Audouin's Gull Research Articles

Modeling temporal and spatial colony‐site dynamics in a long‐lived seabird

AbstractWe studied the determinants of colony site dynamics in Audouin's gull, Larus audouinii, breeding in a small archipelago of the western Mediterranean. Data on island occupation were available for a series of 25 years, since first colonization of the archipelago in 1973. Group behavior was studied in relation to the components of dispersal: permanence or abandonment (extinction) on an island previously occupied and permanence or occupation (colonization) of another island. Generalized Linear Mixed Models (GLMMs) were used to identify the relative contribution of each explanatory variable to the probability of colony abandonment. Gulls showed a low probability (3%) of abandoning one of the islands (Grossa I.), especially when the colony was increasing in numbers from time ti‐1 to ti. However, the probability of abandoning Grossa increased up to 31% when the colony was declining. The probability of island abandonment was very high for all other islands (range 66–99%) when the colony was declining, but much lower (range 36–82%) when it was increasing. Hence, we suggest that island abandonment by Audouin's gull is at least a two‐step process. The first step (dispersal of a portion of the colony) probably takes place at random, as an evolutionary load typical of a species evolved in unstable habitats. The second step, a further loss of breeding pairs, seems to feedback on the first loss of members of the colony (public information), likely perceived as a loss of colony quality. Colonization of islands by gulls abandoning Grossa I. was marginally and negatively affected by the density of breeding yellow‐legged gulls, a predatory species. Results apply to conservation ecology since they highlight the need to protect not only occupied patches but also those empty at present.

Kleptoparasitism, disturbance and predation of yellow-legged gulls on Audouin's gulls in three colonies of the western Mediterranean

SUMMARY: The impact of yellow-legged gulls on Audouin’s gulls was studied by means of observations from a blind, at the Ebro Delta, the Chafarinas Islands and the Columbretes Islands colonies, during different stages of Audouin’s Gulls breeding cycle. The rates of predation (upon eggs and chicks) and kleptoparasitism (aerial and on courtship and chick-feeding regurgitates) were recorded to evaluate this impact. Kleptoparasitism and predation rates recorded at the three colonies may be considered low when compared with similar studies dealing with other gull species. It seems that interactions did not represent a threat to the population dynamics of Audouin’s Gulls in any of the three colonies. The highest rates of aerial kleptoparasitism, courtship feeding and chick mortality were recorded at the Columbretes Islands, and the lowest at the Ebro Delta. The ratio of the number of yellow-legged gulls to the number of Audouin’s gulls seems to influence the rate of disturbances. Food availability was also likely to play an important role in the number of interactions, which increased when food was in shorter supply. The study at the Ebro Delta and Chafarinas Islands was restricted to some subcolonies and hence results may not be necessarily representative of the entire colonies. The existence of some yellow-legged gull individuals specialised as predators was recorded. Finally, we discuss the suitability of several conservation measures commonly applied in colonies where both species breed syntopically.

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.

Growth of Audouin's gull chicks: the role of prehatch and posthatch factors

We compared the growth rates of Audouin´s Gull ( Larus audouinii ) chicks from the Ebro Delta and Columbretes Islands. Chicks from the Columbretes Islands were reared in captivity and fed ad libitum. Wild chicks from the Ebro Delta weighed more than chicks hatched from Columbretes eggs but attained a lower weight at fledging. Chicks from Columbretes eggs hatched with a longer wing length but rates did not differ between samples during the early stages of growth. Eggs from the Ebro Delta were significantly larger than eggs from Columbretes. Hence, differences in growth appeared not to be related to differences in egg size (prehatch factor) but only in parental quality (posthatch factor) through the amount of food delivered to chicks. These differences could be due to the incapacity of parents to provide enough food during the last stages of chick development, in part because of reduced food availability at the Ebro Delta during the study period. Alternatively, differences could be attributed to a trade-off between present and future reproduction. Demographic consequences of reduced food availability for the 1993 Ebro Delta cohort should be explored in future work.

Species Action Plans for the conservation of seabirds in the Mediterranean Sea: Audouin's gull, Balearic shearwater and Mediterranean shag

BirdLife International has so far developed Action Plans for 37 bird species. The plans, although not legally binding, have been approved by the Ornis Committee (EU DG Environment) and most of them have also been endorsed by the Standing Committee of the Bern Convention. Three of the species covered by the SAPs are Mediterranean seabirds: Audouin´s gull (Larus audouinii) , Balearic shearwater (Puffinus mauretanicus) and Mediterranean shag (Phalacrocorax aristotelis desmarestii) . The documents, which have been produced through a participatory process involving tens of experts across the range of the taxa, provide information on the biology, ecology and threats, but the most important feature is the list of specific objectives and actions which are ranked according to their priority and urgency. Alongside actions addressing species-specific conservation needs, such as site protection and management, a number of activities aimed at the conservation of these species are common and refer to wide policies such as land-use planning, fishery, tourist development and pollution control. The documents represent the most up-to-date and scientifically-based source of information for planning and implementing conservation activities across the Mediterranean, to be implemented by both governmental and non governmental organisations.

Impact of fisheries on activity, diet and predatory interactions between yellow-legged and Audouin's gulls breeding at the Chafarinas Islands

Interactions between yellow-legged Larus cachinnans and Audouin´s Larus audouinii gulls and fisheries operating around the Chafarinas Islands, located 4.5 km off the Moroccan Mediterranean coast, are reviewed. At the Chafarinas archipelago two distinct types of fisheries operate: trawlers and purse seines. Gulls take advantage of both fisheries. They scavenge trawler discards and congregate around shoals of fish attracted to the surface by the purse-seine lamps. When both trawlers and purse-seine boats are in operation, the diet of both gull species is similar, with epipelagic fish accounting for over 60% of the biomass, partially collected in association with the purse-seine fishery. When only trawlers operated yellow-legged gulls, but not Audouin´s gulls, augmented their diet mainly with human waste from refuse dumps, suggesting that competition for food between the two species is mainly limited to the periods when resources made available by fishery activities are abundant. Likewise, when only trawlers operated, there was an increase in the predation pressure on eggs and chicks of Audouin´s gulls. In particular, during the week of celebrations for the holy lamb festival when neither fishery operated, egg losses of Audouin´s gull increased dramatically, suggesting that severe food shortage caused by the cessation of fishing can result in an increased predation pressure by yellow-legged gulls, affecting Audouin´s gull productivity. These results suggest a novel socio-ecological link between gulls, fisheries and local feasts.

Offspring sex ratios in subcolonies of Audouin's gull, Larus audouinii, with differential breeding performance

At the Ebro River delta colony in the western Mediterranean Sea, Audouin's gull, Larus audouinii, breeds in discrete aggregations called subcolonies, which showed strong differences in breeding parameters such as egg volume or breeding success. Egg parameters (such as size of both eggs and clutches) are strongly influenced by food availability. As all subcolonies are in the same area, differences in egg parameters might reflect different individuals' foraging efficiency. We measured mean clutch volumes in different subcolonies and chose those subcolonies that showed the greatest differences in this measure, which should indicate differences in parental body condition. Between these subcolonies we would expect, in turn, differences in offspring sex ratios. We took blood samples at hatching and fledging from chicks at these subcolonies and compared offspring sex ratios by means of molecular sexing. The proportions of young breeders differed between these subcolonies, and the subcolony with the greater proportion of young breeders produced smaller eggs and had lower breeding success. However, we did not detect any bias in progeny sex ratio, which probably indicates that if parental condition is not extremely reduced, selective pressures are insufficient to overcome the constraints imposed by Mendelian segregation of chromosomes.

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.

Differences in diet between the two largest breeding colonies of Audouin's gulls: the effects of fishery activities

We analysed and compared the diet of Audouin´s gulls Larus audouinii between their two largest breeding sites in the world: the Ebro Delta and the Chafarinas Islands (western Mediterranean). These two localities showed marked differences in the features of the commercial fishing fleet: in the Ebro Delta area a large fishing fleet produced large amounts of discards, while in the Chafarinas the fleet discarded smaller amounts of fish and marine invertebrates, due to the smaller number of vessels. It is also likely that the percentage of discards from total catches is also lower around the Chafarinas than at the Ebro Delta. We distinguished two types of fishing to compare diet compositions: diurnal (only trawling activity) and diurnal and nocturnal (trawling and purse-seine activity, respectively). We also differentiated regurgitates from young nestlings (up to 20 days old) and from older nestlings or adult birds. At the two localities, fish was the main food of Audouin´s gulls, with epipelagic prey (mainly clupeoids) being more important when both diurnal and nocturnal fisheries were operating. This confirms that epipelagic prey either caught actively by the gulls or linked to fisheries was particularly important in the feeding habits of Audouin´s gulls. Nevertheless, differences between the two colonies appear mainly when only trawlers operated: while at the Ebro Delta gulls showed higher consumption of benthic-mesopelagic prey (probably linked to a higher trawler discard availability), gulls from the Chafarinas Islands consumed higher biomass of epipelagic prey probably caught actively at night. When both fleets operated around the two colonies, the average biomass of prey in a regurgitate of younger chicks was significantly higher at the Ebro Delta than at Chafarinas, and the opposite trend was recorded for older nestlings and adults. Niche width was broader in Chafarinas than in the Ebro Delta for both age classes and for any fishing fleet schedule, suggesting again that the exploitation of discards was higher at the Ebro Delta than at the Chafarinas, where gulls showed a more varied diet. Despite the fact that availability of discards was probably higher at the Ebro Delta than at Chafarinas, the per capita availability was not so different at both localities due to the increasing seabird community population at the Ebro Delta, which ca. doubled that at Chafarinas in the last decade.

Significance of nocturnal purse seine fisheries for seabirds: a case study off the Ebro Delta (NW Mediterranean)

Seabirds are known to make extensive use of fishery waste, a phenomenon that has been particularly well studied in relation to demersal fisheries, especially when operatingduringdaylig ht hours. Contrarily, very little is known about the importance of predominantly nocturnal fisheries in providingfeedingopportunities to seabirds. We considered the particular case of purse se- iningfor small pelagic fish, which takes place basically at night, and assessed the significance of this fishing practice for seabirds off the Ebro Delta (NW Mediterranean). Fieldwork was conducted on board commercial purse seiners (nocturnal activity) in 1997-1998, and was com- plemented with observations performed on board both commercial bottom trawlers (diurnal activity; 1997- 1998) and a research vessel (1999-2000). The purse seine fleet targets small clupeoids, which are attracted and concentrated by the light of a powerful lamp, and then captured usingan encirclingnet. Purse seiners frequently changed fishing area, took very variable catches, and were strongly influenced by the weather. This made the fishery unattractive to most seabird species, which primarily at- tended purse seiners duringthe discardingprocess. This process took place on the way back to port after dawn (i.e. with daylight) during daylight and was quite irregu- lar, thus attractinglower numbers of seabirds than did trawlers. The threatened Audouin's gull Larus audouinii was the only species attendingpurse seiners reg ularly at night, capturing live fish concentrated at the sea surface duringthe haulingprocess. This is in accordance with the specialisation of this gull in the capture of epipelagic fish at night, which seems facilitated by purse seiners. Indeed, the vessels would favour the direct capture of fish (illu- mination of the sea surface, concentration of the fish), as well as the location of the shoals (light signalling). The purse seiningfishery was especially important for Aud- ouin's gull during trawling moratoria (when trawling discards were not available) and in the non-breeding season. A simple bioenergetic model estimated that in- dividual Audouin's gulls could obtain a mean of 669 kJ haul -1 , which would represent far more than half of the daily energy requirements of breeding birds. Provided that Audouin's gulls feeding in a given area could attend more than one haul in a short time, birds attendingpurse seiners at night could easily meet their energy require- ments. However, there was strongvariability in our esti- mate (from 0 to 1659 kJ bird -1 haul -1 , 95% CI), and feedingat purse seiners would only sometimes be profit- able for Audouin's gulls. Purse seine fisheries could be of importance for other nocturnal seabirds in other regions such as the SE Pacific, and this deserves further research. It is important to note that purse seiners could also be detrimental for many seabirds, through direct competi- tion and eventual depletion of fish stocks.

Mercury levels in seabirds and their fish prey at the Ebro Delta (NW Mediterranean): the role of trawler discards as a source of contamination

We determined mercury levels in internal tissues and feathers from corpses of Audouin's Larus audouinii and yellow-legged gulls L. cachinnans michaellis, common terns Sterna hirundo and European shags Phalacrocorax aristotelis desmarestii, as well as from fish representative of trawler discards, collected at the Ebro Delta (NW Mediterranean) between March and July (sea- bird's breeding season) in 1997 to 1999. The levels of mercury were significantly lower in epipelagic (Clupeiforms) than in demersal fish. When representation of each species in the discards is taken into account, the mean mercury concentration from this resource is more than double that of epipelagic fish (the main natural prey for most seabirds in the area). The shag was the only species with direct access to benthic fish, as it can dive to the seabed, and shags presented high levels of mercury even though they do not feed on discards. The other seabirds showed mercury levels in accordance with their seasonal use of discards. Audouin's gull, which exploits discards extensively during the breed- ing season, had the highest levels in those tissues reflecting mercury intake during the breeding sea- son (liver and 1st primary feathers). In contrast, the common tern makes little use of discards and pre- sented the lowest levels of mercury. For those samples reflecting the intake of mercury during the winter (mantle feathers), when only the yellow-legged gull exploits discards extensively, this species presented the highest values. Audouin's gull and the common tern showed similarly low concentra- tions of mercury for this period. We conclude that consumption of discarded demersal fish strongly influenced mercury contamination of surface-feeding seabirds.

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.

A note on the potential role of philopatry and conspecific attraction as conservation tools in Audouin's Gull Larus audouinii

This short communication reports on the first record of Audouin's Gulls Larus audouinii breeding in captivity and on the behaviour of some individuals released as fledglings. During the period 1995–2000 a hand-reared male returned to its natal site and responded, year after year, to the translocation of a captive group of Audouin's Gulls for nesting site selection. The potential role of philopatry and conspecific attraction as conservation tools for this vulnerable seabird after an eight year experiment are discussed.

PCBs and Organochlorine Pesticide Residues in Eggs of Audouin's Gull ( Larus audouinii) in the North-Eastern Mediterranean

Levels of eight PCB congeners (IUPAC 8, 20, 28, 52, 101, 118, 138, 180) and 13 organochlorine pesticides (α-BHC, β-BHC, lindane (γ-BHC), heptachlor, heptachlor epoxide, aldrin, dieldrin, endrin, 2,4 ′-DDT, 2,4 ′-DDD, 4,4 ′-DDT, 4,4 ′-DDD, 4,4 ′-DDE) were measured in unhatched Audouin gull eggs from Aegean Sea (northeastern Mediterranean) colonies in 1997 and 1998. Levels of more persistent congeners 138, 180 and in some colonies 118, predominated among PCBs. Between years significant differences were found in the levels of a variety of contaminants in the same regional colonies (Lipsos, Agathonisi, and Fourni) attributable to temporal changes in diet. Within years differences were limited (1997: PCB congeners 52, 118, 180, heptachlor and 4,4 ′-DDD; 1998: PCB congener 8 and heptachlor epoxide) which in combination with cluster analysis suggest a temporal rather than spatial pattern of pollution in the Aegean Sea. Maximum median levels of five PCB congeners, total PCBs and eight organochlorine pesticides were found in the Agathonisi and Fourni colonies suggesting an elevated nearby pollution probably from the polluted Menderez River. Levels of all contaminants were too low to have any adverse reproductive effects on the Audouin's gull of the Aegean colonies.

Testing predictions from flight mechanical theory: a case study of Cory's shearwater and Audouin's gull

Predictions from flight mechanical theory concerning optimal flight speeds were tested in the field in two Mediterranean seabirds, the Cory’s shearwater Calonectris diomedea and the Audouin’s gull Larus audouinii. Both species were commuting off the coast of Isola di San Pietro, 6 km south-west of the coast of Sardinia. Heading and airspeed were obtained by vector calculation of flight tracks and measured wind. The Cory’s shearwater used a mixture of gliding and active flight. At low wind speeds the proportion of active flight was large but it decreased with increasing wind speed. The mean airspeed was 12.0 m s–1, which is not significantly different from minimum power speed (V mp) in active flight or the speed for best glide (V bg) used in gliding flight. However, the shearwaters showed a significant response to wind increment/decrement, indicating that they were not flying at V mp, which should be unaffected by head and tailwind. Furthermore, shearwaters can potentially reduce induced drag by the ground effect while flying close to the sea surface at weak winds, which leads to a reduction in characteristic flight speed. We suspect that the predictions for gliding flight are most valid for shearwaters at moderate to high wind speeds, when they should be maximising distance by using V bg. Audouin’s gulls used active flight exclusively, with a mean airspeed of 11.3 m s–1 that was significantly different from the predicted V mp. Interestingly, though, the gulls did not show any significant wind response, indicating that they were flying close to their true V mp when foraging along the coast.

Mercury in feathers of Audouin's gull (Larus audouinii) chicks from northeastern Mediterranean colonies.

Feathers of Audouin's gull chicks from three Aegean island areas (north Dodecanese, Cyclades, Kythera) Greece, were sampled in 1997 and 1998 and analyzed for mercury. Mean concentrations varied from 0. 94 microg/g (Lipsos, Dodecanese, 1998) to 2.14 microg/g (Paros, Cyclades, 1998). Significant differences between years occurred in some regions (Lipsos, Fourni) but not in others (Paros). Within each year, especially in 1998, mean mercury concentrations differed among colonies. Results did not support the prediction that mercury levels would be higher in the north Dodecanese area due to the proximity of the polluted Menderes delta. There was no relationship between estimated chick age and feather mercury contents (r = -0.04, NS). Detected mercury levels do not seem to pose any toxic hazard to the Aegean Audouin's gull populations. However, the ease of sampling from gulls indicates that they may be a useful biomonitor of mercury contamination in this region.

Variability of mercury and selenium levels in clutches of Audouin's gulls (Larus audouinii) breeding at the Chafarinas Islands, Southwest Mediterranean.

We determined mercury and selenium in 43 eggs (eggshell, albumen, and yolk) which belong to different clutch sizes of Audouin's gull from the Chafarinas Islands. The results were compared with those obtained previously with the same species at the Ebro Delta. Both, the intra- and the interclutch sources of variability have been examined. There is an effect of the female on mercury and selenium concentrations in a clutch, which supports the use of eggs as monitoring tools. The distribution pattern of mercury among albumen, yolk and eggshell, the dynamics of this element during the laying process, as well as data concerning egg formation strategies suggest that the mercury in the albumen corresponds mainly to the mercury acquired by the female while feeding in the breeding area. The mercury and selenium levels of the eggs from the Chafarinas Islands were lower than those of the Ebro Delta, which can be due to differences in both the marine contamination and the diet in the two colonies.

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.

Helminth Communities in Audouin's Gulls, Larus audouinii from Chafarinas Islands (Western Mediterranean)

A survey of intestinal helminth communities of Audouin's gulls Larus audouinii, from their breeding colonies in Chafarinas Islands, western Mediterranean, Spain was conducted to determine the abundance and species diversity of intestinal parasites of these birds. The sample of 58 gulls harbored intestinal helminth infracommunities composed of species that are gull generalists, including the digeneans Cardiocephalus longicollis, Knipowitschiatrema nicolai, Condylocotyla pilodora, and Aporchis massiliensis, and the cestode Tetrabothrius cylindraceus. Two nematodes are waterfowl generalists (Cosmocephalus obvelatus and Paracuaria adunca), whereas the digenean Acanthotrema armata is an Audouin's gull specialist. The relative high values of species richness and diversity of the helminth infracommunities are comparable to those of other gulls (Larus philadelphia, Larus canus), probably reflecting the specialized, nonselective fish diet of L. audouinii.

Testing the intergenerational conflict hypothesis: factors affecting adoptions in Audouin's gulls, Larus audouinii

We studied chick adoption in Audouin's gull, Larus audouinii, a ground-nesting seabird, to test several predictions made by the intergenerational-conflict hypothesis (ICH). The effects of food availability on adoptions were considered by comparing two breeding seasons with differing food supply. As predicted, adoptions were significantly more frequent when food was in shorter supply (48% of broods affected) than when it was more available (18.5%). Adoptions occurred during the first week of the chick-rearing stage; after that, aggression by adults towards intruding chicks increased significantly, which suggests that mechanisms of parent-offspring recognition were established. Foster parents incurred certain costs, since their offspring had lower chances of survival than those in control broods. However, contrary to the ICH hypothesis, adoptees did not select foster broods in relation to the age of the resident chicks. Survival of wandering chicks was no higher than that of chicks who stayed in their natal brood. Results suggest that chicks move only as a response to neighboring chicks being fed by an adult, and in Audouin's gulls, adoptions might be considered reproductive errors.