Yellow-legged Gull Larus Cachinnans Research Articles

Heritability of resistance to oxidative stress in early life

Oxidative stress has recently been suggested to play an important role in life-history evolution, but little is known about natural variation and heritability of this physiological trait. Here, we explore phenotypic variation in resistance to oxidative stress of cross-fostered yellow-legged gull (Larus cachinnans) chicks. Resistance to oxidative stress was not related to plasma antioxidants at hatching, which are mostly derived from maternal investment into eggs. Common environmental effects on phenotypic variation in resistance to oxidative stress were not significant. Heritability was relatively low and nonsignificant in hatchlings, but interestingly, the chicks of age 8 days showed high and significant heritability (h(2) = 0.59). Our results suggest that resistance to oxidative stress is determined mainly by the genotype as chicks grow. Further work is required to explore the genetic role of oxidative stress in life-history evolution.

Antimicrobial resistance and phylogenetic groups in isolates of Escherichia coli from seagulls at the Berlengas nature reserve

Fifty-three faecal samples from yellow-legged gulls (Larus cachinnans) at the Berlengas nature reserve in Portugal were cultured on Levine agar plates not supplemented with antimicrobial agents, and one Escherichia coli...

The Feeding Behavior of the Black Kite (Milvus migrans) in the Rubbish Dump of Rome

We studied the feeding behavior of Black Kites (Milvus migrans) in a rubbish dump in Rome, Italy, from April–September 2005. The earliest kites reached the rubbish dump at dawn and the last left just after dusk. The number of individuals foraging in the dump increased during the course of the day and also from April to August. The number of kites in the rubbish area of the dump was usually small and tended to show rapid increases and equally quick declines. Kites searched for food while directly standing on the rubbish or, more often, while flying low over the ground. The first strategy, which was never observed in April but was more common during the following months, was used when there were few foraging gulls or trucks in the dump. Cleptoparasitism was the technique most frequently used to acquire food (76% of all foraging events). It was directed toward conspecifics in 35% of the observed cases, toward Yellow-legged Gulls (Larus cachinnans) in 57% and toward Carrion Crows (Corvus corone) in 8%. The success rate of cleptoparasitic attempts was 32% against conspecifics, 73% against Yellow-legged Gulls and 66% against crows. The higher frequency of cleptoparasitic attempts against gulls may thus be explained by its higher success rate, although its efficiency declined with increasing gull numbers. The frequency of intraspecific cleptoparasitism paralleled variations in kite density and its effectiveness increased progressively from April–August.

Evidence of a seamount effect on aggregating visitors

It has been suggested that seamounts hold higher abundances of some 'visiting' ani- mals, such as tuna, sharks, billfishes, marine mammals, sea-turtles and even seabirds, but this has been based on sparse records, and warrants further examination. In this paper we use data from a fishery observer program to examine whether the predicted higher abundances of tuna, marine mammals, sea turtles and seabirds actually occur around Azores seamounts and to map the distribu- tion of the various species. Our results indicate that some marine predators (skipjack Katsuwonus pelamis and bigeye tuna Thunnus obesus, common dolphin Delphinus delphis and Cory's shearwater Calonectris diomedea borealis) were significantly more abundant in the vicinity of some shallow- water seamount summits. Our methodology, however, failed to demonstrate a seamount association for bottlenose dolphins Tursiops truncatus, spotted dolphin Stenella frontalis, sperm whale Physeter macrocephalus, terns Sterna hirundo and S. dougalli, yellow-legged gull Larus cachinnans atlantis and loggerhead sea turtles Caretta caretta. Seamounts may act as feeding stations for some of these visitors. Not all seamounts, however, seemed to be equally important for these associations. Only seamounts shallower than 400 m depth showed significant aggregation effects. These seamounts may be considered hotspots of marine life in the Azores, and a special effort should be made in order to ensure a sustainable management of these habitats.

Sjeldne fugler i Norge i 2006 - Rapport fra Norsk sjeldenhetskomité for fugl (NSKF)

This is the 31st annual report from the Norwegian Rarities Committee (NSKF). The majority of records treated here are from 2006, but some older records are also included. The Norwegian rarities committee follow advises given by the Taxonomic Advisory Committee (TAC) of the Association of European Record and Rarities Committees (AERC) on issues concerning taxonomy and nomenclature. The systematic list of accepted records is set out in the following system: (1) The bird’s Norwegian name, (2) scientific name, (3) the numbers in brackets, e.g. (2, 30/32, 1), refers respectively to the ( i ) total number of records/total number of individuals up to and including 1974, ( ii ) total number of records/total number of individuals from 1975 (the foundation year of NSKF), except ( iii ) records/individuals in 2006, (4) any breeding, (5) year, (6) county, (7) number of individuals, age (1K = first calendar year etc.), sex (if known) and sometimes circumstances («rm» means that the bird was ringed), (8) site and municipality, (9) date, (10) * indicates if the record was complemented by positive evidence, and type of the evidence (F = photo, V = video-recorded, L = tape-recorded, DNA = DNA-analysis, D = found dead), (11) observers’ names in brackets, main observer(s) is specified first and emphasized with *, (12) roughly gives the species’ distribution and (13) comments concerning the records. Species, subspecies and hybrids new to the Norwegian list: With 245 accepted records presented in this report, 2006 was another good year for rare birds in Norway. A total of 42 accepted records from previous years complete the status of rare birds on the Norwegian list. There are five records presented that have been shelved. Also presented are 26 records that for some reason have not been accepted. All previous records of Booted Warbler Hippolais caligata ssp. and Bonelli’s Warbler Phylloscopus bonelli ssp. have been reassessed following the recent split of these two species. The two formerly accepted records of Snow Bunting Plectrophenax nivalis of the subspecies vlasowae have also been reassessed based on new knowledge on the variation within the nominate subspecies. Three new species to the Norwegian category A list are presented in this report. This brings the official Norwegian list to 477 species. Included in this list are accepted records in category A-C. The new species in 2006 were Masked Shrike Lanius nubicus (Utsira, Rogaland 2nd to 15 th October) and Blackpoll Warbler Dendroica striata (Reve, Rogaland 2nd to 12th November). In addition the first two Sykes’s Warblers Hippolais rama (Forus, Rogaland 11th September 1983 and Monstermyr, Vest-Agder 20th September 1997) were accepted during the reassessment of older records. A Red-necked Grebe Podiceps grisegena collected at Traena, Nordland 28th January 1949 has been accepted as the first Norwegian record of the holboellii subspecies. Two records of hybrid Blue-winged Teal Anas discors x Northern Shoveler A. clypeata (Aheim, More og Romsdal 7th – 9th June 2003 and Lista fyr, Vest-Agder 31 st July – 5th August 2006) have also been accepted as the national first and second records. General trends: Record numbers for Norway was recorded for the following species during 2006: Great Bittern Botaurus stellaris (eight records), Black Kite Milvus migrans (nine records), Mediterranean Gull Larus melanocephalus (ten records), Citrine Wagtail Motacilla citreola (nine records) and Lesser Grey Shrike Lanius minor (three records). Also, 2006 saw the highest number (six records of eleven birds) for Corn Bunting in recent times, including the first successful breeding since 1939. Six records involving ten birds of Arctic Redpoll Carduelis hornemanni of the nominate subspecies was also an all time high. Also published here are the 4th & 5th Egyptian Goose Alopochen aegyptiaca for Norway, 4 th American Black Duck Anas rubripes , 3rd Piedbilled Grebe Podilymbus podiceps , 2nd Little Shearwater Puffinus [assimilis] baroli , 3rd Swinhoe’s Storm-petrel Oceanodroma monorhis , 5th Collared Pratincole Glareola pratincola , 4 th American Golden Plover Pluvialis dominica , 8th & 9th Bonaparte’s Gull Larus philadelphia , 4th, 8th & 9th Yellow-legged Gull Larus michahellis , 10th & 11th Caspian Gull Larus cachinnans , 7 th Gull-billed Tern Sterna nilotica , 4th Isabelline Wheatear Oenanthe isabellina , 2nd Eastern Orphean Warbler Sylvia crassirostris and 2 nd Black-faced Bunting Emberiza spodocephala (cat. D).

Age-dependent changes in plasma biochemistry of yellow-legged gulls ( Larus cachinnans)

The study of avian plasma chemistry is providing useful reference values for the management of endangered and game species, supporting veterinarians in their diagnostics, and also bringing to light relevant physiological adaptations during periods of food-shortage. Age is an important source of variability for plasma chemistry. Here I report plasma chemistry of yellow-legged gulls Larus cachinnans from different ages, between post-independence and adulthood, a 5-year interval. Increase in plasma cholesterol concentration and decreases in uric acid, inorganic phosphorus and alkaline phosphatase values were seen. Body mass corrected by body size (i.e. body condition) increased with age, plasma cholesterol being positively correlated in females, but not in males. Moreover, cholesterol was also positively correlated to gonad size in both sexes. Long-term developmental changes in this species, such as gonad development and the acquisition of an optimal body mass for reproduction, could explain these findings. Finally, inorganic phosphorus and alkaline phosphatase, both traditionally related to osteogenesis, were not associated to deferred skull ossification, as originally was suggested in other species.

Organochlorine contaminants in eggs of the yellow-legged gull (Larus cachinnans michahellis) in the North Eastern Mediterranean: is this gull a suitable biomonitor for the region?

Levels of eight PCB congeners and thirteen organochlorine pesticides were measured in eggs sampled at four yellow-legged gull colonies from the Aegean Sea (NorthEastern Mediterranean) in 1997. There were no significant differences among colony areas in the median concentrations in any of the pollutants whereas cluster analyses did not generally reveal reasonable pollution patterns. The maximum concentrations of four congeners were found at Kinaros colony and of nine compounds were found at Lipsos colony. Fingerprints in both groups were similar in all areas. Of PCBs, congener 28, 118, 138, 180 and of pesticides β-BHC and 2,4′-DDD were prominently dominant suggesting a particular pollution pattern in this region. Statistically significant correlations were found between most of the higher PCBs in all areas studied. The DDT metabolites correlated mostly with other OCs. We suggest that regional pollution by both groups is not adequately reflected in the eggs of this gull probably due to its extensive scavenging habits and, though information is needed from more colonies, it seems to be a poor biomonitor for organochlorines in this region.

Nesting seabirds in SE Spain: distribution, numbers and trends in the province of Almería

Little is known on the abundance of seabirds breeding in southeastern Spain. This study reports on the distribution, numbers and population trends of breeding colonies of this avian group in this region. Principally from 1996 to 2000, the coast of the province of Almeria was surveyed during the breeding season. In 2000, the last year of survey, we counted 1,731 seabird breeding pairs, corresponding to 8 species. The yellow-legged gull Larus cachinnans was the most abundant seabird with more than 50% of the total, followed by Audouin´s gull Larus audouinii , the little tern Sterna albifrons , the black-headed gull Larus ridibundus and the common tern Sterna hirundo . Cory´s shearwater Calonectris diomedea , the shag Phalacrocorax aristotelis and the European storm petrel Hydrobates pelagicus bred in small numbers. The black-headed Gull, yellow-legged gull and common tern showed increasing trends over the last decade. The most important breeding sites were Alboran Island and the sea cliffs of Cabo de Gata, La Higuera and Cabrera. A main factor influencing increasing population trends was the lack of human disturbance at legally protected breeding sites.

Geographical patterns of seabird attendance to a research trawler along the Iberian Mediterranean coast

SUMMARY: The attendance of seabirds to a research trawler along the Mediterranean coast of the Iberian Peninsula was monitored during six demersal surveys (MEDITS project). Cruises were performed in late spring during the period 19941999. Twenty seabird species were recorded, including both breeders and migrants. The four most common species behind the boat were local breeders, namely the Balearic shearwater Puffinus mauretanicus, Cory’s shearwater Calonectris diomedea, Audouin’s gull Larus audouinii and the yellow-legged gull Larus cachinnans. Most seabirds made use of trawler discards to some extent, with the exception of the Puffin Fratercula arctica. Geographically consistent patterns were identified in the six years surveyed. Seabirds were most abundant along the eastern coast of Iberia, especially off the Ebro Delta and around the Columbretes Islands, coinciding with one of the major areas of primary productivity and one of the largest trawling fleets in the western Mediterranean. The location of the main seabird colonies also influenced the distribution of breeders.

Predation on common tern eggs by the yellow-legged gull at the Ebro Delta

The Ebro Delta holds a large seabird community, including a common tern ( Sterna hirundo ) local population of 3,085 pairs in 2000 which breeds scattered in several colonies. At El Canalot colony, 1,178 (1999) and 1,156 pairs (2000) of this species bred distributed in 32 and 38 sub-colonies respectively. These sub-colonies varied in size from 1 to 223 pairs and were placed near the main breeding colonies of yellow-legged gulls (Larus cachinnans) and Audouin´s gulls (L. audouinii), which are potential egg-predators of terns. We studied egg predation during 1999 (6 sub-colonies) and 2000 (27 sub-colonies). Overall, we found that 10.6% of the nests in 1999 and 16.7% in 2000 suffered partial or total egg predation, being total in 81.1% of the predatory events. Predation was significantly higher in small sub-colonies (< 11 pairs): 49.4% in 1999 and 75.5% in 2000. Only attacks from yellow-legged gulls were observed, and defence behaviour of terns was significantly more frequent against this gull species (40.5 hours of observation), suggesting that in most cases the egg predation recorded was due to this species. Probability of egg predation was significantly and negatively correlated with distance to the nearest yellow-legged gull sub-colony, although this relationship was no more significant after adjustment for sub-colony size. On the other hand, distance to the nearest Audouin´s gull sub-colony did not show any effect. Our results suggest that the impact of large gulls (at least yellow-legged gulls) upon smaller seabirds breeding in the area might be important, especially when they are breeding in small sub-colonies. Further studies are needed to analyse the general impact of large gulls upon the breeding populations of other colonial bird species in the area.

Attendance of scavenging seabirds at trawler discards off Galicia, Spain

The occurrence of scavenger seabirds at fishing trawlers was studied off Galicia, Spain. A total of 9,368 seabirds of 23 species were recorded during 92 fishing operations in 1998 and 1999. The most common species were the yellow-legged and lesser black-backed gull ( Larus cachinnans and L. fuscus ), Sabine´s gull ( L. sabini ), the northern gannet ( Morus bassanus ), the great shearwater ( Puffinus gravis ), sooty shearwater (P. griseus), the Manx and Balearic shearwater ( P. puffinus and P. mauretanicus ), the great skua ( Catharacta skua ) and terns (mainly Sterna hirundo and S. paradisaea ). Other species occurred in small numbers: Leach´s petrel ( Oceanodroma leucorhoa ), the storm petrel ( Hydrobates pelagicus ), the little shearwater ( Puffinus assimilis ), Cory´s shearwater ( Calonectris diomedea ), the parasitic skua ( Stercorarius parasiticus ), the pomarine skua ( S. pomarinus ), the black-headed gull ( Larus ridibundus ), the glaucous gull ( L. hyperboreus ), the kittiwake ( Rissa tridactyla ), the sandwich tern (Sterna sandvicensis), the black tern (Chlidonias niger), the guillemot ( Uria aalge ) and the little auk ( Alle alle ). The maximum number of seabirds recorded at a haul was 320. The maximum number of a particular species ranged from 120 great shearwaters to 250 yellow-legged/lesser black-backed gulls during a single haul. The differences in ship-follower species abundance are related to migratory movements but fisheries could also have a strong influence at a smaller scale on the distribution of seabirds off Galicia. The degree to which seabirds rely on fishery discards as food was not quantified, but may be important for several species.

Effect of refuge supplement on Audouin's gull chick survival

One of the main threats to Audouin´s gull ( Larus audouinii ) at Chafarinas Islands is predation on eggs and chicks by the yellow-legged gull ( Larus cachinnans ). During the 1999 and 2000 breeding seasons we tested the effects of supplementary refuge availability on chick survival. Well before the laying period, nine adjacent enclosures were erected inside Audouin´s gull´s breeding grounds and some of them were supplied with artificial refuges. Our results showed significantly greater chick survival in the enclosures with artificial refuges than in control ones. In 2000, an overall lower breeding success of Audouin´s gull and a dilution of the refuge effect were observed. These results were probably related to food-shortage that year. We suggest that, along with other factors, an increase in refuge availability may significantly enhance Audouin´s gull chick survival.

The incidental capture of seabirds by Spanish drifting longline fisheries in the western Mediterranean Sea

The western Mediterranean Sea is an important fishing area for the Spanish drifting longline fleet, targeting swordfish ( Xiphias gladius ), bluefin tuna ( Thunnus thynnus ) and albacore ( T. alalunga ). Some seabird species can be captured incidentally as bycatch marine fauna. From July 1999 to August 2000 information on 554 surface longline sets was collected by onboard observers of the Spanish Oceanographic Institute (IEO) on 18 longline fishing boats working within the Spanish Mediterranean fishing area. Eleven yellow-legged gulls ( Larus cachinnans ), 9 Cory´s shearwaters ( Calonectris diomedea ) and 1 Northern gannet ( Morus bassanus ) were captured incidentally. Bycatch rates ranged from 0.002 to 0.023 birds/1000 hooks.

Colonial seabirds change beetle assemblages on a Mediterranean island

Island communities are very sensitive to disturbances. Such disturbances include, in particular, pressure from colonial seabird colonies. Seabirds are a major ecological factor driving both species assemblage patterns and community dynamics of plants on islands. However, very little is known about the response of insect assemblages to seabirds, despite the important ecological and functional role of insects within all ecosystems. Here we aimed to assess the response of beetle communities to the influence of a Yellow-legged gull (Larus cachinnans) colony on a small Mediterranean island. The study was conducted on Bagaud island, which is a fully protected reserve within Port-Cros National Park (France), where the yellow-legged gulls are the most densely concentrated. The experiment was designed on a 12-plot transect extending from a low gull-induceddisturbance area to the highly disturbed centre of the colony. Beetle communities were sampled three times during spring and autumn 2001. Beetle patterns across the gradient were analyzed with respect to species ordination, species richness, abundance, and biomass. A total of 255 beetle specimens from 28 species and 15 families were collected. Surprisingly, species richness and the abundance of beetles were not significantly affected by gulls. However, ordination of beetle assemblages showed that there was a marked change in species composition along the gradient of gull influence. The gull colony also affected distribution of feeding-types of beetles. Tenebrionidae species, in particular, seemed to draw strong benefit from such disturbance. Tenebrionidae species, which are polyphagous, markedly increased at the expense of phytophagous species, which decreased. Collected species were likely to be found in other, gull-free environments and were not strictly endemic to Mediterranean islands. However, some species did present particular conservation interests.

Can rats prey on gull eggs? An experimental approach

Black rat Rattus rattus populations can reach high densities on the Mediterranean islands, as has been the case on the Chafarinas Islands (Western Mediterranean coast) in the last decade. This archipelago holds the second largest breeding population of Audouin's gull Larus audouinii and an important population of yellow-legged gull Larus cachinnans. Circumstantial evidences of rat predation upon Audouin's gull eggs led to conservation concerns about the impact of rat predation on gull productivity. We studied the impact of rats on gull nests experimentally by building artificial nests into the breeding area of yellow-legged gulls. Two kinds of eggs were placed in the artificial nests: manipulated eggs, in which the shell had been previously opened, and intact (unmanipulated) eggs. Rats preyed extensively on opened eggs but only once on intact eggs, apparently due to lack of skills to prey on intact eggs, suggesting that rats do not normally have access to fresh gull eggs. Rats inhabiting breeding areas of gulls can scavenge on broken eggs rolled out of the nest or on eggs with softened shells after nest desertion, but it is unlikely that they prey on natural gull nests guarded by parents. We suggest that introduced black rats on Mediterranean islands can only marginally affect the productivity of gulls owing to egg predation.

Phylogeography and colonization history of Lesser Black-backed Gulls (Larus fuscus) as revealed by mtDNA sequences

Abstract Because of the differential amplitude of climatic oscillations, species living at northern latitudes are subject to more frequent and more severe range oscillations than species at southern latitudes. As a consequence, northern populations should, on average, be phylogenetically younger and possess less phylogeographical structure than closely related taxa further south. To test these predictions, we studied the mitochondrial-genetic population structure of NW Palearctic Lesser Black-backed Gulls (Larus fuscus group [=LBBG], five taxa) breeding at temperate to boreal latitudes from Iceland to the Taimyr Peninsula. Results were compared with those previously obtained (Liebers et al. 2001. Mol. Ecol.10: 2447) for more southerly breeding Yellow-legged Gulls (Larus cachinnans group, six taxa from the Atlantic Islands to Mongolia). Sequences of the hypervariable region I (HVR-I) of the mitochondrial control region revealed low within- and between-taxon sequence divergence, little genetic variation, a shallow haplotype phylogeny and poor phylogeographical structure in LBBGs compared with Yellow-legged Gulls. Haplotype frequencies among the five northern taxa formed a stepped cline with significant gene flow restriction between the forms heuglini and fuscus, probably indicating a secondary contact with (partial?) reproductive isolation. Western forms of LBBG, among which mitochondrial gene flow appears unrestricted, show genetic signs of postglacial range expansion and population growth. The Larus fuscus group is derived from a cachinnans-like ancestral population, probably in the Aralo-Caspian basin, and spread from east (NW Siberia) to west within the Palearctic.

The effects of testosterone manipulation on the body condition of captive male yellow-legged gulls

Persistently high testosterone levels are believed to be costly to males due to their negative effect on body condition. However, this assumption could not be validated when we analysed birds isolated from all social interactions. The hypothesis was tested on birds kept in isolation in order to analyse the effects of testosterone per se, and thereby exclude the influence of social interactions. Adult male yellow-legged gulls ( Larus cachinnans) were captured, and after a period of adjustment, some individuals were subcutaneously implanted with testosterone, while the rest were used as controls. The gulls received ad libitum food for 10 days and were then fasted for 4 days. Thyroid hormones, body-mass change, daily food intake, hematocrit and several plasma biochemical parameters were analysed. Treated (T)-males maintained constant levels of plasma total protein throughout the experiment, whilst control (C)-males showed a decrease. We did not find any other differences between groups for the other variables analysed. Since the implanted birds sustained high testosterone levels for a number of days, any cost to body condition would have been revealed if these costs levels were actually important. Our results do not support the hypothesis that a reduction in body condition can be directly produced by plasma testosterone, although total protein changes do suggest different anabolic patterns in testosterone-treated gulls.

Changes in Plasma Biochemistry and Body Mass During Incubation in the Yellow-legged Gull

Abstract The “Incubatory Reserves Constancy” hypothesis asserts that incubation could be a departure from breeding stress that allows for the maintenance or recovery of body reserves after laying effort (females) or territory defense (males) in those species with bi-parental incubation such as gulls. The plasma composition and body mass of incubating Yellow-legged Gulls (Larus cachinnans) were analyzed and related to the number of days after egg-laying. Female gulls showed an increase in uric acid and cholesterol levels, whereas males showed only an increase in uric acid values throughout this period. Moreover, females increased while males maintained their body masses. These results could reflect a recovery process after the laying effort supporting the Incubatory Reserves Constancy hypothesis in females. Uric acid and urea levels are positively correlated to body condition in Yellow-legged Gulls, which could be the result of a change in diet composition. This disagrees with recent findings on body compo...

Effects of testosterone implants on pair behaviour during incubation in the Yellow‐legged Gull Larus cachinnans

The Yellow‐legged Gull Larus cachinnans is monogamous with bi‐parental incubation. In this study, the effects of high levels of plasma testosterone in male Yellow‐legged Gulls during the incubation period were analysed. Free‐living male gulls were implanted with testosterone (T‐males), and their sexual behaviour within the pair was observed and compared with that of control pairs. Egg temperatures, length of incubation and hatching success were also analysed. T‐males and their mates displayed more sexual behaviour than the controls. T‐males engaged in mounting behaviour with their mates, whereas control males did not. Proportionally less time was spent incubating (in relation to time present in the colony) by T‐males than control males. However, the mates of T‐males did not spend more time incubating than control females to compensate for male neglect, although they did spend more time on the territory. Egg temperature in T‐male nests was significantly lower than in control nests, but no significant difference in the length of incubation or hatching success between the two groups was found. In birds, the effects of high testosterone levels on male behaviour during incubation have only been analysed in a polyandrous species whose females usually do not contribute to incubation. The present results thus suggest that those males of a monogamous species with bi‐parental incubation that sustain high testosterone levels after laying, thus reducing their contributions to incubation, will be confronted with a lack of compensation from their mates during incubation. Finally, this lack of female compensation seems to be mediated by behavioural interactions with the male rather than by her absence from the colony.

Competition between the yellow-legged gull Larus cachinnans and Audouin's gull Larus audouinii associated with commercial fishing vessels: the influence of season and fishing fleet

Competition between the yellow-legged gull Larus cachinnans and Audouin's gull Larus audouinii while foraging at commercial fishing vessels off the Ebro Delta (NW Mediterranean) was assessed in 1997 and 1998. Observations were performed on board two kinds of fishing vessels with different timetables: bottom trawlers (diurnal activity) and purse seiners (nocturnal activity). Three situations were distinguished with respect to the season and the fishing regime: (1) breeding season, both fleets operating; (2) breeding season, only purse seiners operating due to a trawling moratorium; (3) non-breeding season, both fleets operating. Overall, the yellow-legged gull behaved as an opportunist species and exerted pressure over Audouin's gull through kleptoparasitism and agonistic interactions (i.e. contest competition). Despite this, Audouin's gull was more efficient at capturing discards through scramble competition and was able to take profit from fishing vessels when the capture of fish required high skill, in accordance with its higher specialisation. Competition varied in intensity according to the fishing fleet and the season. Indeed, Audouin's and the yellow-legged gulls only interfered at trawlers, since only Audouin's gull attended purse seiners. During the breeding season competition at trawlers was not severe and Audouin's gull preferentially attended these vessels. Purse seiners acted as a secondary food resource and only attracted important numbers of Audouin's gulls during trawling moratoriums. Out of the breeding season the number of Audouin's gulls strongly declined in the area. Furthermore, the intensity of kleptoparasitism increased at trawlers, and the average size of the fish discarded was larger and less suitable. In parallel with these changes, Audouin's gull shifted to attend purse seiners preferentially, thus avoiding the high levels of competition at the trawlers. The lower representation of Audouin's gull in other breeding areas in the Mediterranean, as well as the less important fishing fleets in these areas, would probably reduce the attractiveness of trawlers for this species, even during the breeding season. Moreover, changes in fishing policies aimed to reduce discarding practices would lead to a globally less favourable situation for Audouin's gull.