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Research on Defence Behaviour in a Common Gull(Larus Canus) Colony during BreedingRimantas R. Budrys a & Remigijus Gegelevičius a

a Department of Zoology, Vilnius University, M. K. Čiurlionio 21, 2009, Vilnius, LithuaniaVersion of record first published: 23 Jul 2012.

To cite this article: Rimantas R. Budrys & Remigijus Gegelevičius (2002): Research on Defence Behaviour in a CommonGull (Larus Canus) Colony during Breeding, Acta Zoologica Lituanica, 12:1, 10-17

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10 Acta Zoologica Lituanica, 2002, Volumen 12, Numerus 1

ISSN 1392-1657

RESEARCH ON DEFENCE BEHAVIOUR IN A COMMON GULL (LARUSCANUS) COLONY DURING BREEDING

Rimantas R. BUDRYS, Remigijus GEGELEVIÈIUS

Department of Zoology, Vilnius University, M.K. Èiurlionio 21, 2009 Vilnius, Lithuania

Abstract. The research was carried out on the Didþioji Island, Lake Kretuonas (eastern Lithuania). Astuffed Raven (Corvus corax) was displayed in a Common Gull (Larus canus) colony during differentbreeding stages and attacks on the stuffed Raven made by Common Gulls flying higher and lower than1 m above it as well as attacks of a direct contact were recorded. The stuffed Raven was demonstratedoutside the colony, in the periphery of the colony, and in the centre of it. The most intensive defencebehaviour was demonstrated during the second half of the breeding cycle and during nestling hatching.Especially frequent direct attacks, made by several dozen of birds, were observed in the centre. Stages ofthe breeding cycle in the centre and in the periphery of a Common Gull colony are different, whichaffects defence behaviour in the birds. During later stages of the breeding cycle offsprings of the pairsthat have bred in the centre of the colony can be defended by a kind of a buffer zone, the periphery wherebreeding Common Gulls, occurring in earlier stages of the breeding cycle, demonstrate active defencebehaviour.Key words: Common Gull, defence behaviour in a colony, a stuffed Raven, attack frequency, stages ofthe breeding cycle

INTRODUCTION

Colonial breeding is typical of the majority of Larinae.A Larinae colony might be regarded one of severalequivalent bird breeding strategies which adaptivity initself is not clear and comes out as a certain compro-mise between impacts of positive and negative factors.Two types of colonies can be singled out: obligatoryand facultative (Panov 1983). Birds breeding in a fac-ultative colony retain elements of the territorial breed-ing character typical of solitary breeding pairs: nest ter-ritory is defended, birds are able to breed in solitarypairs, the nest is usually disguised, there are no faeceson its edge and egg-shells are removed from the nest.Facultative colonies are dense or diffusive. ManyLarinae breed in diffusive facultative colonies. Suchcolonies may be formed by Common Gulls (Laruscanus), too.One of adaptive breeding features in colonies of birdsmight be defence behaviour on taking care of clutchesand offspring. Such a behavioural aspect in animals isoften considered a breeding optimization problem. Be-haviour is regarded adaptive if advantages exceed costs(Coleman 1987). Defence behaviour is typical of manyLarinae and depends on various factors. Different spe-cies exhibit different intensiveness of such behaviour.In Great Black-headed Gull (Larus ichthyaetus), Laugh-ing Gull (L. atricilla), and Audouin�s Gull (L. audonii),

for example, defence behaviour is not so distinct(Mierauskas & Buzun 1991; Cavanagh & Griffin 1993;Oro 1996), whereas Herring Gull (L. argentatus), Com-mon Gull, and Common Tern (Sterna hirundo) are moreaggressive species in this respect (Buzun & Mierauskas1989).It was established that Common Gull nests in the pe-riphery of a colony compared to solitary nests are de-fended better (Gotmark & Andersson 1984). The high-est breeding success in solitary breeding Common Gullpairs is observed at collective nest defence (Vermeer &Devito 1986). Defence effectiveness in a Black-headedGull (L. ridibundus) colony increases during the eggincubation period, under increasing individual risk inbirds, the number of the gulls participating in clutchdefence remaining, though, unchanged (Allaine 1991).The dependence of offspring defence intensiveness ona reproduction stage was established for Herring Gull(Kilpi 1987), Great Black-backed Gull (L. marinus;Kilpi 1988), Black-headed Gull (Malickienë & Budrys2000), Arctic Tern (Sterna paradisea), Common Tern,and Roseate Tern (S. dougallii; Lemmetyinen 1971;Kaverkina & Roshchevsky 1984; Shealer & Burger1992). Experiments were conducted with solitary birdpairs that also showed offspring defence to be depend-ent on a stage of the breeding cycle (Westmoreland1989; Hogstad 1991; Halupka & Halupka 1997).Behaviour of the majority of Larinae depends on a

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11Research on defence behaviour in a Common Gull colony during breeding

predator species. Black-headed Gulls respond to a Crow(Corvus corone) flying above a colony more often thanthey do to Herring Gull. Crows flying above a colonyare always attacked (Kruk 1964), whereas based on thedata of our observation Black-headed Gulls almost didnot defend their clutches from Marsh Harrier (Circusaeruginosus). According to Samorodov (1978), whenan aerial predator appears above a Common Gull colonyonly part of the birds take off, of which merely severalattack the intruder. If a colony is entered by a mammal,a wild boar (Sus scrofa), a badger (Meles meles), or afox (Vulpes vulpes), Common Gulls take off all togetherand defend their nests very intensively. Our data hadshown Common Gulls to demonstrate quite intensivedefence behaviour towards Marsh Harriers, Crows,Ravens, and Herring Gulls, so we used a stuffed Ravenfor experimental research on the dynamics of defencebehaviour during breeding.The purpose of our research was to establish the inten-siveness of defence behaviour in Common Gulls bydemonstrating a stuffed potential predator in differentpositions as to a colony during three different stages ofthe breeding cycle, and to find out whether collectivedefence behaviour is displayed in a diffusive faculta-tive colony of Common Gulls, like in that of Black-headed Gull (Malickienë & Budrys 2000), and whethernests are defended equally effectively in all sites of thecolony.

MATERIAL AND METHODS

To find out the dynamics of defence behaviour in Com-mon Gulls during the breeding period, experiments wereconducted on the Didþioji Island, Lake Kretuonas,Aukðtaitija National Park (eastern Lithuania), in 1999.In the colony, there were 68 Common Gull nests madein high luxuriant grass (about 0.5 m high) with pre-dominant meadow-sweet (Fillipendula ulmaria), North-ern bedstraw (Galium boreale), tufted hairgrass(Deschamsia flecsuosa), water avens (Geum rivale), andimperforate Saint-Johnswort (Hypericum maculatum).Average minimal distance between the nests in thecolony was 5.61 ± 0.78 m, the distance variability co-efficient was as high as 57.9%.To investigate defence behaviour in Common Gulls, astuffed Raven was used, since Ravens had been mostoften observed flying above the Common Gull colonyon the Didþioji Island attempting to raid nests. The ex-periments were carried out in several bird breedingstages: during incubation, nestling hatching, and dur-ing parental care of offspring. During each of the abovestages a series of experiments was performed by dem-

onstrating a stuffed Raven in three different sites: (1)outside the colony, 10 m from its edge, (2) in the pe-riphery of the colony, and (3) in the centre of the colony.A cloth-covered roosting stuffed Raven was fixed on a1.5-high stand stuck in the earth in a chosen site. Thefishing-line fixed to the cloth ran to the observers� hide-out made of camouflage material where the observerswaited for the birds to calm down and land to their nest-ing territories. After the cloth was pulled off from thestuffed Raven, responses of Common Gulls to thestuffed predator were recorded for 10 minutes. Basedon the character and intensiveness of attacks, the de-fence behaviour demonstrated by Common Gulls wasdivided into three types: (1) in response to the stuffedRaven a Common Gull flew higher than 1 m above it,(2) lower than 1 m above it, or (3) an attack was a di-rect contact with the stuffed predator.After the demonstration of the stuffed bird in a certainsite was over, all the observers would leave the colonyfor 30 minutes before another experiment. Attacks ofCommon Gulls were observed by three observers eachof whom recorded defence behaviour of different in-tensiveness. For the observation, terrestrial hideouts andone hideout arranged in a 6-m-high tower built at thecolony were used. The experiments in the centre of thecolony were shot with a video camera from the tower.The attacks were counted analysing the shot material,since due to a high intensiveness of defence behaviourit was difficult for the observers to make precise recordsunder natural conditions.In 1999, in the Common Gull colony where a stuffedRaven was demonstrated reproduction was asynchro-nous and prolonged, so Common Gulls with nests inthe periphery and those with nests in the centre occurredin different breeding stages. Nestling hatching startedin the centre, incubation had only come to the middlein the periphery. When the stuffed Raven was demon-strated outside the colony or in the periphery, it wasattacked only by birds from peripheral nests, occurringin earlier breeding stages compared to those breedingin the centre. That breeding asynchronicity was takeninto account on analysing the dependence of defencebehaviour on stages of the breeding cycle in differentsites of the colony. For the experiments, the followingstages of the reproductive cycle were chosen: the be-ginning of full clutch incubation, the second half ofincubation, and the first days of hatched out nestlings�life. At that time in the centre of the colony the stagesof nestling hatching and parental care of offspring tookplace.For statistical processing of the data, a single-factordispersal analysis for non-parametric values, theKruskal-Wallis test (Siegel & Castellan 1988), was used.

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The reliability of the differences in the frequencies ofCommon Gull attacks of different types was tested withthe Mann-Whitney U-test.

RESULTS

Response to the stuffed Raven outside the colony

During the experiments outside the colony the stuffedRaven was attacked by Common Gulls breeding clos-est to it, in the periphery. Common Gulls breeding inthe other parts of the colony did not take part in theattacks. During all the reproduction stages they calmlyincubated or sat within their nesting territories demon-strating no response to defending Common Gulls� criesof danger.The total number of Common Gull attacks significantlychanged depending on a breeding stage outside thecolony (H = 18.80, p < 0.0005) based on the Kruskal-Wallis test.During the experiments outside the colony it was es-tablished that the mean of the number of joint attacks(the cases of flying at a height above and below 1 m, aswell as direct attacks) was reliably lower at the begin-ning of incubation compared to the other breeding stages(Mann-Whitney U-test, p < 0.01; Fig. 1). The compari-son of the attack frequency during the second half ofincubation and during the nestling hatching stage didnot show a significant difference (Mann-WhitneyU-test, p > 0.05).The number of attacks of a different type changed sig-nificantly depending on a breeding stage, too (above1 m � H = 7.69, below 1 m � H = 18.96; p < 0.0005).Divided Common Gull attacks according to the char-acter of intensiveness, it was established that outsidethe colony the birds flying above 1 m attacked thestuffed Raven reliably more frequently during the sec-

ond half of incubation compared to the other breedingstages. The lowest attack frequency with the birds fly-ing below 1 m was observed at the beginning of incu-bation compared to the other stages. This testifies to avery low level of defence behaviour in the CommonGulls incubation in the periphery of the colony duringthe given stage of breeding. The low level of CommonGull responses to the stuffed Raven outside the colonywas indicated by a very low number of direct attacksduring the second half of incubation and nestling hatch-ing (Fig. 2).

Figure 1. The number of Common Gull attacks on the stuffedRaven outside the colony. Reproduction stages: BI � the be-ginning of incubation, SHI � the second half of incubation,NH � nestling hatching. * � p < 0.05 based on the Mann-Whitney U-test compared to the BI stage.

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Figure 2. Changes in the number of Common Gull attacks ofdifferent types on the stuffed Raven outside the colony. Re-production stages: BI � the beginning of incubation, SHI � thesecond half of incubation, NH � nestling hatching. * � p < 0.05;** � p < 0.001 based on the Mann-Whitney U-test comparedto the BI stage.

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At the beginning of incubation Common Gull attackfrequency above and below 1 m was similar (differ-ence not significant when p > 0.05). During the secondhalf of incubation attack frequency above and below1 m did not differ significantly (Mann-Whitney U-test,p > 0.05). During the nestling hatching stage the numberof attacks below 1 m compared to that above 1 m wassignificantly higher (Mann-Whitney U-test, p < 0.05).The lowest level of Common Gull aggressiveness onattacking the stuffed Raven outside the colony was re-corded for Common Gulls breeding in the periphery atthe beginning of incubation. Though the total numberof attacks in the colony during the second half of incu-bation and during nestling hatching was similar (Fig.1), the level of defence behaviour in Common Gullsincubating in the periphery was higher during nestlinghatching. This was reflected by a decreased number ofattacks above 1 m during the nestling hatching stage(Fig. 2).Investigation on Common Gull attacks outside the colonyrevealed the highest level of aggressiveness to be in pe-ripheral nests, the birds from which were the ones thatparticipated in the attacks, during nestling hatching.

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Response to the stuffed Raven in the periphery

In the periphery of the colony, after the cloth was pulledoff from the stuffed Raven, in the beginning all theCommon Gulls of the colony took off, but then the birdsfrom the centre and further parts of the colony landedat their nesting territories and did not take part in theattacks. Those were exclusively the birds from the pe-riphery, where the stuffed Raven was, that defendedtheir nests. Herring Gulls, which bred some 15 m fromthe stuffed Raven, did not join the attacking birds andremained within their nesting territory.The total number of Common Gull attacks in the pe-riphery changed significantly depending on a breedingstage (H = 14.56; p < 0.001).The mean of the total number of attacks at the begin-ning and during the second half of incubation did not

differ significantly (Mann-Whitney U-test, p > 0.05;Fig. 3). During nestling hatching the number of attackswas reliably higher compared to the second half of in-cubation (Mann-Whitney U-test, p < 0.01) and to thebeginning of it (Mann-Whitney U-test, p < 0.01).Divided the attacks according to their type, it was es-tablished that the frequency of attacks above 1 m in theperiphery of the colony was significantly higher at thebeginning than during the second half of incubation(Fig. 4). In the periphery, based on a breeding stage asignificant change was observed in attacks above 1 m(H = 5.95; p < 0.05) and below 1 m (H = 15.04;p < 0.0001). The number of direct attacks did not de-pend on a breeding stage (H = 1.86; p > 0.05).The increased number of attacks below 1 m duringthe nestling hatching stage compared to the secondhalf and the beginning of incubation reflected an in-creased defence in Common Gulls from the periph-ery. The frequency of attacks below 1 m recorded atthe beginning and during the second half of incuba-tion was similar. In the periphery, direct attacks ofCommon Gulls on the stuffed Raven were very rareduring incubation; the frequency of direct attacks dur-ing all the breeding stages differed not significantly.Though the means of the numbers of joint attacks atthe beginning and during the second half of incuba-tion in the periphery did not differ, during the secondhalf of incubation the aggressiveness level of Com-mon Gull attacks was slightly higher. This is indicatedby a decreased number of attacks above 1 m duringthe given breeding stage.

Response to the stuffed Raven in the centre of thecolony

After the cloth was pulled off from the stuffed Ravenin the centre of the colony, all the Common Gulls of thecolony took off, made a dense flock above the stuffedbird and took part in the attacks until the experimentwas over. We failed to find out whether the stuffedRaven was intensively attacked exclusively by the birdsincubating in the centre, since during rapid and veryfrequent attacks it was impossible to establish in whatparts of the colony did the attacking birds nest.The total number of Common Gull attacks in the cent-re changed significantly depending on a breeding stage(H = 22.35; p < 0.0001).In the centre of the colony, the number of joint attackson the stuffed Raven was significantly lower during thebreeding stage than at the beginning of nestling hatching(Mann-Whitney U-test, p < 0.01; Fig. 5). At the begin-ning of nestling hatching attack intensiveness was

Figure 3. The number of Common Gull attacks on the stuffedRaven in the periphery of the colony. Reproduction stages:BI � the beginning of incubation, SHI � the second half ofincubation, NH � nestling hatching. * � p < 0.05 based on theMann-Whitney U-test compared to the BI stage.

Figure 4. Changes in the intensiveness of Common Gullattacks of different types on the stuffed Raven outside thecolony. Reproduction stages: BI � the beginning of incubation,SHI � the second half of incubation, NH � nestling hatching.* � p < 0.05; ** � p < 0.001 based on the Mann-Whitney U-testcompared to the BI stage.

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significantly higher compared to that during the parentalcare stage (Mann-Whitney U-test, p < 0.01). Thus, inthe centre an increased level of defence behaviour inadult Common Gulls during the beginning of nestlinghatching is reflected by a higher attack frequencycompared to the other breeding stages.The number of attacks of a different type significantlychanged depending on breeding stages (attacks above1 m � H = 15.15, below 1 m � H = 21.86, direct at-tacks � H = 18.80; for all the cases � p < 0.0005).Divided Common Gull attacks according to their type,it was established that in the centre of the colony at-tacks above 1 m were significantly less numerous atthe beginning of nestling hatching than during theother breeding stages (Fig. 6). The number of attacksbelow 1 m was significantly higher at the beginningof nestling hatching compared to the stages of bree-

ding and parental care. The intensity of that type at-tacks was lower during incubation than during paren-tal care. Direct attacks were significantly less numer-ous during parental care than during incubation andthe beginning of nestling hatching. During the breed-ing stage the number of direct attacks and of thoseabove 1 m differed not significantly (Mann-WhitneyU-test , p > 0.05).During Common Gull incubation the total intensive-ness of attacks (the cases of flying above and below1 m, as well as direct attacks) on the stuffed Ravenoutside the colony, in the periphery, and in the centre ofit differed significantly (H = 25.60; p < 0.0001). Simi-lar results were obtained on comparing the data of theexperiments carried out in different colony sites at thebeginning of nestling hatching (H = 23.42; p < 0.0001)and during parental care (H = 25.85; p < 0.0001). Thus,the intensiveness of attacks during each experimentdiffered with colony sites. The highest number of at-tacks during each breeding stage was observed in thecentre of the colony compared to the other colony sites(Mann-Whitney U-test, p < 0.01), whereas it was low-est of all outside the colony (Mann-Whitney U-test,p < 0.01).Divided Common Gull attacks according to their type,it was established that during all the reproductionstages the number of attacks significantly differeddepending on the site of the colony where the experi-ment was carried out (above 1 m � H = 31.43, below1 m � H = 71.49, and direct attacks � H = 48.39; in allthe cases � p < 0.0001). The stuffed Raven was at-tacked significantly more frequently by the birds fly-ing above and below 1 m from it in the centre com-pared to the other colony sites (Mann-Whitney U-test,p < 0.01). Direct attacks, too, were significantly morenumerous in the centre than in the other sites of thecolony (Mann-Whitney U-test, p < 0.01). Accordingly,the highest aggressiveness level demonstrated by Com-mon Gulls was observed in the centre. That high fre-quency of aggressive attacks in the centre of the colonytestified to a great activity of Common Gulls partici-pating in the attacks.

DISCUSSION

Collective defence of clutches might be regarded oneof the adaptive features of colonial breeding. Sinceadult birds may potentially become predation victimsthemselves, due to natural selection there forms thekind of defence behaviour that ensures a greater sur-vival possibility for defended offspring at the lowestrisk on the part of the parents. The determiner decid-

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Figure 5. The number of Common Gull attacks on the stuffedRaven in the centre of the colony. Reproduction stages:I � incubation, BNH � the beginning of nestling hatching,PC � parental care. * � p < 0.05 based on the Mann-WhitneyU-test compared to the I stage.

Figure 6. Changes in the intensiveness of Common Gull at-tacks of different types on the stuffed Raven in the centre ofthe colony. Reproduction stages: I � incubation, BNH � thebeginning of nestling hatching, PC � parental care.** � p < 0.001 based on the Mann-Whitney U-test comparedto the I stage.

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15Research on defence behaviour in a Common Gull colony during breeding

ing the intensiveness of this kind of behaviour may beoffspring age (Montgomerie & Weatherhead 1988).There is an opinion that from egg laying until plum-age formation offspring value gradually grows, there-fore the time and energy used by the parents as wellas the risk run by them increase until their offspringare self-dependent and able to escape predation ontheir own. This is why semiprecocial hatchlings aredefended most intensively before they leave the nest,whereas chicken offspring � on hatching out (Barash1975; Andersson et al. 1980). Our results showed thaton demonstrating the stuffed Raven in different sites(outside the colony, in the periphery, and in the cen-tre) the total number of attacks depended on a stageof the breeding cycle; the most intensive defence be-haviour was recorded during the second half of incu-bation and during nestling hatching, though in the pe-riphery that difference was, however, not significant.Results of our experiments do not contradict the hy-pothesis of offspring age in evaluating the adaptiveimportance of collective defence behaviour (Anders-son et al. 1980).Another determiner that may decide defence intensive-ness is offspring vulnerability (Dale et al. 1996). Someauthors suggest that offspring may become overheatedor get cold if their parents defending the nest from apredator leave them alone for too long (Bureð & Pavel1997; Bureð & Horaèkova 1998). According to Kilpi(1987, 1988), due to the fact that after offspring hatch-ing Herring Gulls respond to an intruder into a colonymore intensively and return to their nests considerablyquicker and more synchronically there decreases a pos-sibility of cannibalism. The hypothesis of offspringvulnerability suggests that during the utmost offspringsusceptibility the parents protect the nest more inten-sively in order to return to it quicker. The intensivenessof defence behaviour is well illustrated by the frequencyof direct attacks. During nestling hatching the frequencyof direct attacks was reliably higher merely in the cen-tre of the colony. Nestling hatched out there, the fre-quency of other kinds of attacks (above and below 1 m)also increased. A statement could be made that the ex-periments carried out by demonstrating the stuffedRaven in the centre revealed the most intensive phaseof defence behaviour, observed at the beginning of nes-tling hatching, which as soon as during parental carestarts to reduce.Larinae offspring are semi-chicken; they spend just thefirst several days of their life in the nest and then areable to hide out on their own nearby. Therefore, de-fence in Larinae is most intensive right after nestlinghatching, during the period of their utmost vulnerabil-ity. In the centre of the colony, throughout the demon-

stration of the stuffed Raven (10 minutes), CommonGull attacks, with the participation of several dozen ofbirds, did not stop or did not reduce, so it was impossi-ble to record how long did it take for adult birds toreturn to their nests during different stages of the breed-ing cycle.To generalize the data of the investigation in the cen-tre during Common Gull breeding, there could bestated that the birds showed the most pronounced re-sponse to the stuffed Raven at the beginning of nes-tling hatching, i.e. exactly on the hatching of nestlingin the centre, in some nests offspring being alreadyseveral days of age. High level of aggressiveness and,simultaneously, defence behaviour in adult birds at thattime were reflected by the frequency of close (<1 m)and direct attacks. Though the total number of attacks(Fig. 5) was higher during parental care compared tothe breeding stage, the reduced frequency of directattacks indicated less pronounced defence behaviourin Common Gulls. The diminished level of aggres-siveness in Common Gulls towards an intruder dur-ing that stage was also displayed by the reduced fre-quency of direct and close attacks and increased dis-tant attacks (>1 m) in comparison with the frequencyof different type attacks at the beginning of nestlinghatching.In the centre, all the birds of the colony took an activepart in nest defence, and attack intensiveness washigher there than elsewhere in the colony. Such a com-paratively intensive nest defence in the centre mighthave reduced predation influence there. On demon-strating the stuffed predator outside the colony, de-fence behaviour was displayed just by the CommonGulls whose nests were in the periphery, and that kindof behaviour was most intensive during nestling hatch-ing.During all the experiments not a single agonistic re-sponse of Common Gulls was observed towards an in-dividual of the same species irrespective of where thestuffed Raven was demonstrated. Some authors con-sider (Kilpi 1989) collective defence behaviour to benot effective in colonial breeding sites of large Larinae,since hosts of a territory attack both a predation intruderand hosts of the same Larinae species from neighbour-ing nesting territories.In a facultative diffusive Common Gull colony, collec-tive defence behaviour came out, which was most ef-fective in the centre. There all type attacks on the stuffedRaven were most frequent. The most active collectivedefence of offspring is during nestling hatching and atthe beginning of parental care, the same as in Black-headed Gull colonies, and in each concrete case it islikely to depend on colony size and density. The stages

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of the breeding cycle are different in the centre and inthe periphery of the colony, which influences defencebehaviour in Common Gulls. During nestling hatchingreduced defence behaviour in the birds that have bredin the centre is already weakened and their offspringmay be defended by a kind of a buffer zone, the periph-ery of the colony, where breeding Common Gulls oc-curring in earlier stages of the breeding cycle demon-strate active defence behaviour. They make a kind of adefence barrier for the offspring of the birds that havebred in the centre.

ACKNOWLEDGEMENTS

The authors� cordial thanks are due to Dr D. Malickienë,a research fellow at the Institute of Ecology, for hervaluable commentaries on preparing this article. We areequally indebted to J. Sorokaitë and M. Starodubaitë �they helped us a lot in field works during the experi-ments.

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Barash, D.P. 1975. Evolutionary aspects of parental behav-iour: distraction behaviour of the Alpine Accentor.Wilson Bulletin 87 (2): 367�373.

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PAPRASTØJØ KIRØ (LARUS CANUS) GYNYBINËS

ELGSENOS TYRIMAI VEISIMOSI METU

R.R. Budrys, R. Gegelevièius

SANTRAUKA

Tyrimai vyko Kretuono eþero Didþiojoje saloje,Aukðtaitijos nacionaliniame parke (Rytø Lietuva).Difuzinëje paprastøjø kirø (Larus canus) kolonijojeskirtingø veisimosi stadijø metu buvo demonstruojamakranklio iðkamða ir tiriama ðiø paukðèiø gynybinëelgsena, fiksuojant jø atakas á iðkamðà, praskrendanttoliau ir arèiau 1 m, bei tiesioginius uþpuolimus. De-monstruojant kranklio iðkamðà ávairiose pozicijose � uþkolonijos, jos periferijoje ir centre � bendras atakødaþnumas priklausë nuo veisimosi ciklo fazës. Inten-syviausia gynybinë elgsena buvo antroje perëjimo pusëjeir jaunikliø ritimosi metu. Vëliau jø gynybinis elgesyspasireiðkë silpniau. Kolonijos centre buvo ypaè didelistiesioginiø atakø daþnumas (jose dalyvavo keliasdeðimtpaukðèiø). Pusiau viðèiukiniai kirø jaunikliai tik keliaspirmas dienas praleidþia lizde. Didþiausio paþeidþia-mumo metu jø gynyba yra intensyviausia. Perëjimo ciklostadijø nesutapimas paprastøjø kirø kolonijos centre irperiferijoje turi átakos jø gynybinei elgsenai. Vëlesnësedauginimosi ciklo stadijose kolonijos centre perëjusiøporø jaunikliai gali bûti apsaugoti savotiðkos �buferinës�kolonijos periferinës zonos, kurioje perintys paprastiejikirai, esantys ankstesnëse veisimosi ciklo stadijose,demonstruoja aktyvià gynybinæ elgsenà.

Received: 15 October, 2001Accepted: 01 March, 2002

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