Hunting Methods of the Glaucous Gull and Escape Maneuvers of Its Prey

Vol.54, No. 3 General.Votes [329

Hunting Methods of the Glaucous Gull and Escape Maneuvers of its Prey, the Dovekie.--The GlaucousGull (Larushyperboreus) is the most important predator of the Dovekie(Plautus alle) on Spitsbergen,but there is no informationconcerning the hunting strategyof thisspecies (Bateson, Br. Birds54:272-277, 1961;L6venskiold, Avifauna Sval- bardensis,Norsk Polarinst.Skrifter, Oslo, 1964; N orderhaug,Antarctic Ecol. 1:558-560, 1970). When studyingthe breedingbiology of the Dovekiein the region of Hornsund, south-westSpitsbergen in 1974and 1975 (Stempniewicz,Acta Ornithologica18:141-165, 1981), I observedabout 200 attackson Dovekiesby GlaucousGulls during 300 h of field investigations.In 1980 I quantifiedGlaucous Gull predationon Dovekies(Table 1). These observationswere all made during the breedingseason ot' the Dovekie,i.e., from 15June to the end of August.For detaileddata concerningthe studyarea, nestinghabitat, nest density, chronologyof breeding activitiesof the Dovekie, as well as the significanceof GlaucousGull predation, see Stempniewiczloc. cit.). This note describesthe hunting strategyof the GlaucousGull and the behavior of the Dovekiesduring the gulls' attack and pursuit. GlaucousGulls constantlypatrolled the Dovekie coloniesthroughout the summer. The coloniesappeared to be dividedby the gulls into separatehunting territoriesfrom whichall other gullswere immediatelychased (except for the membersof the particular gull'sfamily). When patrollingits territory, a gull flewagainst the wind,low aboveterrain, along the slope. The folds in the terrain and rocks protruding from the mountainside enabled the predator to appear suddenlyin the colony,surprising the Dovekies.On noticinga gull the Dovekiesimmediately took flight, uttering loud cries.They flew away from the gull and diagonallydown-slope to gain speed,then the whole Dovekie flock circled the colony,returning when the danger had passed.On hearing the criesuttered by flushedbirds, the nestlingson the surfaceof the colony,and the adults enteringor leavingthe nestingcrevices at that time, immediatelytook cover within nest burrowsand reappearedat the surtaceonly after the return of the circlingflock. It seemsthat contin- uous vocal contactbetween the birds on the surface,in the nestingcrevices, and in the air, kept them oriented as to the existingsituation and helped them to avoid danger. Repeated patrol flights over the colony flushed Dovekiesand enabled the Glaucous Gull to searchout an objectof attack.Any Dovekiewhich behaved differently from the rest of the flock (e.g., a single bird, sloweddown at the moment of taking to the air) becamethe objectof interestto the gull. Sucha pattern of selectivechoice of the potential prey was particularly evident in the period when the young Dovekieswere leaving the colony.At that time the gullsignored adults completely. The predator did not chasethe flock which frequentlyconsisted of hundredsor thousandsof individualswhich flew at great speedand gave piercing cries.By pursuing its quarry, the gull prevented it from joining the flock.The flock therebyabandoned the bird chasedby the gull. As a rule, the victim flew down the slopeto gain the maximum speed,its only maneuver being fairly regular zigzags.At this stageof the chase,the distancebetween the birds wascrucial. If it wasgreater than about 10 m, either the victim reachedthe water, or after a longer or shorter chase,the predator gave up (despiteits greater speed).If the distancebetween the birdswas less than about 10 m, the gull continuedthe chaseand graduallycaught up with its victim. I observedsuch pursuits to cover 200 m to more than 3 km. In eachof 89 casesobserved, the moment the predator waswithin 1 or 2 m of its prey, the Dovekie (whetheradult or youngbird flyingfor the first time) Iblded its wingsand droppedalmost vertically. The gull was unable to carry out such an evolution, therefore first it slowed down and then plungedafter its prey. At the sametime the Dovekiedove into the water if thiswas nearby, or landedon the groundand squeezeditself into a handy crevice,or-- not finding any hiding place--it took off again and tried to escapeby flying low over the land. In the lastcase, the Dovekie'slate wassealed, in the first two cases,its fate depended upon the depth and extentof the wateror kind of crevice.The consequenceof the diving maneuver carried out over the land was sometimes that the bird struck the rocks. The greater the height at which the flying Dovekie was attackedby the Glaucous Gull, the greaterthe time gainedto find a shelter,after carryingout the diving maneuver. A distinctdifference was observed in the height at whichDovekies flew over the seaand over the land. Over the sea,where there was not only relativelylittle danger from the 3 3 O] General.Yotes j. F,eldOrnithol. Summer 1983

T^BLE 1. Dovekieskilled by GlaucousGulls in Hornsund region, Spitsbergen,1980.

Young birds Young birds leaving Age-class Adults testingwings the colony Period 16-18 June 6-14 August 12-21 August Area under observa- Whole southern Part of slopeof Whole southern tion slope of Arie Arie mountain, slopeof Arie mountain, area of area of breeding mountain, area of breeding colony, colony,ca. 5500 breeding colony, ca. 22,000-25,000 m 2 ca. 22,000-25,000 m z m z Number of Dovekies 10 birds per 15 h 33 birds per 36 h 32 birds per 21 h killed by gulls observation,i.e., observation, i.e., observation, i.e., 0.6-0.7 birds/24 4.0 birds/24 h/ 1.5-1.7 birds/24 h/1000 m" 1000 m 2 h/1000 m 2

GlaucousGulls but alsothe opportunityfor a fast dive into the water, the Dovekiesflew very low,just abovethe surfaceof the water. Over the land however,the Dovekiesmore often flew 50-200 m over the ground, or even higher (estimatedin relation to terrain of a known height). In my opinion this was related, among others, to the greater danger from the gulls and to difficultiesin Dovekiesfinding safe shelterquickly. The sight of a gull chasinga Dovekieencouraged other gullsin the vicinityto join in. The momentthe victimwas captured by one of the gulls,the othersattacked it fiercely in an attempt to steal the quarry. As a result, the Dovekie may passfrom one gull to another before it is finally swallowed.Occasionally the Dovekie took advantageof the confusionand escaped.Holding the quarry in its beak, the gull would shake it several times and swallowit (whole), either in flight, or after settlingon the ground. The strongwind distinctlyfavored the successof the gull predation.During windy weather Dovekieshad a difficult time taking wing, gaining sufficient speed, and landing. On 16 June 1980 during 3 h of observation(strong wind) the GlaucousGulls killed 4 adult Dovekies,while on 18 June 1980 during 3 h of observation(calm weather) gulls killed only 1 adult bird in the sameobservation area. In a similar way gulls hunted young Dovekiesflying from the breeding colony,but successusually took lesstime and effort. Gomparedto the adultsmore fledglingDovekies were caught while still airborne than were driven to the ground and caught there. The situationdiffered in the caseof flightless,young Dovekies testing their wingsin the colony. The suddenappearance of a GlaucousGull gave rise to rapid attemptsto escapeand the chicks--sometimesunable to find or reach their own nestingcrevice--tried to squeeze into the nearestavailable hole. If this wastoo tight or shallow,the gullshad no difficulty in capturingthe nestlings.Gulls were frequentlyobserved walking around the colonyin searchof poorly hidden chicksand eggs as well as birds stricken againstthe rocks or injured by the gulls. Young Dovekiesleaving the colonywere killed by gullsmore frequentlythan adults and lessfrequently than nestlingstesting their wingson the surfaceof the breedingcolony (Table 1). Data included in the table enable only an estimationof the proportionsof the age-classesof the Dovekiekilled by gulls. The observationsdescribed suggest that the following factorsmay influencethe predation of the GlaucousGull on the Dovekie:(a) the well-developedvocal warning system of the Dovekie;(b) the flock behaviorof the Dovekie making it more difficult for the gull to selectthe objectof an attack;(c) the numberof Dovekiesstaying in the breedingcolony or flying in the flock; (d) the distancebetween the colonyand the sea--the shorterthe distance,the greaterthe chanceof escapefor the Dovekie(especially young birds) pursued Vol.54, No. 3 Ge•eralNotes [33 1

by the gull; (e) the altitudeat whichthe Dovekie'scolony is situated--thehigher a colony is situated,the greaterthe chanceof escapefor the Dovekiewhen chased by the gull, after carryingout the diving maneuver;(f) the exposureof the colony--themore open the colonyterrain, the lesspossibility of surprisingthe Dovekiesstaying in the colonyby the gulls;(g) the weatherconditions--strong wind increasesthe effectivenessof the gull predation.--L•cH STEMPNIEWICZ,Department of AnimalEcology, Institute of Biology,Gda•isk Universit),,Czolgistbw 46, 81-378 Gdynia,Poland. Received 10 July 1980;accepted 16 Sept. 1982.

GlaucousGulls Stealing Spoil from Parasitic Jaegers.--Piracyis a manner of foraging,particularly common in thejaegers (Stercorariidae) and gulls(Laridae) (e.g., Fisher and Lockley,Sea birds, Collins,London, 1954; Belopolski,Ekologia morskich kolonial- nych ptic BarencevaMoria, [in Russian],Izdat. Akad. Nauk SSSR,Moskva-Leningrad, 1957; Hatch, Auk 87:244-254, 1970).Jaegers feed largelyby harrying other seabirds,in particulargulls and terns,until they disgorgeand drop food whichthe jaegers then snatch. Caseswhere gulls steal food fromjaegers are much rarer. Parmeleeand MacDonald(Natl. Mus. Can. Bull. 169:61, 1960) reported GlaucousGulls (Larushyperboreus) attacking and stealingfood from Long-tailedJaegers (Stercorarius longicaudus) which had beenforaging at the dump at Eureka, EllesmereIsland. Belopolski (op. cit.) describedMew Gulls(Larus canus)snatching fish droppedby birdsbeing pursued by a jaeger. Morrison(Wilson Bull. 90:649-650, 1978) observedHerring Gulls (Larusargentatus) stealing spoil from Parasitic Jaegers(Stercorarius parasiticus) by direct attack. When studyingthe breedingbiology of the Dovekie(Plautus alle) in the region of Hornsund,south-west Spitsbergen (Stempniewicz, Acta Ornithologica18:141-165, 1981) I was also interestedin the GlaucousGull which is the most important predator of the Dovekie. During field studieson Spitsbergen,in summer 1974 and 1975, I observed severalcases of GlaucousGulls stealing food disgorgedby a Kittiwake(Rissa tridactyla) that was being pursuedby a ParasiticJaeger. In each of 12 observations,the GlaucousGull joined the jaeger in harrying the Kittiwake.Next, the gull flew a few metersbelow and behind the Kittiwake,whereas the jaeger was usuallyto be found over and behind its quarry. The appearanceof the GlaucousGull did not noticeablyaffect the behaviorof the other two birds. After disgorgingof the food by the Kittiwake,both the Glaucous Gull and the ParasiticJaeger tried to catch it. However, the GlaucousGull won this competitionin 9 of the 12 casesbecause of the shorter distanceit had to cover to retrieve the dropping spoil.After either of the birds grabbedthe food, the other one ceasedto be interested in it. The foraging manner of the GlaucousGull describedabove takes less time and effort from the gull than hunting the Dovekies,especially the adult Dovekies.However, it was used fairly rarely, despitethe high frequencyof the Kittiwakesharried by the Parasitic Jaegersin the region of Hornsund.The food disgorgedby the Kittiwakes(consisting of marine macroplankton)probably did not constitutespoil large enoughto be worthwhile for the GlaucousGulls to make it the main target for foraging.--L•cH STEMPNIEWICZ, Departmentof Animal Ecology,Institute of Biology,Gda•isk University, Czolgist6w 46, 81-378 Gdynia,Poland. Received 10 July 1980; accepted16 Sept. 1982.