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Home , Dolphin gull, Larus

The Condor98:208-2 15 8 The Cooper Ornithological Society 1996

BREEDING BIOLOGY OF THE DOLPHIN AT PUNTA TOMBO, ARGENTINA ’

PABLO YoRIo,~,~J P. DEE BOERSMA,*,~AND SCOTT SWANN* 2Wildhfe ConservationSociety, Bronx, NY 10460 Department‘ of Zoology, Universityof Washington,Box 351800, Seattle, WA 98195-1800

Abstract. Dolphin ( []scoresbii) at Punta Tombo, Argentina, aggregatedin small coloniesof 16-24 pairsto breed.Colony location varied among years. Nesting density was 1.02 to 1.6 pairs/m2. Mean clutch size was 1.9 and 2.0 in 1987 and 1989, respectively, and egg volume was similar among years. Incubation took between 24 and 27 days, with chicks hatching from mid to late December. When chicks were two to five days old they followed their parentsaway from the nestingarea and, as they grew older, they began creching in one or more groups. Egg mortality in 1987 and 1989 was 41% and 46%, respectively,mainly through disappearanceand presumedpredation. Breedingsuccess from 1984 to 1990 was variable, ranging from complete failure to 0.86 chicks surviving to three weeks of age per active nest. After nest lossesin 1986 and 1990 renested.During the breeding seasonDolphin Gulls were specialized scavengers,mainly eating southernsea lion (Otaria Jlavescens)excrement and scrapsof food dropped when Magellanic Penguins (Spheniscusmagellanicus) and Imperial (Phalacrocorax atriceps) fed their chicks. Dolphin Gulls alsofed in the intertidal zone, stolecormorant and (Larus dom- inicanus)eggs, and scavengedremains of penguineggs already preyed upon by other . They nested with other speciesand left the breedingarea in February after the chicksfledged. They did not return to the breeding area in any numbers until September. The breeding biology of the appearsto be largely influenced by their specializedscavenging habits and vulnerability to predation. Key words: Dolphin Gulls; Larus [Leucophaeus]scoresbii; breeding biology; reproductive success.

INTRODUCTION unique coloration for gulls;gray with a large bright The Dolphin Gull (Larus [Leucophaeus] scores- red-orange and a dark hood in the non- bit) is restricted to the southern coastsof South breeding plumage (Woods 1975, Devillers America. Its breeding distribution on the Atlan- 1977a). In addition, juvenile plumage is uniform tic coast ranges from Tierra de1 Fuego north to and chick plumage is grey with black spots (De- Punta Tombo (44”02’S, 65’1 I’W), Argentina, in- villers 1977a). Dolphin Gulls are fairly small gulls cluding the Malvinas (Falkland) Islands, and and exhibit several behavioral traits not found north to Chiloe Island (42”) Chile, on the Pacific in most species of gulls. They are specialized coast (Goodall et al. 195 1, Harrison 1983). Dol- scavengers,nest in closeproximity to each other, phin Gulls have been variously assignedto the adults and chicks leave the nest shortly after genera Gabianus, Larus, and Leucophaeus. Sib- hatching, and chicks form creches(Woods 1975, ley and Monroe (1990) placed it in Larus, but Devillers 1977a). Burger and Gochfeld (in press) retain it in Leu- Few studies have focused on this unusual gull cophaeus becauseof its unique plumages and bill although some aspectsof their general behavior shape as well as behavior. Dolphin Gulls have (Moynihan 1962, Devillers 1977a) and plumage (Dwight 1925, Woods 1975, Devillers 1977a) have been described.Its diet and scavenginghab- its have been briefly discussedby Murphy (1936), ’ Received 20 July 1995. Accepted 14 December Humphrey et al. (1970), Kury and Gochfeld 1995. (1975), Woods (1975) and Devillers (1977a). 4 Present address: Fundaci6n Patagonia Natural, However, very little is known about the Dolphin M.Zar 760, 9 120, Puerto Madryn, Chubut, Argentina. 5 Mailing address:P. Dee Boersma. Department of Gull’s breeding biology and behavior. This paper Zoology, University of Washington, Box 35 1800, Se- describes the breeding biology of the Dolphin attle, WA 98195-1800. Gull at Punta Tombo, Argentina.

PO81 DOLPHIN GULL BREEDING ECOLOGY 209

METHODS We studied the breeding biology of Dolphin Gulls at Punta Tombo between 1985 and 1990. In ad- dition, in 1983 and 1984 we made some oppor- tunistic observations on breeding. During 1986 and 198 7 we captured incubating adults at their nests using the Weaver and Kadlec (1970) tech- nique. We banded birds with U.S. Fish and Wild- life Service metal bands on the right leg and with large numbered yellow laminated plastic bands on the left leg. The plastic bands were readable from up to 50 meters with a 20 x spotting scope. At banding, we weighed adults using a 1,000 g Pesola scale, and used calipers to measure head length, culmen, bill depth and tarsus to the near- est 0.1 mm. We measured wing chord to the FIGURE 1. Breeding sites of Dolphin Gulls (A,B,C) 1983 to 1987, B: 1988 nearest mm. at Punta Tombo, Chubut. A: to 1990, C: re-nesting 1986. During 1985, 1986, and 1990, we visited the colony irregularly to measure eggsand determine breeding success.During the 1987 and 1989 breeding seasons,we visited the colony every two Kelp Gulls (Larus dominicanus) and (Ca- to five days, from the beginning of November to tharucta sp.) (Fig. 1). A colony of 250-500 south- mid January, to record settlement patterns, tim- ern sea lions (Otariaflavescens)is located on a ing of nest initiation, egglaying dates, clutch size, small islet about 500 m north of Punta Tombo. hatching dates and breeding success.We weighed Magellanic Penguins and Imperial Cormorants eggsand chickswith 100 g and 500 g Pesolascales lay eggs in mid-October, hatch chicks in mid- to the nearest 1 and 5 g respectively, and mea- November and fledge chicks beginning in mid- sured them with calipers to the nearest 0.1 mm. January. We calculated eggvolumes (V) as: V = length x Values listed in the text are means * SD (n). width2 x 0.52 (Preston 1974). We measured the distance between nests (center to center) to the RESULTS nearest cm during 1987. We marked chicks at hatching with a fiber-tape band. We calculated ADULT SIZE AND GENERAL BEHAVIOR breeding successas the number of chicks sur- Dolphin Gulls are fairly small gulls, with an av- viving to three weeks of age per nest with eggs. erage adult weight of 524 g (SD = 5 1, n = 42). We watched the colony from a high point 20 They are sexually dimorphic (Table l), with the m from the nests. From this distance birds could presumed male being significantly larger than its be easily observed without apparent disturbance. mate in weight, bill, and tarsus. Wing chord did We made continuous observations during day- not differ significantly between larger and smaller light hours (0600-2000) on 30 December 1987 members of a pair. Like other gulls, sexesappear and 6,9, 13, and 17 January 1988. We recorded similar in behavior with both taking part in in- incubation shifts, feeding frequency, feeding be- cubation, chick feeding, and the defense of the havior, and parental behavior. nest and offspring. During 1986 and 1987, we surveyed the Punta Tombo area every four days to determine the COLONY SIZE, SETTLEMENT AND NESTING foraging areas of the Dolphin Gulls and to ob- At Punta Tombo, the number of breeding pairs serve their feeding activities. We used opportu- of Dolphin Gulls varied from a high of 24 in nistic sightings to determine winter dispersal. 1989 to a low of 16 in 1990. The location of the Several other speciesbreed at Punta Punta Tombo Dolphin Gull breeding colony Tombo: Magellanic Penguins (Spheniscusma- changed among seasons(Sites A to C, Fig. 1). gellanicus),Imperial Cormorants (Phalucrocor- Two of these sites have been used in the past, ax atriceps)( “albiventer” color morph, Ras- site B in the 1970s (Conway, pers. comm.) and mussen 199 l), Rock Shags (P. magellanicus). site C in 198 1 (Passera,pers. comm.). All nesting 210 PABLO YORIO ET AL.

TABLE 1. Body measurements(mean + SD) of Dolphin Gull breedingpairs (n = 16) at Punta Tombo, Argentina.

Variable Larger of pa,r Smaller of pair Paired t-test

Weight(g) 544 & 32.1 489 f 30.0 t = 5.27*** Culmen (mm) 40.1 * 1.4 37.9 + 2.5 t = 3.63* Bill depth(mm) 15.6 * 0.7 14.5 + 0.8 t = 4.11** Head length(mm) 91.1 i 1.9 86.9 f 2.2 t = 5.92*** Wing chord(mm) 339.4 f 8.9 339.4 + 10.5 t= 1.36NS Tarsus(mm) 61.1 f 1.9 63.3 & 2.4 t = 4.51*** *P < 0.002. **P < 0.001. *** P < 0.0001

sites were close to or among other colonial sea- As a result of this close nesting proximity Dol- birds, Kelp Gulls, Imperial Cormorants or Mag- phin Gulls were commonly engagedin physical ellanic Penguins, and all were on bare irregular contact, such as jostling and grabbing one an- volcanic rock, a few meters from the edge of the other’s bills. water. During early September, only a few birds (one EGG LAYING AND INCUBATION to seven) were regularly present at Punta Tombo. Time between the commencement of nest build- By early October, more Dolphin Gulls arrived ing and the laying of the first egg in the colony and started to attend the Magellanic Penguin and was only two to three days. Egg laying in most Kelp Gull colonies. Even though some Dolphin years started during the last days of November. Gulls were at their colony site by late September Although nestswere not checkedregularly during (generally one to five birds), most did not regu- egglaying in 1985, the first hatching datessuggest larly attend their nesting area until the first or the eggswere laid a week earlier in 1985 than in secondweek of November. Dolphin Gulls began the other years. Egg laying continued for nearly courtship feeding in November. The few food a month, with the last eggslaid on 23 December pellets from courtship feeding that we obtained in 1987 and 30 December in 1989. About 60% contained only partly digested fish. of the nests had complete clutches by 9 Decem- The first nests appeared in late November, in ber in 1987, and 50% had complete clutches by both 1987 and 1989. Nests were made mainly 7 December in 1989. with algae, but grasses,feathers, bones and sticks Mean clutch size was 1.9 f 0.6 (n = 21) in were also used. Mean external diameter of nests 1987 and 2.0 f 0.6 (n = 23) in 1989. In 1987 during 1987 was 28 cm + 2.3, (range = 25-32, and 1989, respectively, 9.5% and 20.8% of the n = 20) and mean intemest distance measured clutches contained three eggs.Eggs averaged 6 1 from center to center of the nestswas 41.3 f 5.3 x 42 mm (Table 2). Calculated egg volume was cm (n = 2 1). During all years, nestswere as close significantly correlated with eggweight (Pearson together as substrate irregularities allowed and r = 0.73, P < 0.001, n = 136). Egg volume was were often touching one another. Nesting density similar among years (ANOVA F = 0.21; df = 3, in 1987 was 1.O pairs/m2, although the density 132: P = 0.89) and between first and secondeggs of the main group of 18 nests was 1.6 pairs/m*. within the same clutch (Wilcoxon Z = 1.68, P =

TABLE 2. Dolphin Gull egglength, width, volume and weight(mean + SD) at Punta Tombo, Argentina.

YeaI n Length (mm) Width (mm) Volume (cm’) Weight (EC) 1983 60.8 * 2.2 42.2 & 1.0 57.0 t 4.1 56 * 5 1985 60.9 + 2.5 42.2 ? 1.1 56.5 + 4.1 56 & 6 1986 25 60.6 + 2.3 42.2 + 1.5 56.3 & 4.9 52 * 6 1987 39 61.0 + 2.0 42.1 & 0.9 56.3 & 2.9 57 * 3 Total 136 60.9 f 2.2 42.3 + 1.1 56.6 -c 3.9 55.4 f 5.2 DOLPHIN GULL BREEDING ECOLOGY 211

0.09, n = 10). In seven clutches that had three TABLE 3. Dolphin Gull chick weight and size at eggs,one egg,presumably the third one laid, was hatching during 1987 (n = 14, except weight n = 13). significantly smaller than the other two eggs (Kruskal-Wallis x2 = 9.5 1, P = 0.009, n = 21). Variable Mean f SD The two larger eggsin each of theseclutches were Weight (g) 41 f 3.0 similar in volume (Mann-Whitney U = 17, P = Culmen (mm) 15.1 f 0.4 0.34). Bill depth (mm) 7.0 f 0.3 Wing (mm) 33.4 + 1.6 The length of the incubation period in 1987 Tarsus (mm) 29.9 f 2.4 was estimated to be between 24 and 27 days. Like other gull species,both sexesincubated the eggs. In four nests where pairs were followed throughout the daylight hours (14 hours), the approachedand performed aggressivehead-toss- birds traded incubation duties on average 6.0 & ing. 0.8 times, (range = 5-7). On occasion,gulls flew directly towardsus when we were still approximately 50 m away from the CHICK STAGE colony, swooping and vocalizing before return- Chicks were first seen on 16,22, and 29 Decem- ing to their nest. Such intimidation behavior, ber in the 1985, 1986, and 1987 breeding sea- performed individually or in groups, also oc- sons, respectively. In 1987, chicks weighed at curred away from the colony in areaswhere birds hatching an average of 41 g (Table 3). were feeding or resting. The average number of brooding exchanges Adults followed their chicks closely as they during the daylight hours for pairs with chicks moved. When the creche was approached by a under 5 days old was 10.33 ? 3.08 (n = 6) and potential predator such as Kelp Gulls or skuas, mean number of chick feeding bouts per day was adults actively defended their chicks. Defense 7.5 f 2.43 (n = 6). Chicks grabbed regurgitated behavior consisted of head-tossing, calling from food from the parent’s beak, and food not taken the ground, mock charges,physical attacks, and/ was again swallowed by the parent. During each or flights towards the intruder, defecating and meal, a parent regurgitated one to seven times circling while calling before returning to the group (X = 2.27 + 1.46 (n = 44). A meal lasted 9 to of moving chicks. The frequency of calling and 80 minutes (n = 45.5 * 2 1.9, n = 18). Most food aggressivedisplays increasedwhen the group was appeared to be Southern Sea Lion dung or fish disturbed, and chicksgathered more closelywhen probably scavengedfrom Magellanic Penguins. approached by an intruder. Chicks older than When chicks were two to five days old they two to three weeks sometimes fled briefly into followed their parents a few meters away from the water, followed by adults. the nesting area down toward the water where they remained with their parents but separate BREEDING SUCCESS from other broods. As chicks grew older, they Egg mortality in 1987 and 1989 was similar and creched in one or more groups. Chicks grouped more than 40% of the eggsdid not hatch (Table tightly together and moved as a unit, often sur- 4). Since there were no large storms and no egg rounded by adults. They remained in the vicinity remains were found within or near the colony, of the colony, where they were guarded and fed eggsthat disappeared were probably taken away by their own parents until fledging. by predators,such as Kelp Gulls and skuas,which were regularly seen in the vicinity of the colony. ANTIPREDATOR BEHAVIOR Similarly, no chick carcasseswere found. Unlike most other gulls, incubating Dolphin Gulls Breeding successwas highly variable among stayed at their nest when approached, and often seasons, ranging from complete failure during did not leave their nest until we were less than 1984, 1986, 1988, and 1990 to 0.86 chicks sur- a meter away. In some instances, only the birds viving to three weeks of age per active nest in closestto us moved, while birds a meter or two 1985 (Table 5). In both 1984 and 1986, complete away remained incubating. Birds that left gen- failure was caused by storms that washed away erally flew up, vocalizing loudly, defecating, dive all nestsand their contentsduring late December, bombing, and even hitting us. Individuals that before hatching in the former year and during flushed often landed a few meters away, called, hatching in the latter. Renesting occurred after 212 PABLO YORIO ET AL.

TABLE 4. The percentof Dolphin Gull eggsaddled, broken, or lost to predatorsat Punta Tombo, Argentina. Samplesize in parentheses.

YeaI Eggslaid % Addled (n) % Broken(n) % Predation(n) Total (n) lost 1987 39 10.25 (4) 2.56 (1) 28.20 (11) 11.03 (16) 1989 47 8.51 (4) 4.26 (2) 34.04 (16) 46.81 (22)

nest losses from storms in 1986 and 1990, al- four times. During interspecific feeding interac- though replacement clutches were also lost. tions, Dolphin Gulls were always displaced by skuas,Kelp Gulls, and Snowy Sheathbills(Chionis ADULT SURVIVAL &a). Dolphin Gulls occasionally preyed upon Of the 37 birds banded at Punta Tombo during and Kelp Gull eggs,and were partic- the 1986 breeding season,33 (89%)were sighted ularly adept at taking advantage of a disturbance in the following year. During 1989, 20 (54%) of to gain accessto eggs.Kury and Gochfeld (1975) the gulls originally banded in 1986were resighted and others report predation on cormorant eggs, at the breeding colony, but this is undoubtedly whenever the incubating cormorants are dis- an underestimate of survival becausesurveys of turbed by human intruders. The gulls anticipate the banded population were not made through this food source by associating with humans as the whole breeding season.During 1994,at least they approach the cormorant colony. seven birds banded in 1986 bred at the Punta Dolphin Gulls regularly fed on the scraps of Tombo colony (Boersma, pers. obs.), showing food dropped while Magellanic Penguins and that some breeding adults survived more then cormorants were feeding their chicks. Usually, eight years. Dolphin Gulls moved quickly around a penguin that was feeding chicks until it dropped the food, FEEDING ECOLOGY which commonly happened when the penguin During the breeding season, Dolphin Gulls interrupted feeding the chick to attack the gull. mainly scavengedwithin the southern sea lion, When food was dropped, the gull often waited Magellanic Penguin, and Imperial Cormorant until the penguin moved, allowing it to safely colonies. Individual birds were seen repeatedly dart in and steal the dropped food. at the same place, suggestingindividuals have From September to February, Dolphin Gulls an affinity for particular foraging locations. also scavenged on southern sea lion excrement Dolphin Gulls were never seen stealing eggs and fed in the intertidal zone. Dolphin Gulls from penguin nests, although they fed on eggs were more likely to feed in the intertidal zone stolen by other predators such as Kelp Gulls or before Magellanic Penguin or Imperial Cormo- skuas. When Dolphin Gulls tried to steal the egg rant chicks hatched (x2 = 10.77, df = 1, P -C or scavengethe remains (n = 41) from another 0.005). Occasionally, they picked through kelp predator, they got some egg contents one out of that washed ashore after storms.

POST BREEDING DISPERSAL After they finished breeding in late February, TABLE 5. Breeding successof Dolphin Gulls breed- most Dolphin Gulls left Punta Tombo, appar- ing at Punta Tombo, Argentina. ently moving north. By late March, only a few

Nests Meall birds were seen near the colony. Sightings of with Empty Chicks Chicks chick banded individuals were only to the north of the YeaI ew nests 23 hatched fledged per nest breeding colony (n = 11). However, human hab- 1984 - 0 0 0 itation and our sighting efforts were mainly 1985 22 0 z 20 0.86 northward, soit is possiblethat somebirds moved 19861987 2115 06 2639 23B (*) 140 i66 south. Dolphin Gulls were regularly seen at the 1989 24 0 47 25 21 0:83 southern sea lion and Imperial Cormorant col- 1990 16 1 28 3 0 0 onies at Punta Leon (43”04’S, 64”29’W). At Pun- (*) Storm washedaway nestsbefore all chickshatched. ta Leon, of 2 1 adults seenon 17 September 1987, DOLPHIN GULL BREEDING ECOLOGY 213

eight were banded Dolphin Gulls that bred at ties (Buckley and Buckley 1977, Siegel-Causey Punta Tombo in 1986. Dolphin Gulls from Pun- and Kharitonov 1990). ta Tombo were also seen at the southern sealion For most gulls, clutch size is three eggs(Lack colonies on Peninsula ValdCs at Puerto Piram- 1968, Reid 1987), with a few exceptions where ides (4235’S, 64”19’W) and Punta Delgada clutch size is two eggs(Cullen 1957, Beer 1965, (42”43’S, 63”38’W), 200 km north ofPunta Tom- Lack 1968, Mills 1973). Dolphin Gulls at Punta bo during the nonbreeding season.A dead band- Tombo lay a clutch of two eggs,with less than ed was recovered from the Rio Chubut 20% of nestshaving three eggs.Three-egg clutch- (43”21’S, 65’03’W) in 1987, and a live banded es have been reported at other sites such as the gull was seen along the beach near the mouth of Malvinas (Falkland) Islands (Vallentin 1924, this river in 1989 (Passera, pers. comm.). Dol- Woods 1975) and Chile (Goodall et al. 195 1). phin Gulls started returning to Punta Tombo in In contrast to most gulls where broods begin September and by October most of the birds were to leave the vicinity of the home territory at sev- present. eral weeks of age or at fledging (Tinbergen 1953; Burger 1974, 1984) Dolphin Gull chicks at Pun- DISCUSSION ta Tombo abandon the colony area at two to five Dolphin Gulls at Punta Tombo exhibit several days of age. In most gulls, movement of young behavioral traits that are atypical for most gull chicks is restricted to the vicinity of the nest site species. Their nests are packed close together, (Burger 1984) and chicks that trespassinto other commonly have a two-egg clutch, adults and territories are pecked and may be killed (Hunt chicks leave the nesting territories when the latter and Hunt 1976). Like Dolphin Gulls, adults and are a few days old, chicks form well-defined chicks of Black-billed and Olrog’s Gulls per- creches,birds remain at the nest when disturbed manently leave their nesting territories when only mobbing when approached within a few chicks are a few days old (Beer 1966, Devillers meters, and are primarily specialized scavengers 1977b). Once Dolphin Gull chicks leave the nest, during the breeding season. they aggregateand form crechesvery closeto the The dense nesting of more than one pair/m* colony site. Chick creching is also an uncommon observed at Punta Tombo is a common behavior gull feature, and has only been reported for the throughout the Dolphin Gull’s range (Philippi et Slender-billed Gull L. genei(Isenmann 1976) and al. 1954; Humphrey et al. 1970; Woods 1975; the Black-billed Gull (Beer 1966). Creche for- Devillers 1977a; Yorio, pers. obs.). Most other mation may reduce predation (Evans 1980, Da- species of gulls, even small gulls, have much vis 1982). Dolphin Gull chicks group tightly to- greater inter-nest distances. The dense nesting gether, move as a unit and are often surrounded aggregationof Dolphin Gulls is most comparable by adults, which should reduce their vulnerabil- to the density reported for the Black-billed Gull ity to skuas or Kelp Gulls. (Lams bulleri), Red-billed Gull (Lams novae- Dolphin Gulls at Punta Tombo also differ from hollundiae scopulinus)(Beer 1966), and Olrog’s most other gulls in that they are onshoreforagers, Gull (Lams atlanticus) (Devillers 1977b). mainly feeding by scavengingduring the breed- Small internest distance and thus, high density ing season on food dropped by other colonial in this speciesmay be important in predator de- and on sea lion excrement. Scavenging fense. Predator defense should be more effective appears to be a common trait of the Dolphin when nestsare more clumped (Kruuk 1964, Wit- Gull throughout its range (Reynolds 1935, Cas- tenberger and Hunt 1985). Predation of Dolphin tellanos 1935, Woods 1975, Devillers 1977a, Gull eggsby Kelp Gulls was observed at Punta Parmelee and MacDonald 1975). This species Tombo, and more than 25% of the eggswere lost always breeds in association with other colonial each season to predation. High predation from seabirdsand marine mammals, mainly with cor- skuas was reported in the Malvinas (Falkland) morants, penguins, gulls and sea lions (Vallentin Islands (Vallentin 1924). Nesting habitat usedby 1924; Woods 1975; Devillers 1977a; Yorio, pers. Dolphin Gulls at Punta Tombo, i.e., bare vol- obs.). Their arrival at their breeding colony at canic rock closeto the water and to other colonial Punta Tombo occurs after egg laying of other species,is not limited and thus, habitat limita- seabirds has begun and their departure occurs tions for the Dolphin Gull cannot explain the before chicksof other seabirdshave fledged.Their small intemest distancesas in other seabird spe- timing of breeding at Punta Tombo coincides 214 PABLO YORIO ET AL. with the peak breeding activity of both Magel- ACKNOWLEDGMENTS lanic Penguins and Imperial Cormorants. Dol- Research was funded by Wildlife Conservation Soci- phin Gulls started breeding earlier during 1985, ety, and made possible by a joint agreement between which was also an early year for penguins (Boers- the Society and the Organism0 Provincial de Turismo ma et al. 1990), further suggestingtheir depen- of Chubut. Gabriela Bidart, Dianne Decker, Gene Fowler, Liz Gray, Anne Handley, Arthur Kettle, Maria dence on penguins at Punta Tombo. Elena Lizurume, And&s Maldonado and Javier Lopez Most Magellanic Penguins at Punta Tombo de Casenave,helped gather these data. We also thank feed their chicks early and late in the day, al- William Conway for helpful comments on earlier drafts though some penguins arrive with food at other of this paper. times (Boersma, unpubl.). Food from penguins is therefore not available uniformly throughout LITERATURE CITED the day. Cormorant food scrapsand sea lion ex- BEER, C. G. 1965. Clutch size and incubation behav- crement, in contrast, are available throughout ior in Black-billed Gulls (Law bulleri).Auk 82: the day. Becausethese food sourcesare relatively 1-18. BEER,C. G. 1966. Adaptations to nesting habitat in predictable in spaceand time. Dolphin Gulls may the reproductivebehaviour of the Black-billedGull have strongerassociations with sealions and cor- Larusbulleri. Ibis 108:394-410. morants than with penguins. BOERSMA,P. D., D. L. STOKES, AND P. M. YOFUO. 1990. Colony size at Punta Tombo was relatively Reproductive variability and historical changeof small. Dolphin Gulls along the eastern coast of Magellanic Penguins(Spheniscus magellanicus) at Punta Tombo, Argentina, p. 15-43. In L. S. Davis Patagonia commonly nest in small colonies of and J. T. Darby [eds.],Penguin Biology. Academic less than 25 pairs, although colonies range be- Press, San Diego. tween 2 to 210 pairs (Yorio, unpubl.; Schiavini BUCKLEY,F. G., AND P. A. BUCKLEY. 1972. The and Yorio, in press.).In the Malvinas (Falkland) breeding ecology of Royal Sterna(Thalas- sew) maxima-kxima: Ibis 114:344-359. Islands, 85% of the known colonies are lessthan BUCKLEY.P. A.. AND F. G. BUCKLEY. 1977. Hexae- 100 pairs (Croxall et al. 1984). At Punta Tombo, onal’packing of Royal nests. Auk 94:3645. colony size throughout the last three decadeshas BURGER,J. 1974. Breeding adaptationsof Franklin’s never been more than 29 to 31 pairs (Conway, Gull (Laruspipixcan) to a marsh habitat. Anim. pers. corn.). Unlike most gulls and more similar Behav. 22521-567. BURGER,J. 1984. Pattern, mechanism, and adaptive to terns, the location of the colony at Punta Tom- significanceof territoriality in Herring gulls(Laws bo changed among seasons. argentatus).Ornithological Monographs, 34. Colonial breeding and colony size can be in- BURGER,J., AND M. GOCHFELD.In press. - fluenced both by predation and feeding ecology Gulls. In Handbook of the Birds of the World, J. de1 Hoyo, A. Elliot and J. Sargatal [eds.]. Lynx (Wittenberger and Hunt 1985). The observedco- Edicins, Barcelona, Spain. lonial and foraging habits of the Dolphin Gulls CA.STELLAN~~,A. 1935. Observacionesde algunasaves is consistent with G&mark’s (1982) hypothesis de Tierra de1Fuego e Isla de 10sEstados. Homer0 that colony size in gulls increaseswith foraging 6~22-37. range and unpredictability of food sources.This CAWKELL,E. M., AND J. E. HAMILTON. 1961. The birds of the Falkland Islands. Ibis 103a:l-27. positive correlation between coloniality and for- CONWAY,W. G. 1971. Predation on penguinsat Pun- aging range has also been found in other seabirds ta Tombo. Kingdom 74:2-6. (Lack 1967, 1968, Erwin 1977, 1978). CROXALL,J. P., S. J. MCI-, ANDP. A. PRINCE.1984. Predation appearsto be a strongselective force The status and conservation of seabirds at the Falkland Islands, p. 271-29 1. In J. P. Croxall, P. for the Dolphin Gull, as evidenced by several G. H. Evans, and R. W. Schreiber [eds.], Status aspects of their breeding biology. Many of the and conservation of the world seabirds. ICBP behaviors Dolphin Gulls exhibit, such as chang- Technical Publication No 2. ing nest location, high nesting densities, reduced CULLEN,E. 1957. Adaptations in the to mobbing response,early abandonment of the nest cliff nesting. Ibis 99:75-302. CULLEN,E. 1960. Some adaptations in the nesting site and creching, are also present in Crested behaviours of terns. Proc. Int. Om. Congr. 12: Terns, Sterns bergii (Buckley and Buckley 1972, 153-l 57. Helsinki: Tilgmannin Kirjapaini Veen 1977, Langham and Hulsman 1986, Yorio DAVIS.L. S. 1982. Crechinabehaviour of Adelie Pen- and Quintana, unpubl.). The unusual aspectsof guin chicks(Pygoscelis~eliae). N.Z.J. Zool. 9:279- 286. the breeding biology of the Dolphin Gull appear DEVILLERS,P. 1977a. Comments on plumages and to be largely influenced by their dependency on behaviour of Scoresby’sGull. Gerhaut67:254265. scavenging and to some extent by predation. DEVILLERS,P. 1977b. Observationsat a breedingcol- DOLPHIN GULL BREEDING ECOLOGY 215

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