BIRD-BANDING A JOURNAL OF ORNITHOI•OGICAI, INVl•,STIGATION VOL. 49, NO. 1 WINTER 1978 PAGES 1--100 NESTING BEHAVIOR OF THE GULL-BILLED TERN BY HAROLD F. SEARS INTRODUCTION The Gull-billed Tern (Gelochelidonnilotica) has a scattered distribution, and its breeding behavior has been little studied in the past (e.g., deWaard, 1952; Lind, 1963a; M•511er,1975). I have therefore made a relativelybroad studyof the breedingbehavior of thisspecies. I present my observationson nestingbehavior here and discussthe antipredator strategyof the Gull-billed Tern. The species'display behavior will be discussed elsewhere. STUDY AREA AND METHODS I spent the springsand summersof 1971, 1972, and 1973 on an unnamed island 0.5 km south of the port at Morehead City, Carteret County, North Carolina.Beaufort Inlet liesto the southeast,and a Spar- tina marsh and Bogue Banks lie to the southwest. Early during eachbreeding season, I watchedthe ternsfrom exposed but distant positionsusing 8 x 24 binocularsor a 15-60 power zoom telescope.As colonieswere established,I positionedburlap blinds at their borders, and observations were made from within the blinds. I usedtime-lapse photography to studyincubation behavior. A Braun- Nizo S-80 movie camera was positionedon a tripod 20 to 30 m from the nest.It wasset to exposeone frame everyfour to five seconds(later determinedexactly) and wasleft in positionfor up to five hours. Gull-billedTerns are quite sensitiveto disturbance;therefore, I did not trap or mark any terns. This meant I could only rarely recognize individuals.The sexeswere not distinguishable,although Lind (1963a) thought he could distinguishmales by their heavier bill. No terns were naturally marked in any way, and only twice did I study pairs whose memberscould be distinguishedby tone of voice.I did band mostchicks with a Fish and Wildlife Serviceband, and on a few occasions,I placed a red dye in solution in a small mollusc shell in the bowl of the nest, thereby temporarily marking an incubatingadult. Such a mark faded after a few days. RESULTS ColonySite and Composition The islandconsisted of recentlydredged spoil, which provided a va- riety of nestingsubstrates, including loose sand, sand mixed with shells, 2] H. F. Sears Bird-BandingWinter 1978 and firmer sandmixed with siltthat supportedlow vegetationof variable density.Gull-billed Terns in this area appearedto prefer a sitethat had a substrateof fairly rough texture (consistingof shells,sticks, and other debris as well as sand), that was sparselydotted with vegetation,and that was situatednear an area of relativelydense, low vegetation.The initial colonies of 1971 and 1972 were established in such sites. No Gull- billed Terns nestedin densevegetation, and nestingon bare substrate was associatedwith disturbedconditions. After widespreadnest deser- tion during unusuallycold wet weatherin 1972, presumedrenesting attemptswere made on virtually bare sand. Then, during the fall of 1972,the southwesternhalf of the islandwas largely buried under fresh spoil, and in 1973, one of the two major colonieswas establishedon bare substrate. Most of the colonies I studied contained both Common Terns (Sterna hirundo) and Black Skimmers (Rynchopsniger), as well as Gull-billed Terns. During 1971, the first colonyestablished contained at least 20 Gull-billed nests.Later, a secondand a third colonywere established, containingabout 9 and 3 Gull-billed nests,respectively. These were probably renesting attempts. During 1972 and 1973, two colonies formed initially (9 and 10 Gull-billed nestsin 1972 and 9 and 7 Gull- billed nestsin 1973), and again those that abandonedtheir nestsap- parently renestedin smallercolonies in other locations. In 1973,the averagedistance between a Gull-billednest and its near- estneighbor was about 10 m (Table 1). If only conspecificnearest neigh- bors are considered,the distancewas a little greater, 12 m. Of the 32 Gull-billed nestsconsidered, 16 had conspecificnearest neighbors, 10 had BlackSkimmer nearest neighbors, and 6 had CommonTern near- est neighbors.I collectedno quantitativedata on the total frequencies TABLE l Averagedistance of Gull-billedTern nestsfrom their nearestneighbors in sevendifferent coloniesestablished during 1973•. Neighbor Colony BlackSkimmer CommonTern Gull-billedTern Far SW 2.7 (3) -- 7.0 (6) 5.6 W (14 May) 8.5 (2) 10.0 (3) 16.5 (2) 11.4 W (6 June) 2.0 (2) -- 4.0 (3) 3.2 Near SW 3.0 ( 1) -- -- 3.0 N 5.0 (2) -- 8.0 (1) 6.0 NW -- 5.0 (2) 10.0 (1) 6.7 S -- 42.0 (1) 29.3 (3) 32.5 • Distances in meters. Number of Gull-billed nests on which each mean is based is given in parenthesis. Vol.49, No. I NestingGull-billed Terns [3 of the three specieson the island,but the Gull-billedTern wascertainly in the minority,outnumbered by eachother speciesby at leasta factor of five.Thus, the Gull-billedTern tendsto nestnear conspecifics. TerritoryEstablishment Nestingterritories are establishedno more than a day or two before the first eggis laid. Establishmentof the territoryinvolves rapid local- izationof a pair'scourtship activity, at whichtime the nestis scraped out by both membersof the pair. The first eggswere laid about 5-7 May in 1971, 1972, and 1973. Nest Construction The Gull-billedTern tendsto nestnear an objectthat standsout from the substrate,such as a tuft of grass,a stick,or other pieceof flotsam. My studyarea was unusually bare, being composed largely of freshspoil, but nearlyhalf the nestsestablished during 1973were positioned near such a feature. The nestconsists of a depressionin the sand,a surroundingrim of sand raised above the level of the substrate, and an accumulation of shells,sticks, or grassblades on and around the rim. The exactnature of the nestvaries considerably from nestto nestand f¾omday to day, largely dependingupon the area in which the nest is located.At a nest on an open shellyarea, many shellsare incorporatedinto the nestand nestrim. At a neston a shrubby,grassy area, twigsand grassesare used. In open areas,nests become well defined and complexduring periods of calm weatherbut may be obliteratedduring heavywind or rain. In protectedareas, nests remain deep and the rims conspicuous. The constructionof the nestinvolves three relativelydistinct behavior patterns:scraping, sideways-throwing, and sideways-building.Nest ma- terial is not broughtto the nest from a distance.A tern beginsnest constructionby scrapingout a shallowdepression in the sand.The tern kicksits feet alternatelywhile restingon its breastand so pushessand backwardsand onto the nestrim. Subsequentdeepening of this bowl and later clearingit of blown sandinvolve the samemovements. During sideways-throwing,shells or other items are tossedfrom the immediate vicinity of the nest toward the nest. The tern standswith wingsslightly abducted and drooped. The tern stretchesits neck out horizontally,reaches down to the substrate,grasps the item in its bill, and tossesit to one sideand backwardwith a quickand relativelyster- eotyped turn of the head. The item is tossedas far as a meter to the rear of the bird. Sideways-throwingis occasionallyperformed by an incubatingtern thatrises from the nestand walksaway, sideways-throw- •ng repeatedly. The tern then walks back to the nest and resumesin- cubation.Sideways-throwing isalso commonly performed during or im- mediatelyfollowing the nestexchange. With the repeatedperformance of thisbehavior through the incubationperiod, material is slowly moved from the surroundingarea toward the nest. 4] H. F. Sears Bird-BandingWinter 1978 Sideways-buildingis a more variableand more well-directedbehavior wherebymaterial is actuallyplaced onto the nest rim. It is performed by incubatingterns or by ternsstanding near the nestbefore beginning to incubate. Although the sideways-throwis relatively stereotypedin form, it is frequently oriented with respectto the nest,just as sideways- building is. Perhapssideways-throwing and sideways-buildingare two ends of a continuum. At a distance from the nest, the movement is relatively stereotypedand undirected, but the closerthe tern is to the nest, the more deliberate and well-directed the movement becomes (Harrison, 1967). Clutch Size The Gull-billedTern producesclutches of one to three eggs(• = 2.0, n = 41). Most of the pairs that establishednests at the beginningof the 1973 season(before 15 May) produced clutchesof two or three (• = 2.5, n = 16). Most of thosethat nestedlater producedclutches of one or two (• = 1.7, n = 25). A decreasein the average clutch size during the nesting seasonis a trend found in many birds (Bergman, 1953; Cullen, 1956; Langham, 1968, 1974). However, these data probably give a biased picture of clutch size in thisspecies because some clutches could have been reduced by predation before I discoveredthem or could have been abandonedbefore laying was completed.Pemberton (1927) describesone nest containingfour eggs.Bent (1963) reportsthat the clutchsize is two or three, rarely four. The eggswere laid at a rate of lessthan one per day. Five three-egg clutcheswere laid over the courseof at least four days,and three were laid over the courseof at leastfive days. IncubationSpell An incubationor sitting spell is an uninterrupted period of incuba- tion, separatedfrom the precedingand following spellsby suchoff-the- nest behavior as nest exchanges,sideways-throwing, defecation, or alarm flight. Using time-lapsephotography, I monitoredincubation behavior for two to five hours on each of 36 separateoccasions at 12 different nests. The duration of the incubationspell ranged from lessthan 1 minute to over 194