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 indi- viduals which is self-sustainingand relatively free from association with other groupsduring the nestingseason. The very small inter- changeof 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 popula- tion 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 break- downof 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 migra- tion 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 breed- ing 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