j. Vield Ornithol., 52(3):191-209 THE DISPLAY BEHAVIOR OF THE GULL-BILLED TERN By HAROLD F. SEARS The displaybehavior of the Gull-billedTern (Gelochelidonnilotica) has not been describedin detail, although Lind (1963) discussesmany of the displaysof a Danish population. In this paper, I discussthe form, variation, function, and taxonomicimplications of displaysobserved during the breedingseason. STUDY AREA AND METHODS This studywas done from March to July, 1971 to 1973, on an un- named island 0.5 km S of the port at Morehead City, Carteret Co., North Carolina(34.7øN lat., 76.7øWlong.). The siteis describedfurther in Sears (1978). I made observationsfrom exposed positionsearly each seasonand from blinds after colonyestablishment. Periodically I photographedbe- haviorswith a Braun-Nizo S-80 camera and super-8 film at 18 frames per sec,and recordedvocalizations with an Uher 4000 Report L tape recorderand M514 microphoneat a tape speedof 19 cm per sec.A few recordingswere made at a distancewith a Dan GibsonElectronic Par- abolicMicrophone. All recordingswere spectrographicallyanalyzed on a Kay Elemetrics Corporation Sona-Graph, model 7029A, using the wide band filter. To obtain data unbiasedwith respectto temporal parameters,I tape- recordeddescriptions of socialinteractions at severalsites for predeter- mined periodsof time (Sears1976). I tentativelydefined as independent those sequencesof behavior separatedfrom each other by over 15 sec of non-display(cf. Nelson1964, Rades•iter1975), and I workedwithin these sequencesto determine the minimum period of time between independentbehaviors. I measuredthe interval from the termination of the initial behavior in each sequenceto the initiation of each subse- quent behaviorof that sequence.Each resultingdatum consistedof a "precedingbehavior" (the initial behaviorof eachsequence), a "follow- ing behavior,"and the interveninginterval. For each interval class(10, 15, 20, 25, 30, 35, 40, 45 sec), the associateddata are independent, drawn from independentsequences, and eachof thesedata subsetscan thereforebe analyzedby chi-squarefor statisticalindependence between the variables,preceding and following behavior (analysesperformed using IBM 360 computerand programsfrom Nie et al. 1970). I found behaviorsseparated by 10 secto be clearlystatistically dependent upon eachother, but there wasno indicationof dependencewhere the inter- val was greater; thus, adjacentbehaviors are statisticallyassociated and a seriesof suchbehaviors both preceededand followedby over 15 sec of rest posturecan be referred to as an independentsequence. To identify associationswithin theseindependent sequences, I tallied eachtransition according to its precedingand followingbehaviors (Fig. 191 192] H.F. Sears J.Field Ornithol. Summer 1981 1). I then compared, using chi-squareanalysis, the relative occurrence of each followingbehavior in the presenceof each precedingbehavior with its relative occurrencein the absenceof that precedingbehavior (Table 1). Finally, the data are too few to permit the study of many transitions,therefore I repeatedthis analysisusing more generalcate- goriesof behaviors(Fig. 2, Table 2). The entire sequenceanalysis and the resultingpicture of the temporal organizationof Gull-billedTern displaybehavior is unbiasedonly to the degree that the data are independent of one another. To test the un- biasednature of this sequenceanalysis, I repeated it using an indepen- dent subsetof the data, consistingof the first transition of each inde- pendent sequence(n = 428). This revealed only 2 differences that cannotbe explainedby the smallsize of the independentsubset or by the early positionin the sequenceoccupied by all membersof the in- dependent subset(Table 1). AERIAL DISPLAY The terns that first arrive in the colony area do not appear to be paired, they avoid contactwith one another and with the future colony sites,and the first interactionsseen are aerial interactions.First, groups of up to 6 or 8 terns fly about the colony area. The leader seemsto vocalize more than the followers and sometimes carries food in its bill. Over the courseof a few days,these group flightsgive way to paired flights,and finally, pairsposture more and more on the groundand on colonysites. The followingpatterns are thoseI've observedin the air. V-flying.--The tern flies slowly at a relatively low altitude, its wings moving from about 45ø abovehorizontal to horizontal. Aerial bent.--The tern glideswith its wingsheld stationaryin a v-po- sition over its back, each 45o-60 ø above horizontal, and its head bent downward so that the head and bill form an angle as great as 90ø with respectto the body. Aerialstraight.--The tern glides,wings motionless, with the body and head in line or slightlybowed, head and tail slightlylower than the center of the back, and the wingsas much as 30ø below horizontal. Swaying.--The tern precedesits partner and flies back and forth acrossthe relativelysmooth course of the secondtern. The aerial bent, aerial straight,and swayingall occur in the contextof paired flight. Hairpin course.--Twoor 3 terns flying togetherfrequently fly a hair- pin-shapedcourse 100-200 m long. They fly straight, turn 180ø, and continue in this manner back and forth, the same individual leading throughout the flight. Simplepass.--The followingtern overtakesthe leader and flies past, either above, below, or to one side of the leader. The performance of a simplepass almost always results in the former leader initiatinga turn and again precedingthe other along the other sideof a hairpin course (21/23 observations).In addition to theserelatively stereotyped interac- tions, many brief encountersoccurred in which a tern flew along and a Vol. 52, No. 3 Gull-billedTern Display [193 HERD LANDING BENT ß ß ß ß ß MOUNT '",, UP W/TURN ............. •FORWARD W/TURN MQUNTED (MALE) DOWN' •-FOR VyAR D f '.. \•, ? /' \ I '"'".. \\'"', .-"'"' / \\ • % \,, ..' / x. COPULATEDISMOUNT \ ',..-" • ...,,,F•._Y ..." . \', 1..- HEADTOSS-'""'Z-'""-.;,REST•. LAND B•Et•T / / ",,' I! ',", HUNCHED / " I ß I / ,' I ', I // " I " • • ,,' I ',, SCRAPEß ,," OBLIQUE• ............. ;.ATTACK ß ß NEST FIOUREl. Sequential associationswithin the behavior of the Gull-billed Tern. Short dashesindicate that ! 0-I 9 transitionsbetween the behaviorswere tallied. Long dashes indicate 20-29 transitions. A solid line indicates 30 or more transitions. (n = !296). secondtern swoopedpast. Usually, the secondtern made a few passes, the first did not react,and the 2 ternsthen separated. Complexpass.--The followingtern overtakesthe leader and glidespast in the aerial straight;the tern beingpassed performs the aerialbent. At the point of closestpassage, the wings reach the highestin the aerial bent and the lowestin the aerial straight. Arc soar.--Both terns assumethe aerial straight posture and glide togetherin a wide levelarc, usuallyat a high altitude.One is positioned above,to the side,and within a few body widthsof the other. Most aerial interactionsare relativelysimple, involving paired flight and sometimesthe simple passor hairpin course (104/128). I saw the 194] H. F. Sears J. F•eld Ornithol. Summer 1981 LANDING BENT I I I MAPLECOP'•............. •?•R iT ,.FE M'A •E COP BE NT•.--- I I / S•CRAPE•// AGGRESSIVE FIGURE2. Sequentialassociations among broad categoriesof behavior.See Fig. 1 for key. morecomplex patterns only a few times.I sawv-flying and swayingonly once. TERRESTRIAL DISPLAY Oblique.--Thecharacteristic structural features of this postureare ab- duction of the wings,extension of the neck, and direction of the bill toward an intruder. The degree of abductionand of extensionvaries little, but the orientationof the neck rangesfrom almostvertically up- ward ("high" oblique) to horizontal ("low" oblique).The oblique is al- mostalways accompanied by the ackor rattle callor by the gape(below), and it (especiallythe low oblique)is associatedwith approachand with attack(Figs. 1, 2). The Gull-billedTern attackswith its bill and feet. Crouch.--Thebody is horizontal,and the head is withdrawn closeto the shoulders.The crouchis not performedoften, but it occasionally occursin associationwith the obliqueand attack. Gape.--The bill is held open silentlyfor at leasta secondor 2 and is orientedtoward the intruder. The gape is associatedwith the oblique and with the ack, the latter associationusually taking the form of oc- casionalcalls emanating from a continuouslyopen bill. Head toss.--Thehead is rotated as high as verticallyupward and is then droppedto its originalposition. It is usuallyperformed a few times in quick succession,and it can be performed in a normal standingpo- sition,in the erect,or in the bent (seebelow). Often, the tern givesack- chirupcalls (below), and sucha performanceelicits erect posturing from nearbyterns; occasionally, the head tossprecedes attack. Vol.b2, No. • Gull-billedTer•t Di,•play [ 195 Sleekedupright.--The neck is extendedvertically upward, and the bill is horizontal. The feathers of the neck are sleeked,and the wings are held tightly against the body. In this posture, the tern often looks around sharplywith the eyelidsunusually wide open in reactionto dis- turbancecaused by, for instance,an airplane flying overheador other terns fighting nearby. Footlook.--Thismovement is usuallyperformed in a normal standing posture.It consistsof a singlerotation of the head down and up. Like the sleekedupright, the footlookoccurs relatively rarely and in contexts that are probablymildly disturbing. Landingbent.--This is a distinctivemanner of landing. During the