Behavior of Black Skimmers at Cardwell Island, Virginia 1

THE AUK

A QUARTERLY JOURNAL OF

ORNITHOLOGY

VOL. 54 JVLY, 1937 No. 3

BEHAVIOR OF BLACK SKIMMERS AT CARDWELL ISLAND, VIRGINIA 1

BY OLIN SEWALL PETTINGILL• JR. Plates 16, 17 CXRDWELLISLXND, a narrow strip of land three quartersof a mile in length, lies adjacent to the southwesternend of Cobb Island, Virginia. Scarcelymore than an emergedsand-bar with a blunt, central ridge of grass-tufteddunes, its chief value to the ornithologistlies in the longsouth- easternbeach where a groupof BlackSkimmers (Rynchops nigra nigra) has returnedeach year to nestand rear their young. In 1933, I was on CobbIsland and from June 23 to July 6 made frequent visitsto this colonyby crossingthe narrowchannel which separates the two islands. A shortwalk from this point broughtme to the centralsection of thebroad beach, where the birds were nesting. Here upon the loose, sandy terrain, the continuityof whichis brokenup by piecesof driftwood,small plants,clumps of grassesand innumerablemulticolored shells of mollusks, were approximatelyfour hundredskimmers' nests. Without any lining whatsoever,they were mere depressionsin the soft soil. Two-thirdsof themat the timeof my firstvisit contained young; the remainder contained eggs. Three or four was the usualnumber in the clutch,rarely two or five. Interspersedamong the skimmers'nests were two nestsof the Least Tern (Sterna antillarum antillarum), five of the Gull-billed Tern (Gelochelidon nilotlcaaranea), and twelveof the CommonTern (Sternahiru•do hlrundo). In studyingthe behaviorof the skimmerson thesenesting grounds, a blind was occasionallyused. This was placedin the midst of the colony within easy view of twenty-threeskimmers' nests, as well as one of the Gull-billed Tern and two of the Common Tern. As a contribution to our

, Read at the Fifty-fourth Stated Meeting of the A. O. lJ., Pittsburgh, Pa., October 21, 1936. The author is indebted to Dr. Alfred O. Gross of Bowdoin College, Brunswick, Maine, and Mr. Francis H. Allen of West Roxbury, Massachusetts, for a critical reading of this manuscript. 237 T•v. Amr, Vo•. 54

BLACK SKIMMEH8 FEIGNING INJURY IN THE VICIN•EY OF THEIR 238 r•.•N•LL,Behavior ofBlack Skimmers [jAulU•y knowledgeof the Black Skimmer,I have presentedin the followingpara- graphscertain of my observationspertaining to its behaviorat Cardwell Island and the immediatevicinity. Unless otherwiseindicated, these observations were made from a blind. Mating.--Since the nestingseason was already under way at Cardwell Islandby June23, I did not expectto seeany sort of courtshipbehavior. On June 28, however,I sawit occurin oneinstance. Not twenty feet from me an adult, whichI later ascertainedto be a female,stepped from her nest to greet, a few feet distant, her mate who had just arrived in the vicinity. Walking to within three feet of him, shecame to a stop. After a moment's hesitationthe newcomerbowed his head,leaned forward, and with conspic- uousdeliberation picked up a smallpiece of driftwoodsix inchesin length and nearly half an inch in diameter. In order to seizethis with his mandi- blesof unequallength, he tilted his headslightly to oneside and, openink his mouth, insertedthe tip of the longer,lower mandible in the sandbeneath it. Closing his mandibles, he lifted the stick crosswisebetween them. Then, assuminga stance more erect than usual, the suitor slowly and stiffly approachedhis inamorata,projecting his bill with its contentsdirectly towardher. Within onefoot of her he paused. The next move was hers. With bill pointingdownward, head drawn closely back uponher shoulders, she cautiously,almost coyly, steppedtoward him. Once within reaching distance,she hesitated and, then, with a suddenupward jerk of her head, took the stick gently from his mandiblesand carefullyreturned it to the ground. Copulationimmediately followed. The nest which this female left containedtwo eggs. Two days later I notedthree. Copulationhad apparentlycontinued during the layingof the clutch. This preliminaryact in the dischargeof the sexualfunction in- volvingthe passingof the stick,was undoubtedlya feedingreaction,--an inedibleobject being substitutedfor food,--and servedas an extrinsicin- fluencein the heighteningof the seximpulse. Activitiesduring incubation.--Thebelief that the Black Skimmerleaves its nestand eggsfor the entireday, allowingthe sunto supplythe heat re- quiredfor incubation,was long ago dispelledby Dr. Frank M. Chapman.• The fallacyof sucha beliefis all the moreemphasized if one visitsthis island on a hot summer'sday and walks over the sand barefooted. The intensityof the heat absorbedfrom the solarrays is extremelypainful. Certainlythe skimmerembryo cannot be adaptedto suchhigh tempera- tures. Furthermore, if there is a wind, the dry, sifting sand levelsover the nest depressionsand buriesthe eggsin a very short while. That the skimmeris able to copeto someextent with the constantsift-

• 13amps and Cruises of an Ornithologist. D. Appleton and Co., New York, pp. 64-75, 1908. Vol.1937 541J PETTINGILL,Behavior ofBlack Skimmers 239 ing of the sandwhen the wind blows,was exhibitedon oneoccasion when I first placedmy blind in the colony. For thirty-five minutesa skimmerwas absentfrom a nearby nest. In the interim the eggshad becometwo-thirds coveredwith sand. This apparentlycaused the incubatingbird no concern for, uponits return, it reachedunder each egg with its bill and pried it to the surface. Then, settling upon them, it began to turn round and round in the nest, pushingthe new sand away from the eggsand out of the de- pressionwith backward thrustsof its feet until the nest was restoredto its former depth. A. C. Bent• and othershave indicatedthat only the femalestake part in incubation. Two observationsmade by me showthat both sexestake part in this reproductiveprocess. In one easeI saw an adult leaveits nest and take wingjust as its mate alightedbeside it. The newly arrivedbird paused severalminutes and, as its mate disappearedover the colony,settled upon the eggs. In another instancea skimmer,which had been crouchingfour feet away from its incubatingmate for an undeterminedlength of time, suddenlyambled up to the sideof the nest. But the incubatingbird, unlike the one in the observationpreviously cited, choseto remain upon the eggs. Its reluctanceto leave,however, failed to quell the desireto incubate on the part of its mate. Consequentlyit proceededto crouchbeside the latter and, with an effeetlvepush, dislodgedthe occupantand took possession of the nest. To what extent the male skimmer takes part in incubationis a matter for speculation,since the sexesare undeterminableboth in appearanceand in general behavior. These observationstend to indicate that both sexes take an equal part in the incubationprocess. The secondobservation particularlyshows a strongineubative instinet in both partiesconcerned. Activitiesof young.--Onecan readily see that the chicksof the Black Skimmer are well adapted to the vicissitudesof their hot, sandy environ- ment. When broodingis not affordedby the adults, whichoften leave the nestinggrounds temporarily, the youngresort to the shadeof the nearest grassclumps, plants, large shells, and driftwood,and therefind protection from the sun'srays. Especiallydo I recallone large log lying on the beach in such a fashionas to give shadealong one entire side. Here I counted twenty-one chicks,ranging in age from two to seven days, which had soughtprotection from the direct sunlight. When left alone,the chickswould not permit themselvesto be inundated by the blowingsand. Rather would they continueto move constantly, shakingthe sandfrom their backswith muchannoyance. If I appearedin plain view to them outsidemy blind, they would instinctively remain

• Life Histories of North American Gulls and Terns. Bull. U.S. Nat. Mus., no. 113, p. 314, 1921. 240 r•.•N•, BehaviorofBla• Skimmers [July[Auk motionless. By so doing the fine sand would sprinkle over their buffy down and renderthem all the more protectivelycolored. S.C. Arthur (cf. Bent, Life Histories of North American Gulls and Terns, p. 315, 1921) has called attention to the fact that occasionallythe skimmer chicks will entrench themselvesin the sand of their own accord, lying flushwith the surfaceof the beach. This habit I noticedmany times. NeverthelessI saw that, eventhough thus hidden,they would not allowthe sand to cover their backs and would wriggle continuallyto keep it from collectingthere. Broodingof youngby ad•dts.--Althoughthe skimmerchicks wander away to find shadeunder variousobjects, the parent birds, I observed,do not as a rule join them but return insteadto the vicinity of their originalnesting sites. This has apparentlyresulted in certain of the young returningto parents other than their own and being cared for by them. On June 23, I noted one adult broodingtwo chickswhose ages differed by at least five days. In another, more unusualcase I saw an adult skimmer return to its nest which containedthree small young, presumablyits own. One of them had recently emergedfrom the egg. Shortly thereafter two more chicks, obviouslymuch older, appearedat its side,one followingthe other. Both crawled beneath the old bird which offered no objection. Becauseof the noticeabledifference in agesit wasapparent to me that the chicksbrooded by thesetwo birds werenot in eachcase hatched from the sameset of eggs. More extensive observationsare necessaryto prove conclusively that broodingbirds of this speciesalways tolerate the presenceof youngother than their own as thesetwo adultsappeared to do; but certainlythroughout my hoursof watchingI sawno attempt on the part of the old birdsto pre- vent their neighbors'chicks from intruding upon their territory nor any evidenceof death resultingfrom attacks by adults. Feedingof youngby adults.--Both sexes assist in the feedingof the young; the parent birdsreturn separatelyor sometimestogether to give them food. On two occasionsI saw an adult carry on the feedingactivities while its mate brooded the young. The bird bearing the food alighted two feet away from its broodingmate and uttered severallow, barely audiblesounds, whereupontwo chicks emergedfrom beneath their parent and walked directly to the bird with the food. After beingfed they tried to creepunder this bird but it flew away. Most of the food, as far as I could determine,was held betweenthe mandi- bleswhen broughtto the young. I did not seeany caseswhere it seemed that the food wasregurgitated. The youngtook the food directly from the beak, although,if it were dropped,they were capableof pickingit up and devouringit. Feedingby adults.--Windingthrough the salt marshesof CobbIsland on Vol.1937 54]J PETTINGILL,Behavior ofBlack Skimmers 241 their way to the sea are numeroustidal creeks. At low tide scarcelyany water runs throughtheir muddy basins. If any doesoccur, it is very shal- low and slow flowing. At such a time I was standingat a right-angled bend of one of thesecreeks that containeda ribbon of water averagingtwo feet in width whenI discovereda skimmerflying very low overthe marshes. I could see this bird only at intervals when it swung up above the tall marshgrasses, for most of the time it flew alongbelow them. Finally I realizedthat this bird wasfollowing along one of thesecreeks, the identical one by which I was standing. Soon it appearedover the straightaway that led to the bendbeside me. Its mouth wasopen and its thin, laterally compressedlower mandible was insertedin the stream as it dashedtoward me. Oncenearby, ! noteda sizzlingsound produced by the lowermandible as it cut the surfaceof the water. Without reducingspeed the skimmer approachedthe bend and, continuingthe steadybeat of its long pinions, sailed around it, banking its body as a pilot would laterally incline his plane. As if no sharpturn had occurredat all, it continuedto cut the water of the creek for several feet until it suddenlyrose above the xnarshand headedaway towardshore. For at leastthirty feet this bird had steadily cleaved the water with its lower mandible. The skimmer'shabit of cuttingthe water was notedmany timesevery day. Alongthe shoresof CardwellIsland thesebirds were watched cutting the troughsof the incomingbreakers longitudinally, following each one until it all but flattenedout on the beach,then flying out to cleavethe next one as it rolled in. Their wings were always held high, in the upward strokestheir manusesseeming almost to touch abovetheir backs;in the downwardstrokes the tips of the primariesseldom were lowered below the level of the underparts. The horizontalplane of their bodieswas tilted slightlyforward. Their beakswere usuallyopened at an angleof about twenty degrees. The distal third of their lower mandibleswas immersed, bringingthe tips of the shorterupper mandibles nearly to the water'ssur- face. Occasionally,when only the tips of the lowermandibles were touching the water, their beakswere not opened. S.C. Arthur (Auk, vol. 38, p. 574, 1921) has said: "The skimming and'plowing the main'is usuallydone with the mandiblesclosed save for the timesthey are openedto permitthe cry or 'bark'." I do not wish to imply in the abovedescriptions that water-cuttingwas observedonly in the tidal creeksand in the near vicinity of the shore. Oftentimesit wasseen to take placein tidal pools,even in deepwater. I do not wishto imply, either,that this habit wasobserved continually while the birdswere moving along here. Skimmerscould be seento fly very near the surfaceof the creeksor to fly parallelwith the shore,following closely the troughsof the incomingbreakers, without immersingtheir bills, save, perhaps,at very rare intervals. 242 PETTINGILL,Behavior ofBlack Skimmer3 [Auk[July

I firmly believethat theseskimmers seen along the tidal creekand shore were searchingfor food. For the most part they seemedaltogether too intent uponthe water beneaththem to be flyingalong without such a pur- pose. Furthermoremany of the skimmersseen along the shorewere noticed goingback and forth over the same course,sometimes several of them at a time. Their persistencein this behaviorresembled markedly that of the terns in flying again and again over a certain area where fish are to be found. In this paper I do not proposeto review the extensiveliterature per- mining to the feedinghabits of the skimmer sincethis has beensatisfac- torily doneby S.C. Arthur (Auk, vol. 38, pp. 566-574, 1921) and Dr. R. C. Murphy ('OceanicBirds of SouthAmerica,' vol. 2, pp. 1175-1177,1936). Neither do I attempt a final judgment on the traditional belief that the water-cuttinghabit is a meansof obtainingfood. My studiesin this re- spectare far too limited to justify that. I wish to present,nevertheless, the followingobservation with a few comments. While watchingthe skimmerscutting the water along the shores,three times I noticed an individual thrust its entire beak forward and retrieve a glisteningfish crosswisebetween the mandibles. In eachinstance the bird lifted its bill parallel to the surfaceof the water, swallowedthe fish, and continuedon with its maneuvers. The methodof securingthe prey, each one of which was approximatelytwo inchesin length,was suggestiveof fish-catchingin the tern, exceptthat the bill was already in the water before the lunge and the bird did not immerseits body for the bill and fore- headwere the only partsto disappear.So far as I coulddetermine, no ap- parentadvantage was offered either by the modifiedstructure of the bill or by the fact that the lowermandible was previously cleaving the water. Dr. Witruer Stone• once witnesseda skimmer driving its bill into the water and seizinga fishabout three inchesin length. In this casethe fish- ing resembledeven more decidedlythat of a tern, for the bird was not cutting the water at the time. The Black Skimmer,according to the observationsof S.C. Arthur (Auk, vol. 38, pp. 566-574,1921), feeds in the mannerof heronsby wadingabout in shallowwater and strikingat fish. Arthur (p. 569) has "neverseen a Skimmersecure its fish food while skimmingthe waters with its underman- dibleimmersed" and has concluded (p. 574)that the "BlackSkimmer does not secureits food in the way generallyset forth. ." The observationsjust cited seem to indicate that the skimmerfeeds commonlyby catchingfish while on the wingand that the lowermandible is sometimesbut not always immersedimmediately before the strike is

Auk, vol. 38, pp. 595-596, 1921. Vol...54'11937J PETTINGILL,Behavior ofBlack Skimmers 243 made. Further observations are needed to determine the extent and varia- tions of this habit. Defenseof nestsand youngby adults.--My arrival in the immediatevicinity of certain skimmers'nests, particularly those containingyoung, was greetedwith a typeof behaviorwhich struck me asbeing decidedly different from that of other colonialsea birds whosenests I had visited along the Atlantic Coast. Here I noted that adults feignedinjury in the mannerof certaingallinaceous and limicolinespecies. Sincea part of this behavior hasalready been described by B. R. Bales,x I quotehim asfollows: "The four birds [skimmers]that did not return to their nestsused every meansin their powerto lure the strangeobject [blindl from their eggs. They would alight somedistance from the blind and then toddleaway on wobblylegs, with wingsoutspread; push themselves along by slidingupon their breasts;stagger away with outspreadwings beating upon the sand; sit at a safe distanceopening and closingtheir bills, but not making any soundas thoughthey wereswearing at us undertheir breath. " In the midstof the CardwellIsland skimmercolony a long,narrow tidal poolran alongthe upperpart of the sandbeach. Individualsnesting near this water areafeigned injury by swoopingdown upon it as uponland and draggingtheir belliesalong the surface,or, flying just aboveit, splashing theirfeet in thewater as if walking. Occasionally,too, they would nervously cut the water's surface for short distanceswith their lower mandibles, a habit to which Arthur has called attention. On July 3, furtherstudies of the CardwellIsland colonywere interrupted by a severenortheast storm which, with its almost unprecedentedhigh tides,completely washed the nestinggrounds. From the porchof Captain GeorgeW. Cobb'shouse I watchedthe steadyencroachment of the ocean upon the colony. AlthoughI couldnot seethe nestsor the adults upon them, I knewwhen the first toweringrollers rushed over them;for suddenly the old birdsrose together in the air, hovereda moment,and settledout of sight. Againand againthey took wing until finally they remainedin the air constantly. It wasapparent now that their nestswere completely inundated. The adults were flying about here and there in a frenziedmanner as if humanintruders had suddenlycome upon them. Darknessset in before the tide abated and it was, therefore,impossible to follow further their distressed activities. Early the next morning,on visitingthe colony,I found the loosesandy soil hardenedand flattenedlike a floor, while the vegetation,driftwood, shells,nests, eggs, and youngwere nowhere to be seen. Near the vicinity of my blind a pair of Gull-billedTerns swooped down upon me. They were obviouslyprotesting against my appearancein the vicinity of their nest,

Wilson Bull., vol 31 (new ser., vol. 26), 1•. 85, 1919. 244 r•TT•N•,.,.,Behavior ofBlack Skimmers [July['Auk which had been locatedhere the day before. But, strangely enough, no longerwere the Black Skimmersobjecting to my presence.Instead they wereclustered together on the groundfarther downthe beach. At my ap- proachthe entiregroup flew up and movedfarther away. Unlike the Gull- billed Terns, they had already surrendered"possession" of their nesting territories. A careful searchof Cardwell Island finally revealed a small group of nearly thirty partially featheredyoung scatteredhere and there on the debrisat the baseof the duneson the southernmostpoint. My attention was first drawn to them by the vocal protestationsand the injury-feigning of a few adults watchingover them. Upon making this discovery,I real- ized what had beenthe direct fate of the eggsand small young of the colony. The beach,which faces in a southeasterlydirection, had beenswept by the giant wavesas they camein from the northeast. This sidewisesweep had carriedthe movableobjects of the beachdown to the southernmostpoint and out to sea. Someof the olderchicks of the colonywere strongenough to hold their own againstthe force of the rollersand to take refugeat the baseof the dunes. The adults, insteadof remainingon their nestingarea, had followed their young. Here they stayed,--a quarter of a mile from their originalnesting grounds,--with the few offspringthat had not been carried to sea. Departmentof Zoology,Carleton College Northfield,Minnesota TI•E AUK, VOL. ,54 PLXTE 17

UPPER: YOUNG BLACK SKIMMERS BEING COVERED BY BLOWING SAND. LOWER: BLACK SKIMMER FEEDING IN A LARGE TIDE POOL.