southward.Since they have no previ- ous knowledgethrough experienceof where they are to winter, never beforehaving travelled the interveningterritory, their first journey to the species'customary wintering grounds can be effectedby only two means: first, either an inherited instinct, or directions received in an unknownmanner from their elderson the breedinggrounds; or second, somedegree of guidancein transit. For the latter, which is the more likely explanation,there are three possiblesources. Guidancecould be obtained: (1) from their parents, (2) from continuedassociation with other adultsin the unit, large or small, of which both the young birds and their parentsare members,or (3) from contactwith terns from otherunits usingthe sameflyway. It is not known how long parentscontinue association with their progeny. In August,when many terns exhibit the post-nuptialwander- ing previousto migration characteristicof many other species,adults are occasionallyseen feeding well-grown young of the year miles from the nearestternery. If, as is likely, .theyare parentsand their chicks, they representthe longesttime after hatching such contact can be indicatedto have continued. Neverthelessthe possibilityof parental guidanceto the winteringground cannot be discardedeven though, as will be shownlater, it could not have accomplishedthe collectionof simultaneousrecoveries on the winteringgrounds without the help of groupadherence. During their southwardmigration terns usually string out alongthe shorelinesduring daylight hours. While they do not travel in compact flocks,large groupsof them containingboth adultsand young of the year are often seenresting together on points and sandbars. These observationsare not inconsistentwith the opinionthat most juvena,ls are guidedto their winter sitesby somedegree of continuouscontact with their summertimeassociates. An instancewas shownof young birdsbeing led to unfamiliarterritory by their older affiliateswhen the Little Sipsoncolony was discussed above. _Nobanding evidence shows continued association of adultsand young 12] AUSTIN,Group ,4dherence in the Common Tern Bird-BandingJanuary

fromthe samecolony on the winteringground, 'bu,t the Caperecoveries giveadequate proof that youngfrom the samecolony remain together during their first year. Of the 234 juvenals'recovered, 112 or 47.8 percentwere ta,ken in 33 groupsof two to ten as the followingtable shows:

SIMULTANEOUS WINTERING GROUND RECOVERIES OF CAPE COD TERNS BANDED AS JUVENALS •aiti Porto Rico Martinique Venezuela Peru 1928•4 1931-- 3 1926--2 1928--2 1934--2 1933--3 1933• 3 1932--2 1933--2 1934---4 1934•10 Grenada 1934---7 1935--3 1935-- 4 1944 2 1935--2 1938--2 1938• 4 Trinidad 1939--5 1940•3 1942-- 3 1927-•4 British 1945--2 1945-- 3 1929--3 Guiana 1946--5 1946-- 2 1933--2 1937--3 1947--3 1939--5 1939---4 French Guiana 1934•4 Summary Groups of: 2 3 4 5 7 10 Totals: No. •f groups: 11 10 7 3 1 1 33 groul•s No. of birds: 22 30 28 15 7 10 112 birds

BANDED AS ADULTS H.AITI 2 Ibanded in 1947, recovered in 1947 PO,RTO RI,CO 2 banded in 1936, recovered in 1936 B,RAZIL 2 banded in 1936, recovered in 1939

Thirty•five of thesesimultaneous recoveries, or almostone-third of them,wdre banded on the same day in theTern ,Island co.lony as follows:

TERN ISLAND CHICKS BANDED THE SAME DAY AND RECOVERED TOGETHER ON THE WINTERING GROUNDS Date Randed Place Recovered Date Recovered 9 July 1928 Haiti 27 ,Sep. 1928 .... 3 Oct. 1928 6 July 1933 Trinidad 15 July 1934 .... 22 Aug. 1934 24 June 1934 Porto ltico 7 Sep, 1934 .... 18 Sep. 1934 .... 12 Oct. 1934 24 June 1934 Venezuela 15 Nov. 1934 .... 5 Dec. 1934 .... 9 ,Dec. 1934 26 June 1934 Venezuela 7 Dec. 1934 " " 9 .Dec. 1934 .... 25 ,Dec. 1934 10 July 1934 Peru 1 •ar. 1940

13 July 1934 Haiti 5 Oct. 1934 " " 20 Oct. 1934 Vol.1951XXII AUSTIN,Group Adherence in the Common Tern [13

18 July 1934 Porto .Rico 29 Sep. 1934 .... 5 Oct. 1934 .... 13 Oct. 1934 .... 19 .Oct. 1934 1 July 1935 Porto Rico 28 Sep. 1935 .... 27 Oct. 1935 26 June 1939 Trinidad 1 Mar. 1940 .... 9 June 1940 29 June 1939 Venezuela 16 Nov: 1939

11 July 1939 Venezuela 9 Nov. 1939 .... 16 •Nov. 1939 7 July 1940 Haiti 6 Oct. 1940 .... 7 Oct. 1940 5 July 1946 Haiti 16 Oct. 1946 .... 3 Dec. 1946 18 July 1946 Haiti 29 Oct. 1946 .... 8 Nov. 1946

This table revealsthat Tern ,Islandchicks banded on the sameday were retakenin one group of four, three groupsof three, and eleven groupsof two, at the sameplace on the winteringgrounds, twice on the sameday, otherwisewithin a few weeksof each other. One pair was taken in Trinidad the followingsummer, when the older bird•swere back on the breedinggrounds. The pair taken in Peru had remained togetherfor threeyears. That youngof the year usuallyremain southinstead of returning northwardtheir first summeris verifiedby manyJune, July, andAugust wintering ground recoveriesof year old birds. Fourteen such Tern Island chicks,banded the sameseason but not on the sameday, were recoveredtogether as follows:

SIMULTANEOUS SUMMER RECOVERIES OF TERN ISLAND CHICKS ON THE WINTERING GROUNDS Birds Surmmer Place Sttmmer in Grou,p Banded Recovered Recovered 3 1929 Trinidad 1930 4 1934 French Guiana 1935 3 1937 British Guiana 1938 4 1939 Trinidad 1940

These simultaneousrecoveries are too numerousto be disregarded as fortuitous. They suggestthat group adherencemay originate in family relationship. ß he coloniesof ,CommonTerns which nest along the Atlantic Coast from the Florida Keys to Labradoruse the samemigration route and winteringgrounds. They do not migratecompactly in groupsas aligned on the breedinggrounds, but in smallerflocks. The initial descentof the large coloniesonto their nestingsites on their return from the winteringg•,ounds is usuallymade by at least a majority of their memberships,as hasbeen observed by manyworkers in tern colonies (Palmer;1941, 39). Usuallythe birds collect on nearbybeaches before repairingto the nestingarea, but at Tern Island the colonygathers 14] ?kUSTIN,Group Adherence in the Common Tern Bird-BandingJanuary togetheroff shore. This behaviormust be motivatedby group adherence,for it indicatesthat site tenacity alone doesnot bring about the final alignments. Were it not for these two behavior traits, especiallygroup adherence,there would be a much more frequent interchangeof membershipsbetween the Capegroup and other groups along the Atlantic Coast. From whateverstandpoint group adherence is studied,the conclusion is alwaysthe ultimateinference that kinshipplays a role in its develop- ment and perpetuation,exactly as it is known to do in someother animalaggregations. There is no credibleevidence at handto indicate that family influencedoes not controlto a considerableextent the group behaviorof individualterns, be it parent-offspring,sibling, or an even more distantrelationship. Certainlyit offersthe mostlogical explanation for mostof the occurrencesrecounted herein, particularly the align- ment of individualson the winteringgrounds.

COLONIAL INTERRELATIONSHIPS Tern colonies are never static, but vary continually in size and composition. Terns have nestedat 17 differentsites in the Cape area in the last two decades.Several of thesesites, Billingsgate, Egg Island, and NausetPoint in particular, have washedaway. Othershave be- come equally untenablebecause of ecologicalchanges. Tern Island itself,the nucleusand mainstayof the Capegroup, has been maintained as a breedingsite only by extensiveannual reclamation work. As variouscolonies have broken up, their membershave beenfound nestingwith other flockswithin the circle of the group'stenure. These displacedbirds usuallyjoin one of the larger colonies. Most of the Billingsgateand Egg Island birds went to Tern Island and Plymouth. Tern Island refugeeshave movedto Plymouth,Bird Island, and Ram Island at various times. Such emigreesare seldom found in the smaller colonies. The trend has been constantlytoward coalescence of the smallercolonies with the larger ones. This is advantageousto the maintenanceof the species,for reproductivesuccess is muchgreater in the larger colonies. Until 1930 mostinvestigations of breedingterns were carried on in the largest colonies,such as Tern Island on the Cape, Muskeget, Penikese,and Weepecketin the Vineyardarea, Cobb Island in Virginia, and thosein the Great Lakes. Very little attentionwas paid to the smallercolonies, or to their relationshipwith the large ones,as has beendone comprehensively since then on the Cape. However,a study of the literature,and personal reports from workersin otherareas show that elsewhere,as in the Capegroup, most large tern colonieshave one or severalsmaller satellites near them, with which they probablyform similar self-contained,self-perpetuating groups. In inland fresh-water lakesthese groups are frequentlyseparated so far from the nearestother set of Coloniesthat thereis little questionof their unity. In others,as in the Maine-NewHampshire coastal region, and along the shoresof New Jersey,Maryland, and Virginia, no probableline can be drawn betweenthem, if one exists,without banding evidence. ¾ol. XXII 1951 AUSTIN,Group •4dherence in the Common Tern [15

The recent history of other important colonies shows them as variable and unstableas the Cape colonieshave been. The terneries on Cobb Island have shifted with the changingenvironment many timesin the last half century. When the formerlarger rookeryon Gull Island of[ Long Island was evictedby Army occupationin 1898, the birds apparentlyrelocated in the nearby area. They formed several newlarge colonies of[ LongIsland, each with its complementof smaller ones,usually on siteswhere terns are knownto havenested before. It appearsthat whenrelatively large coloniesare dispossessed,either for a seasonor permanently,they relocate as a group in the same generalarea. While site tenacityis responsibleto someextent for this behavior,group adherence is predominantlyaccountable for the maintenanceof the speciesunder such adverse conditions.

SUMMARY The CapeCod coloniesof breedingCommon Terns form a distinct, self-perpetuatinggroup, free from associationwith other coloniesdur- ing the nestingseason. Similar groupsappear to existelsewhere. Membershipin sucha groupis not determinedby the geographical proximityof coloniesto oneanother, but is engenderedby two behavior traits, site tenacity and group adherence. The former implies the attachmentof terns to specificterritory, the latter the attachmentof terns to each other. Groupadherence functions throughout the wholelife of the species, even on the winteringgrounds. Probably originatingin kinship, it tendsto coalesceterns into progressively larger units in whichreproduc- tive successis greater. Group adherencemaintains these societies for concertedaction when site tenacityceases to functionbecause of ecologicalor other environmentalchanges. Hence it is essentialto the welfareof the species.

LITERATURE CITED AUSTIN, O. L. 1940 Some Aspects of Individual Distribution in .the 'Cape Cod Tern Colo- nies. Bird-Banding,Vol. XI, No. 4, pp. 155-169, 1942 The Life Span of the CommonTern (Sterna hirundo). Bird-Banding, Vol. XIII, No. 4, pp. 159-176. 1946 The Status of Cape Cod Terns in 1944; a Behavior Study. Bird. Banding, Vol. XVII, No. 1, pp. 10-27. 1949 Site Tenacity, A Behavior Trait of the Common Tern. Bird-Banding, Vol. XX, No. 1, pp. 1-39, CROWELL,ETHEL M. and SEARS 1946 The Displacementof Terns by Herring Gulls at the WeepecketIslands. Bird-Banding, Vol. XVII, pp. 1-10. LINCOLN, FREDERICKC. 1927 Notes on the Migration of Young ,CommonTerns. Bull. Northeast. B. B. Assoc.,Vol. III, No. 2, pp. 23-28. PALMER,RALPH S, 1941 A Behavior Study of the CommonTern. Proc. Bas•. $oc. •Vat. H•st., Vol. XXXII, No. 1, pp. 1-119. North Eastham, Cape Cod, Massachusetts.