Audouin''s GullsLarus audouinii associate with sub-surface predators in the Mediterranean Sea

Two observations of Audouin's Gull flocks foraging on small fish forced to rise to the sea surface by tunas are presented. Observations were made just off-shore in the Ebro Delta, and Audouin's Gulls and terns concentrated in these flocks more than other seabirds breeding there, probably due to their higher ability for catching fish actively. The association of Audouin's Gulls with sub-surface predators may be a supplementary foraging behaviour for the species, especially during periods of short food supply.

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Zusammenfassung Vor der Küste des Ebrodeltas wurden zweimal Korallenmöwen bei der Jagd auf kleine Fische beobachtet, die durch jagende Tunfische an die Oberfläche getrieben worden waren. Korallenmöwen und Seeschwalben waren in den beiden beobachteten Trupps häufiger vertreten als andere in der Nähe brütende Seevögel. Die Assoziation der Korallenmöwen mit submersen Prädatoren könnte besonders in Perioden knappen Nahrungsangebots ein zusätzliche erfolgversprechende Form des Nahrungserwerbs darstellen.

     

MIGRATION AND DISPERSAL OF AUDOUIN'S GULL LARUS AUDOUINII FROM THE EBRO DELTA COLONY

Oro, D. & Martinez, A. 1994. Migration and dispersal of Audouin's Gull Larus audouinii from the Ebro Delta colony. Ostrich 65:225-230.

Migratory paths and dispersal patterns of Audouin's Gulls (Larus audouinii) born in the Ebro Delta have been studied, as well as their degree of philopatry. The gulls always disperse southwards from the colony, and only the juveniles seem to disperse in other directions. Dispersal patterns of 2y, 3y and older gulls are similar, wintering mainly on Iberian Mediterranean and Moroccan Atlantic coasts. Juveniles migrate before the rest of the age classes and they winter south of older gulls, in the Senegambia region. However, conclusions on Audouin's Gulls movements are difficult to draw, owing to the large differences in resighting and recovery efforts. Data presented also suggest that Audouin's Gulls do not appear to breed before the age of four years.     

Modelling Terrestrial and Marine Foraging Habitats in Breeding Audouin's Gulls Larus audouinii: Timing Matters

Although the breeding ecology of Audouin’s gull has been widely studied, its spatial distribution patterns have received little attention. We assessed the foraging movements of 36 GPS-tracked adult Audouin’s gulls breeding at the Ebro Delta (NW Mediterranean), coinciding with the incubation period (May 2011). This also coincided with a trawling moratorium northwards from the colony. We modelled the distribution of the gulls by combining these tracking data with environmental variables (including fishing activities from Vessel Monitoring System, VMS), using Maxent. The modelling range included both marine and terrestrial areas. Models were produced separately for every 2h time interval across the day, and for 2 fishing activity scenarios (workdays vs. weekends), allowing to assess the spatio-temporal distribution patterns of the gulls and the degree of association with fisheries. During workdays, gull distribution at sea fully matched with fishing activities, both trawling (daylight) and purse-seining (nightime). Gulls tended to avoid the area under trawling moratorium, confirming the high influence of fisheries on the distribution patterns of this species. On weekends, gulls made lesser use of the sea and tended to increase the use of rice fields. Overall, Audouin’s gull activity was more intense during dailight hours, although birds also showed nocturnal activity, on both workdays and weekends. Nocturnal patterns at sea were more disperse during the latter, probably because these gulls are able to capture small pelagic fish at night in natural conditions, but tend to congregate around purse-seiners (which would enhance their foraging efficiency) in workdays. These results provide important insight for the management of this species. This is of particular relevance under the current scenario of European fisheries policies, since new regulations are aimed at eliminating discards, and this would likely influence Audouin’s gull populations.     

Habitat utilisation, feeding tactics and age related feeding efficiency in the Caspian Gull Larus cachinnans

The feeding behaviour of the Caspian Gull Larus cachinnans was analysed in southern Poland in 2001. During the pre-breeding period, most birds foraged on a refuse dump and some foraged in a river valley. During incubation, similar numbers of birds foraged on fishponds, gravel pits and the refuse dump. During the chick-rearing period, fishponds were the most important foraging grounds. The foraging success of three main foraging tactics was analysed: digging on refuse, fishing and kleptoparasitism. We found that digging success was higher in juveniles than in immature or adult birds. However, older birds moved and ate more items per unit of time than juveniles, which indicates that older birds improved their energy gain simply by a higher speed of searching. The opposite was found for fishing success. As juvenile birds made fewer attempts than immature or adult birds, fishing success was higher in adults. Adults and immature birds interrupted more attacks than juveniles, which indicates that older birds were better able to assess the probability of fish catching than juveniles. Kleptoparasitism was observed almost exclusively on the refuse dump during the pre-breeding period. Young birds kleptoparasitised more frequently than adults, but they had a lower rate of success. However, the lower success in young birds was due to victim choice, rather than differences in flight skills. Young birds kleptoparasitised Black-headed Gulls Larus ridibundus and Jackdaws Corvus monedula more frequently than adults, but none of the attacks towards these species was successful. Generally, Caspian Gulls kleptoparasitised conspecifics more often than expected from species frequency. Only attacks towards conspecifics yielded any success.     

Effects of a trawling moratorium on the breeding success of the Yellow-legged Gull Larus cachinnans

The establishment of a two month commercial fishing moratorium, which overlapped each year with different stages of the breeding cycle of the Yellow-legged Gull Larus cachinnans at the Ebro Delta, Spain, provided the opportunity of testing the effects of food limitation on the breeding success of this species. The study was carried out in 1993, when the chick rearing stage occurred with normal commercial fishing activity and in 1994, when the chick rearing stage overlapped with the trawling moratorium. We recorded diet of the chicks, breeding phenology, clutch size, egg volume, hatching success and productivity of adult birds. There was a highly significant difference in the diet of the chicks between the two years and significantly lower productivity in 1994, whilst the other parameters examined did not change. These results show that a dependence on discards from commercial fishing activities may be a limiting factor in the breeding success of this seabird.     

Genetic and morphological differentiation between the two largest breeding colonies of Audouin's Gull Larus audouinii

We assessed the genetic and morphological differences between the two largest breeding colonies of Audouin's Gull Larus audouinii , an endemic seabird species of the Mediterranean region. The two colonies comprise c. 75% of the total world population and are 655 km apart. The Ebro Delta colony was formed recently and, after dramatic growth mainly due to high rates of both immigration and reproductive success, is now the largest in the world (more than 60% of the total population). The Chafarinas Islands support an ancient colony with relatively little fluctuation in breeding numbers. The two colonies also differ greatly in environmental conditions, with the Ebro Delta being a higher quality breeding site. Very little movement occurs between the two colonies. We collected morphological data and blood samples from both colonies. Polymorphic microsatellite markers were used to study the genetic differentiation. These showed no significant variation between colonies, nor evidence of a founder effect in the Ebro Delta. Individuals from the Ebro Delta were larger than those from Chafarinas, the difference being greater for males. This probably reflects a stronger male susceptibility to worse environmental conditions during chick growth at the Chafarinas Islands.     

KLEPTOPARASITISM OF SANDWICH TERNS STERNA SANDVICENSIS BY BLACK-HEADED GULLS LARUS RIDIBUNDUS

The parasitic behaviour of Black-headed Gulls in a mixed colony of terns and gulls at the Sands of Forvie on the northeast coast of Scotland is described in some detail. Food-stealing occurred with varying frequency throughout the breeding season. Less than 6% of the terns were attacked during incubation and when their chicks were a few days old, but up to 29% were attacked thereafter. At the same time the percentage of successful attacks rose from 1% or less to 6.5%.
Food items brought back to the ternery were predominantly sandeels, clupeids and gadoids. Only a small proportion of terns carrying fish shorter than 7 cm were attacked whether they were sandeels, clupeids or gadoids.
Robbing success was higher with clupeids and gadoids than with sandeels.
The effect of the Black-headed Gulls' kleptoparasitism on the Sandwich Terns' breeding success is thought to have been negligible during incubation and early chick-life, but might have influenced fledging weight and ultimately post-fledging survival.     

Influence of food availability on demography and local population dynamics in a long-lived seabird

Few studies have addressed the effects of food availability as a proximate factor affecting local adult survival in long-lived organisms and their consequences at local population dynamics. We used capture-recapture analysis of resightings of 10 birth cohorts of ringed Audouin's gulls, Larus audouinii, to estimate adult survival and dispersal (both emigration and immigration). For the first time, permanent emigration (the transient effect in capture-recapture analysis) was modelled for the whole population and not only for the newly marked birds. Gulls exploit to a large extent fishes discarded from trawlers, and a trawling moratorium established since 1991 has decreased food supply for the colony. This was used as a natural experiment of food availability to assess its effects on adult survival and emigration. These and other demographic parameters were used in a projection modelling to assess the probabilities of extinction of the colony under two scenarios of lower and higher food availability. Food availability (together with the age of individuals) influenced emigration probabilities, but not adult survival, which was estimated at 0.91 (s.e. = 0.02). When food was in shorter supply during the chick-rearing period, emigration was very high (ca. 65%) for younger breeders, although this rate decreased sharply with age. Probabilities of extinction were very high when food availability was low, and when environmental stochasticity was introduced, and only stochastic immigration from the outside seemed to prevent extinction. The results highlight the importance of dispersal processes in the population dynamics of long-lived organisms.     

Determinants of local recruitment in a growing colony of Audouin's gull

1. Local recruitment of Audouin's gull ( Larus audouinii Payraudeau) was studied between 1988 and 1997 at the Ebro Delta colony (north-western Mediterranean). Since its establishment in 1981, the colony has dramatically grown to include, in 1997, 65% of the total world population. Several hypotheses were tested, involving the effects of a badger predatory event in 1994, and sex, age and cohort (year of birth) on recruitment.
2. Results supported the prediction that colony size influenced recruitment: the probability for any individual to have previously bred increased throughout the study, together with colony size. At the end of the study, 90% of breeders aged 4 years had already been recruited at age 3, the age of first reproduction by Audouin's gulls. As expected by the dramatic increase of breeding numbers, most local recruitment occurred at very young ages, especially when compared with other Laridae.
3. Neither food availability nor reproductive success affected recruitment. Recruitment was not affected by high nest predation by the badger, although after the event, the proportion of Ebro Delta birds nesting on the nearby Columbretes Islands tripled.
4. Probability of first reproduction depended on age: it was the highest at ages 3 and 4, and then decreased sharply with age to stabilize beyond age 6 to a value depending on the year and cohort but always very low (< 5%). Cohort and sex did not influence local recruitment.
5. Annual resighting rates ranged between 35% and 82%, and were higher for females. This may represent a sex-dependent suspension of breeding, probably as a trade-off between early recruitment and future survival.     

A comparison of Herring Gull Larus argentatus and Lesser Black-backed Gull Larus fuscus nest sites: their characteristics and relationships with breeding success

Nest sites used by Herring Gulls and Lesser Black-backed Gulls in a mixed colony were compared. Correlations between features surrounding a nest and breeding success at different stages of the breeding cycle are presented. Lesser Black-backed Gulls nested on more vegetated and flatter areas than did Herring Gulls, even though the latter species had a lower hatching success at less vegetated sites. The difference in the general topography of nest sites between the species suggests that the Lesser Black-backed Gull, through an individual defence strategy, may be better adapted to use sites which are accessible to ground predators than the Herring Gull. The fledging success of Lesser Black-backed Gulls tended to increase with an increased proportion of relatively tall vegetation close to the nest. The Lesser Black-backed Gull may therefore be able to leave its young unattended and rely on their concealment for protection against predators. In contrast, the Herring Gull may rely more on parental vigilance to protect young. More frequent attendance by adult Herring Gulls at the nest site during chick rearing compared with Lesser Black-backed Gulls supports this hypothesis.     

Predation by Kelp Gulls Larus dominicanus at a mixed-species colony of Royal Terns Sterna maxima and Cayenne Terns Sterna eurygnatha in Patagonia

We studied the predator-prey interaction between Kelp Gulls Larus dominicanus and Royal Terns Sterna maxima and Cayenne Terns Sterna eurygnatha at Punta León, Argentina, from 1990 to 1992. Kelp Gulls were the main predator of tern eggs in all 3 years, accounting for 99% of observed predations (n = 454). A total of 99% of predations occurred on peripheral tern nests. Total predation rate on both tern species was highest in 1990 and varied significantly between years, with between 0.01 and 0.03 cases of predation per hour. Predation rates in all years were significantly higher on Cayenne than on Royal Terns probably because the latter were significantly more aggressive towards gulls than were Cayenne Terns. Royal and Cayenne Terns displayed aggressively towards gulls at least once in 64% and 10% of the observation periods, respectively. Defence behaviour, such as attacking and chasing the predator, was used more frequently by Royal than Cayenne Terns (13.2% v 3.6%). Predation on tern eggs was restricted to only a few Kelp Gulls which nested close to the tern colony periphery, and 50% of the eggs that were stolen were taken by only two pairs during 1991 and 47% by three pairs in 1992. This suggests that the current expansion of the Kelp Gull numbers at Punta León might not necessarily result in a proportional increase in predation rate on both tern species. Stealing eggs from the ground was more successful than from the air (77% v 17%). The nesting strategy shown by Royal and Cayenne Terns at Punta León appears to be an important factor which reduced the impact on their breeding success of high potential predation pressure by gulls.     

Food availability and nest predation influence life history traits in Audouin's gull, Larus audouinii

The effects of food availability and nest predation on several life history traits such as adult survival, dispersal, and reproductive performance were assessed in an Audouin's gull (Larus audouinii) colony during the period 1992–1997. The amounts of fish discarded from trawlers were used as a measure of food availability, and a trawling moratorium which partially overlapped with the breeding season of the gulls was taken into account. The effects of nest predation were assessed in 1994, when a terrestrial predator entered the colony and remained for the whole breeding season preying on both eggs and chicks. Using the moratorium and the predatory event as natural experiments, several hypotheses were tested: (a) food supply would affect breeding performance but not adult survival (independently of age and sex), since gulls are long-lived and adult survival is the most sensitive demographic parameter in their population dynamics; (b) the predator would trigger breeding dispersal (although gulls are mostly philopatric, they are known to abandon their natal colony after breeding failure instigated by events such as this). If breeding dispersal occurs, the rate is expected to be higher in females than in males, and higher in new breeders than in more experienced breeding birds, as is usually recorded in colonial seabirds. Probabilities of resighting and survival were estimated separately, using capture-recapture models. As expected, changes in food availability did not affect adult survival, whereas they influenced egg volume, clutch size, and breeding success. Local adult survival was estimated to be 0.908 (SD = 0.007) for males and females, and it did not change significantly with the age of individuals (range 3–8 years). The predator significantly decreased breeding success, and caused the dispersal of a number of adults probably to breed in another colony; this rate was estimated at an average of 0.10 (SD = 0.02). As expected, inexperienced breeders dispersed significantly more (14%) than more experienced breeders (8%) after the predator event, but dispersal was not sex biased. Recapture probabilities after the predator event suggest that birds that left the colony still had not returned. Results confirm that population dynamics of ground-nesting seabirds are sensitive to terrestrial predation, even when predation caused only a partial breeding failure. Received: 16 July 1998 / Accepted: 16 November 1998     

How costly is clutch formation in the Audouin's Gull Larus audouinii?

During the Audouin's Gull's breeding season at the Ebro Delta in 1993, 24 fresh eggs from eight three-egg clutches (modal clutch-size) were collected at the peak of the laying period. Eggs were processed to obtain formalin-fixed yolks, which were halved and stained using the potassium dichromate method. Digitized images of the yolks were examined to assess the daily rates of yolk deposition. We used these data in combination with egg compositional analysis to build a model of energy demands during the formation of an average clutch in Audouin's Gull. To show how the different parameters of clutch formation affect the daily energy investment peak, we performed a simulation analysis in which the rapid yolk development (RYD) period, the follicle triggering interval (FTI), the laying interval (LI) and the albumen synthesis period (ASP) were allowed to vary simultaneously. In our sample, the mean RYD period was seven days with a range from six to eight days. There were no significant differences in yolk volume among eggs in a clutch, but albumen volume was significantly smaller in third eggs. According to our model the albumen synthesis of the a-egg coincides with the energy demand peak for clutch formation. This peak represents an increase by ca. 42% in female energy requirements. Values obtained from the simulation analysis showed that only the ASP of the a-egg and the RYD durations of the second and third follicles produced noticeable reductions in peak energy investment. We predict that in gulls, whose laying intervals seem to be kept constant, significant increases of the durations of the RYD periods of second and third eggs, or even significant reductions of yolk size of these eggs, may operate simultaneously to match the energy demands during clutch formation to the prevailing food conditions.     

FOOD SIZE AND AGGRESSIVE INTERACTIONS BETWEEN TWO SPECIES OF GULLS: AN EXPERIMENTAL APPROACH TO RESOURCE PARTITIONING

Duffy, D. C., Heseltine, S. & La Cock, G. D. 1987. Food size and aggressive interactions between two species of gulls: an experimental approach to resource partitioning. Ostrich 58: 164–167.

In feeding experiments with different sizes offish, Hartlaub's Gulls Larus hartlaubi reached prey first, but lost larger items to the larger, dominant Kelp Gull L. dominicanus. Hartlaub's Gulls took longer than Kelp Gulls to consume prey of the same size.     

Plenty of rooftops with few neighbours occupied by young breeding Yellow-legged Gulls (Larus michahellis): Does this occur at the expense of their health condition?

Gulls, as largely flexible opportunistic individuals, have been increasingly breeding in many cities around the world, but it is still unclear whether urban habitats are of equal or higher quality than traditional natural habitats or represent an ecological trap with immediate reproductive benefits but longer-term detrimental consequences to health. Here we present a study of breeding parameters (nest density, egg dimensions, clutch size, hatching success and adult body condition) and physiological parameters (erythrocyte sedimentation rate, heterophil/lymphocyte ratio, haemoglobin concentration and measurements of oxidative stress) as indicators of the general health condition of Yellow-legged Gull Larus michahellis adults and chicks from natural and urban colonies. Yellow-legged Gulls in the largest urban area (Porto) laid smaller eggs and clutches, showed a significantly lower occurrence of inflammatory processes in chicks, and showed a slower early chick growth than in the natural colony of Deserta. This suggests that urban gulls might be facing important trade-offs between the advantages of breeding in lower density urban colonies, with fewer intraspecific interactions and a lower disease transmission probability, and the disadvantages of having an anthropogenic diet usually lower in nutritional value.     

Trophic niche width and overlap of two sympatric gulls in the southwestern mediterranean

The diets of two potential competitor species, Audouin's Larus audouinii and yellow-legged gulls Larus cachinnans, were examined while they bred at the Chafarinas Islands during 1993, 1994, and 1995. Data were collected during two commercial fishing regimes: (1) trawling and purse seine fisheries, and (2) diurnal trawlers only. Since the food supply for the gulls in this area was heavily reliant on the activity of purse seine fisheries, these contrasting situations allowed us to analyze short-term effects, induced by daily changes in food supply, on niche width, dietary shift, and niche overlap between the two species. Overall, both species relied mainly on fish for food, especially Clupeiforms, in the case of Audouin's gull irrespective of the fishing situation, and in the case of the yellow-legged gull, only when purse seine fishing was in operation. When purse seine boats did not operate (food shortage), yellow-legged gulls broadened their niche, consuming equal amounts of all the feeding resources, and they showed a dietary shift toward a greater consumption of prey from refuse tips. In contrast, Audouin's gulls did not change their niche width, but showed a slight dietary shift away from the consumption of epipelagic fish, compensated by an increase in reliance on benthic-mesopelagic resources. Niche overlap was clearly higher on days when both fishing fleets operated, probably because a superabundant food resource facilitates high overlap without affecting coexistence between the two species. Since our study was developed on the basis of daily variations in food supply, and competition effects are to be expected on a longer-term basis, these changes can be seen as the outcome of the coexistence of two species in stable competitive equilibrium. Received: 4 October 1996 / Accepted: 20 May 1997     

BREEDING SUCCESS AND POPULATION GROWTH IN A COLONY OF HERRING AND LESSER BLACK-BACKED GULLS LARUS ARGENTATUS AND L. FUSCUS

This paper describes the results of investigations into the factors affecting breeding success of the Herring and Lesser Black-backed Gulls Larus argentatus and L. fuscus, in the large colony on Walney Island, northwest Lancashire, between 1962 and 1965. These investigations were concerned with the incubation period, and the first ten days after hatching. The survival of chicks to ten days is 67% in Herring Gulls, and 56% in Lesser Black-backs. Most of these losses occur in the period just after hatching and are due to “cannibalism” by other gulls. This form of predation does not appear to be masking any effects from starvation or disease. The following factors contribute to egg or chick mortality:breeding too late (and, to a much smaller extent, too early); breeding in the open, as opposed to amongst cover; the facts that eggs in small clutches have a lower hatching success than those in large ones and that Herring Gull (but not Lesser Black-back) chicks in small broods are less likely to survive to ten days than are those in large broods. Chick mortality after the first ten days is not certainly known. About 30% of the eggs laid gave rise to fledged young— or about one fledged chick per pair. In the Herring Gull, the average clutch size (2.56) is lower than that of the Lesser Black-back (2.76). Both species show a seasonal decline in clutch size—this occurs earlier in the Herring Gulls than in the Lesser Black-backs. The Walney population, which stood at about 700 pairs in 1950, had reached 12,000 in 1957, and is at present about 18–19,000 pairs. It is suggested that this increase may be linked to the greater availability, or exploitation, of human garbage in the Morecambe Bay area. The population explosion between 1950 and 1957 must have been partly due to massive immigration and could not have come about through natural increase alone. The possible influences of the gulls' behaviour on the population growth are discussed. There is no evidence of any “shock disease”, although the Walney colony is very crowded. “Cannibalism” is regarded, not as evidence of a failing food supply, but as an extension of the normal hunting behaviour of these omnivorous gulls; it will be an economical means of obtaining food only in a large, dense colony, such as Walney. It may be offset by increased breeding efficiency due to social factors.     

Disturbance-dependent Yellow-legged Gull (Larus michahellis) predation on Larid chicks decreases with chick age

Predation is one of the key factors shaping the dynamics of animal populations. In birds, nest loss due to predation can be a significant cause of low reproductive success. Ground-nesting birds are among the bird groups most susceptible to predation, mainly because their nests are easily accessible to a broad suite of potential predators. For these birds, anthropogenic disturbances can generate changes in nest predation risk by altering their antipredator behaviour and also by altering the behaviour of the predator species, i.e. the predator becoming much more aware of predation opportunities due to frequent disturbances and/or motivated to repeat predation attempts when some are successful. To date, most previous studies investigating this have focused on a single effect, either predation or disturbance, on chick survival. It remains unknown how the risk of predation with and without disturbance varies with chick age. In this study, we used behavioural observations to assess how the interaction between predators and disturbance affects predation risk in chicks and how this interacts with chick age. Specifically, we investigated the effect of disturbance caused by humans and stray dogs on the predation of Slender-billed Gull Chroicocephalus genei chicks by Yellow-legged Gulls Larus michahellis, and whether this depended on the age of the chicks. Our results revealed that disturbance had a significant positive effect on predation measures of Slender-billed Gull chicks by Yellow-legged Gulls, but that this effect was mediated both by disturbance type and the age of chicks. Stray dogs entering the colony had a stronger disturbance effect on chicks than passing humans, increasing predation risk by Yellow-legged Gulls. Our results also showed that chick age interacts with disturbance type to determine the predation risk. This is probably mediated by chicks' capacity to escape predation by gathering in a single large crèche that runs into the water when disturbed. To preserve Slender-billed Gull colonies in one of its few remaining breeding sites in Tunisia, and as gulls tend to react even when the disturbance occurs relatively far from the colonies, it is crucial to (1) restrict human access to dikes and islets where large colonies breed and (2) construct artificial islets attractive to gulls and inaccessible to stray dogs.     

Habitat selection and breeding success in Yellow-legged Gulls Larus cachinnans

The habitat selection and breeding performance of Yellow-legged Gulls Larus cachinnans were studied in the Medes Islands colony, northeastern Spain, during 1995 and 1996. Of the three main habitats on the islands (shrubs, grass and bare areas), gulls first occupied those with the highest percentage of tall vegetation. Gulls tended to select nest sites with 20–75% cover despite great differences in the cover in the habitats and territories, suggesting that the presence of a suitable nest site may play a major role in the choice of breeding habitat. Nest-site tenacity did not influence the preferences of gulls at any level since the same pattern of choice was observed in an area subjected to annual culls (i.e. where most of the breeding pairs were culled annually and replaced by naive birds). In spite of great differences in the physical characteristics of the habitats, little difference was found in breeding performance of the gulls between habitats. Gulls nesting in the least preferred habitat (i.e. mainly bare) had smaller clutches than those nesting in the other two habitats, possibly as a result of their later seasonal laying. Despite the similar breeding success in different habitats, gulls did not seem to distribute according to the ideal free model reported for Herring Gulls Larus argentatus since the density in the preferred habitat (i.e. shrubs) was never higher than in the other two. We suggest that the habitat selection by Yellow-legged Gulls within the colony could follow an ideal despotic distribution.