Audouin's Gull Larus Audouinii Research Articles

Drivers of resource allocation for breeding under variable environments in a bet hedger.

The evolutionary theory of life histories predicts that there is a trade-off between survival and reproduction: since adult survival in long-lived organisms is high, then breeding investment is more variable and more dependent on conditions (e.g. food availability and individual experience). Clutch features influence fitness prospects, but how a bet hedger builds its clutch in temporally varying environments is quite unknown. Using 27-year data on 2847 clutches of known-age breeders, we analyse how Audouin's gulls (Larus audouinii), a species showing a combination of conservative and adaptive bet-hedging breeding strategies, can allocate energy by laying clutches and eggs of different sizes. Results show that both food availability and age influenced clutch size and total egg volume in a clutch. Interestingly, we found an interaction between food and age on egg parameters: total volume in two-egg clutches, laid mostly by younger breeders, did not significantly change with food availability and the quadratic pattern in clutch size over the range of ages was less marked as long as food conditions became harsher. With increased food, females invested more by building larger first eggs, whereas they were more conservative on second and third eggs. Furthermore, asymmetries in egg volume within three-egg clutches increased with food availability for old females. Egg size profiles of two-egg clutches suggest that gulls should exhibit progressive reduction of the size of the third egg before shifting to a two-egg clutch size. Food availability influenced all parameters studied, whereas age affected the amount of energy allocated for producing eggs (their size and number) but not the way of allocating those energies (i.e. asymmetries within the clutch). Despite the range of factors affecting the clutch, results suggest that females can allocate the amount of resources in a clutch optimally to increase their fitness under variable environments via bet-hedging.

Target and untargeted screening of perfluoroalkyl substances in biota using liquid chromatography coupled to quadrupole time of flight mass spectrometry

Perfluoroalkyl substances (PFAS) are known to bioaccumulate and trigger adverse effects in marine birds. This study develops an extraction and analytical methodology for the target/untargeted analysis of PFAS in eggs of Yellow-legged gull (Larus michahellis) and Audouin's gull (Larus audouinii) and blood of Greater flamingo (Phoenicopterus roseus), which are used as bioindicators of organic chemical pollution. Samples were extracted by ultrasonication with acetonitrile and cleaned-up with activated carbon, and analysis was performed by ultra-high-performance liquid chromatography coupled to a quadrupole-time of flight mass spectrometer (UHPLC-Q-TOF) with negative electrospray ionization. Data-independent acquisition (DIA) was performed through full-scan acquisition to obtain MS1 at 6 eV and MS2 at 30 eV. In a first step, quantitative analysis of 25 PFAS was performed using 9 mass-labelled internal standard PFAS and quality parameters of the method developed are provided. Then, an untargeted screening workflow is proposed using the high-resolution PFAS library database from NORMAN to identify new chemicals through accurate mass measurement of MS1 and MS2 signals. The method permitted to detect several PFAS at concentrations ranging from 0.45 to 55.2 ng/g wet weight in gull eggs and from 0.75 to 125 ng/mL wet weight in flamingos’ blood, with PFOS, PFOA, PFNA, PFUdA, PFTrDA, PFDoA, PFHxS and PFHpA the main compounds detected. In addition, perfluoro-p-ethylcyclohexylsulfonic acid (PFECHS, CAS number 646-83-3) and 2-(perfluorohexyl)ethanol (6:2 FTOH, CAS number 647-42-7) were tentatively identified. The developed UHPLC-Q-TOF target/untargeted analytical approach increases the scope of PFAS analysis, enabling a better assessment on contaminant exposure and promoting the use of bird species as bioindicators of chemical pollution.

Occurrence and antimicrobial resistance of zoonotic enteropathogens in gulls from southern Europe

Campylobacter spp. and Salmonella spp. are the two most frequent zoonotic bacteria involved in human enteric infections in the European Union. Both enteropathogens have been isolated from a diversity of wild birds in Northern Europe, but there is limited information about gulls as potential reservoirs in Southern Europe. A broad sampling of fledglings from nine colonies of yellow-legged gull (Larus michahellis, N = 1222) and Audouin's gull (Larus audouinii, N = 563) has been conducted in Spain and Tunisia during the late chick-rearing period. Overall, the occurrence of Campylobacter spp. and Salmonella spp. was 5.2% (93/1785, CI95%: 4.2–6.2%) and 20.8% (371/1785, CI95%: 18.9–22.7%), respectively. The most predominant Campylobacter species was C. jejuni (94.6%). A high diversity of Salmonella serovars was isolated and the most frequent were those also reported in human outbreaks, such as Salmonella Typhimurium. A high proportion of Campylobacter and Salmonella isolates showed resistance to at least one antimicrobial agent (20.2% and 51.5%, respectively), while 19.2% of Salmonella isolates were multidrug-resistant. These results show the relevance of gulls as reservoirs of Campylobacter and Salmonella by maintaining and spreading these bacteria, including resistant and multidrug resistant strains, in the environment. Our results suggest that gulls can serve as sentinel species for antibiotic pressure in the environment.

Explorative GPS-Tracking of Foraging Movements by Audouin's Gulls Reveals No Association with Fishing Vessels in Croatia

The Audouin's Gull Larus audouinii is endemic to the Mediterranean, with the majority of the global population concentrated in Spain. It mainly feeds on small pelagic fish during the night; however, there is more variation in the foraging behaviour of the Ebro Delta population, with birds also feeding during the day and inland. This plasticity was identified as one of the main factors involved in the increase in population size since 1980. However, foraging movements have only been poorly studied in other populations. We studied the movements of Audouin's Gulls breeding in the small, endangered Croatian population, foraging at the Adriatic Sea, and compared their behaviour with that of conspecifics in the increasing Spanish population in Ebro Delta. Five incubating adults were equipped with a GPS-GSM solar-powered tracking device. We analysed characteristics of short and long foraging trips, differences in movement patterns between sexes and between night and daytime, and the association of gulls with fishing vessels. Daily average trip lengths were similar for all birds, while individual patterns were very diverse. Birds undertook both short (up to 45 km) and long-distance trips (up to 256 km), with length of trips, trip duration and the maximum distance being greater for females. The comparison of gull GPS-fixes with tracking data (VMS) of purse seine fishing vessels showed that birds from the Croatian population, in contrast to the Spanish birds, did not associate with fishing vessels, nor did their foraging behaviour change as an effect of a fishing moratorium. Our results showed that long-distance movements were longer and more frequent than those recorded in other populations, which suggests that depending on non-anthropogenic resources may result in a high cost of chick-rearing.

Telomere shortening rate predicts species life span.

Telomere shortening to a critical length can trigger aging and shorter life spans in mice and humans by a mechanism that involves induction of a persistent DNA damage response at chromosome ends and loss of cellular viability. However, whether telomere length is a universal determinant of species longevity is not known. To determine whether telomere shortening can be a single parameter to predict species longevities, here we measured in parallel the telomere length of a wide variety of species (birds and mammals) with very different life spans and body sizes, including mouse (Mus musculus), goat (Capra hircus), Audouin's gull (Larus audouinii), reindeer (Rangifer tarandus), griffon vulture (Gyps fulvus), bottlenose dolphin (Tursiops truncatus), American flamingo (Phoenicopterus ruber), and Sumatran elephant (Elephas maximus sumatranus). We found that the telomere shortening rate, but not the initial telomere length alone, is a powerful predictor of species life span. These results support the notion that critical telomere shortening and the consequent onset of telomeric DNA damage and cellular senescence are a general determinant of species life span.

Seasonal and annual differences in the foraging ecology of two gull species breeding in sympatry and their use of fishery discards

Niche segregation between similar species will result from an avoidance of competition but also from environmental variability, including nowadays anthropogenic activities. Gulls are among the seabirds with greater behavioural plasticity, being highly opportunistic and feeding on a wide range of prey, mostly from anthropogenic origin. Here, we analysed blood and feather stable isotopes combined with pellet analysis to investigate niche partitioning between Audouin's gull Larus audouinii and yellow‐legged gull Larus michahellis breeding in sympatry at Deserta Island, southern Portugal, during 2014 and 2015. During the breeding season there was considerable overlap in the adults’ diet, as their stable isotope values of blood and primary feather (P1) did not differ, and their pellets were comprised mainly by marine fish species. However, Audouin's gulls presented higher occurrences of pelagic fish, while yellow‐legged gulls fed more on demersal fish, insects, and refuse. SIAR mixing models also estimated a higher proportion of demersal fish in the diet of yellow‐legged gulls. We also found differences between the two gull species in chicks’ feathers, suggesting that Audouin's gull adults selected prey with lower carbon isotope values to feed their young. Secondary feather (S8) of Audouin's gull presented higher isotope values compared to yellow‐legged gulls, indicating different foraging areas (δ13C) and/ or trophic levels (δ15N) between the two species in the non‐breeding season. During both the all‐year and non‐breeding periods the yellow‐legged gull showed a broader isotopic niche width than Audouin's gull in 2013, and in 2014 the two gull species exhibited different isotopic niche spaces. Our study suggests that both gull species foraged in association with fisheries during the breeding season. In this sense, a discard ban implemented under the new European Union Common Fisheries Policy may lead to a food shortage, therefore future research should closely monitor the population dynamics of Audouin's and yellow‐legged gulls.

Audouin's Gulls Larus audouinii Affected by Sublingual Fistulas

Summary. Injuries have the potential to influence individual behaviour, reproduction and/or survival. One of the least frequently observed and least known injuries in birds is sublingual fistulation, which has only been reported in four species to date. Audouin's Gull Larus audouinii is now added to this list. Based on periodical re-sightings of darvic-ringed gulls at four Mediterranean sites, the prevalence of this injury in Audouin's Gulls was estimated to be 0.13%. No negative effects of fistulas on short-term survival of injured birds were reported; the minimum survival period was 999 days. However, our findings should be treated cautiously because of the low detected incidence of this injury in free-living birds.

Consecutive cohort effects driven by density-dependence and climate influence early-life survival in a long-lived bird.

Conditions during early life, including maternal cohort effects, can influence the future fitness of individuals. This may be particularly true for long-distance migrating birds, because, apart from conditions experienced by cohorts during rearing, conditions during early life in regions far from breeding grounds may also influence their population dynamics. Very little is known about the fitness consequences of those conditions experienced by juveniles after independence, especially in wild populations and for long-lived birds. We used multi-event capture-recapture-recovery models and a unique 26-year dataset for the Audouin's gull (Larus audouinii) to assess for the first time whether survival was influenced by early conditions, both during the rearing period (i.e. a maternal cohort effect potentially affected by density dependence) and the first winter (i.e. a cohort effect driven by climate when birds disperse to wintering grounds). Our results show that juvenile survival was highly sensitive to early-life conditions and that survival decreased with stronger density dependence and harsh climate. The two consecutive cohort effects were of similar magnitude and they may represent a selection filter. Thus, early-life conditions had a strong impact on survival, and neglecting this complexity may hinder our understanding on how populations of long-lived animals fluctuate and respond to perturbations.

Sex-specific foraging behavior in response to fishing activities in a threatened seabird.

Some seabird species have learnt to efficiently exploit fishing discards from trawling activities. However, a discard ban has been proposed as necessary in Europe to ensure the sustainability of the seas. It is of crucial importance for the management and conservation purposes to study the potential consequences of a discard ban on the foraging ecology of threatened seabirds. We assessed the influence of fishing activities on the feeding habits of 22 male and 15 female Audouin's gulls (Larus audouinii) from the Ebro Delta (Mediterranean Sea) during the breeding period using GPS loggers together with Stable Isotope Analysis (SIA), which provided new insights into their foraging behavior and trophic ecology, respectively. GPS data revealed different sex-specific foraging patterns between workdays and weekends. Females were highly consistent in that they foraged at sea throughout the week even though discarding stops at weekends. In contrast, males switched from foraging at sea during the week (when discards are produced) to an increased use of rice field habitats at weekends (when fishermen do not work). This sex-specific foraging behavior could be related to specific nutritional requirements associated with previous egg production, an energetically demanding period for females. However, on a broader time scale integrated by the SIA, both sexes showed a high degree of individual specialization in their trophic ecology. The need to obtain detailed information on the dependence and response of seabirds to fishing activities is crucial in conservation sciences. In this regard, sex-specific foraging behavior in relation to fisheries has been overlooked, despite the ecological and conservation implications. For instance, this situation may lead to sex differentiation in bycatch mortality in longlines when trawlers do not operate. Moreover, any new fisheries policy will need to be implemented gradually to facilitate the adaptation of a specialized species to a discard ban scenario.

Accumulation and maternal transfer of perfluorooctane sulphonic acid in yellow-legged (Larus michahellis) and Audouin's gull (Larus audouinii) from the Ebro Delta Natural Park

This study aimed to determine the accumulation and maternal transfer of perfluorooctane sulphonic acid (PFOS) in yellow-legged gulls (YLG, Larus michahellis) and the protected species Audouin's gull (AG, Larus audouinii), which cohabit in the Ebro Delta Natural Park (Catalonia, Spain). The Estimated Daily Intake (EDI) through diet (fish and crayfish), depuration rates and transfer capacity from blood to first eggs was studied for a set of 44 pairs. For AG, mean (±SD) EDI was of 128±36ng/d in males and 119±32ng/d in females, and for YLG, 170±48ng/d in males and 159±42ng/d in females. PFOS levels in blood were higher in males than females (60.6±21 and 61.1±27ng/g ww in AG and YLG males versus 25.2±12 and 27.3±14ng/g ww in AG and YLG females), with little differences among species. The lower levels in females were attributed to the annual release of PFOS to eggs during the laying period, which was estimated of 5544±1571ng/egg for AG females and 6716±2689ng/egg for YLG. The overall mass balance of PFOS calculated for both species clearly demonstrates that uptake is higher than elimination rates, considering depuration rates and transfer of PFOS to eggs, and this explains that PFOS is accumulated in gulls in a yearly basis.

Levels of antioxidants in breeding female Audouin's gulls and their deposition in eggs across different environments

Diet quality and dietary antioxidants are interrelated factors that influence many animal traits. However, little is known about this relationship in wild birds, especially how it may affect the deposition of antioxidants from the female to the eggs over the laying sequence. In this context, albumen has been far less studied than yolk due to its lack of carotenoids, which are a common focus of dietary antioxidant research. Our study was conducted in the Audouin's Gull (Larus audouinii), a seabird that shows a great dependence on trawl discards, although in the Ebro Delta also exploits resources from neighbouring rice fields, especially the American crayfish (Procambarus clarkii). We examined the relationship between diet (traced through stable isotope analysis) and the antioxidant capacity (not only carotenoids, but also other non-enzymatic antioxidants, like tocopherols and retinol) of plasma in females from two breeding groups and their clutches. In the eggs we analysed antioxidants in both albumen (hydrophilic antioxidants) and yolk (hydrophilic and lipophilic antioxidants) fractions, taking into account the laying sequence. A decrease in the antioxidant capacity of female plasma was found over the incubation period. We found little difference between groups in the antioxidant deposition pattern in the yolk and whole egg over the laying sequence, but a greater variation was observed in the intra-clutch patterns of albumen antioxidant capacity, probably related to quality differences. When taking into account total antioxidant deposition in the clutch, a declining tendency over the laying sequence was found. Both yolk lipophilic and whole egg antioxidant capacity were negatively related to δ15N, indicating that marine fish diets (with depleted δ15N values) contain a higher amount of antioxidants than rice field prey.

From recruitment to senescence: food shapes the age‐dependent pattern of breeding performance in a long‐lived bird

We used a long-term data set (26 years) from Audouin's Gull (Larus audouinii), a long-lived seabird, to address the relationship between the age-dependent pattern of reproductive performance and environmental conditions during breeding. Although theoretical models predict that the youngest and oldest breeders (due to inexperience and senescence, respectively) will perform less well than intermediate age classes, few empirical data exist regarding how this expected pattern varies with food availability. To assess the influence of age and food availability (corrected by population size of the main consumers to take into account density dependence) on a number of breeding parameters (laying dates, egg volume, clutch size, and hatching success), we modeled mean and variances of these parameters by incorporating heterogeneity into generalized linear models. All parameters varied with age and to different degrees, depending on food availability. As expected, performance improved with increased food supply, and the observed age pattern was quadratic, with poorer breeding performances occurring in extreme ages. For most parameters (except for laying dates, for which age and food did not interact), the pattern changed with food somewhat unexpectedly; the differences in performance between age classes were higher (i.e., the quadratic pattern was more noticeable) when food was more readily available than when food availability was lower. We suggest that, under poor environmental conditions, only high-quality individuals of the younger and older birds bred and that the differences in breeding performance between age classes were smaller. Although variances for egg volume were constant, variances for laying dates were highest for the youngest breeders and tended to decrease with age, either due to the selection of higher-quality individuals or to a greater frequency of birds skipping breeding with age, especially when food was in low supply. Our results show that mean and variances of breeding parameters changed with age, but that this pattern was different for each parameter and also varied according to food availability. It is likely that, other than food, certain additional factors (e.g., sex, cohort effects, density dependence) also influence changes in breeding performance with age, and this may preclude the finding of a common pattern among traits and among studies on different taxa.

Dechlorane Plus in eggs of two gull species (Larus michahellis and Larus audouinii) from the southwestern Mediterranean Sea

Dechlorane Plus (DP) and some of its possible degradation products were measured in eggs from the yellow-legged gull (Larus michahellis) and Audouin's gull (Larus audouinii) from a protected area in the southwestern Mediterranean Sea. Statistically significant differences were found between both gull species, with yellow-legged gull eggs showing the highest average total DP concentration (209 pg/g wet weight). According to stable nitrogen and carbon isotope values, variations in DP concentrations in the gull species studied are explained by foraging behavior and diet rather than by the trophic position. Both DP stereoisomers were quantified in all the samples studied, and a slight enrichment of the anti-DP could have occurred in both species. The quantification of anti-[DP-1Cl] only in ∼58 % of yellow-legged gulls support the hypothesis of a species-dependent factor influencing the bioaccumulation and/or biotransformation of Dechlorane-related compounds. This study reports on the first measurements of Dechlorane-related compounds in biota from the North African continent, contributing to the knowledge about DP environmental fate and distribution. In the light of our results, more research on differences in species-dependent bioaccumulation and biotransformation capabilities as well as ecological effects is encouraged in future Dechlorane-related compound studies.

Comparing demographic parameters for philopatric and immigrant individuals in a long-lived bird adapted to unstable habitats

Theoretical models about the benefits of philopatry predict that immigrant fitness can be higher, lower or similar to that of philopatrics depending on habitat heterogeneity, dispersal costs, distance between patches or population densities. In this study, we compared transience rates, local survival and recruitment among philopatric and immigrant individuals of Audouin's gull Larus audouinii, a long-lived bird with high dispersal capacities. Several previous studies have shown that these capacities were probably the result of adaptation to unstable and highly discrete habitats; hence, we tested the hypothesis that fitness components for philopatrics and immigrants were similar. During 1988-2006, ca. 27,800 chicks were marked in 31 colonies in the western Mediterranean metapopulation, and more than 52,000 resightings were made in a single, high-quality colony, to estimate local demographic parameters by capture-recapture analyses. Results suggest that, even though parameters related to site-tenacity (e.g. recapture rates) were higher for philopatrics than for immigrants, survival and recruitment were fundamentally similar. Small differences between philopatrics and immigrants were probably influenced by a highly suitable habitat at the study site, which reduced dispersal costs for immigrants; furthermore, the similarities in most fitness components were also probably the result of a life-history strategy of a species living in unpredictable, unstable habitats with high emigration rates among local populations, and with a relatively low cost of dispersal.

Mercury and Stable Isotopes in Feathers Of Audouin's Gulls as Indicators of Feeding Habits and Migratory Connectivity

AbstractTo better understand migratory connectivity between breeding and nonbreeding populations, we analyzed mercury (Hg) and stable isotope signatures of nitrogen (δ15N), carbon (δ13C), and sulfur (δ34S) in Audouin's Gulls (Larus audouinii) breeding in two different colonies, the Ebro Delta (northeastern Spain) and the Chafarinas Islands (southwestern Mediterranean). Although abundant information is available on the biology and trophic ecology of this gull's breeding populations, little is known about migration patterns, distribution in winter, or conditions faced during the nonbreeding period. Analyses were carried out on first primary feathers, grown during the summer while gulls are on the breeding grounds, and mantle feathers, grown during the winter. Different isotopic signatures (δ15N, δ13C, and δ34S) in summer (primary) feathers from each area agree with the observed differences in diet between the two colonies. In winter (mantle) feathers, isotopic signatures did not differ, consistent with a common wintering ground and common diet, although the lack of isotopic basemaps in marine systems precludes assignment to a geographical area of reference. Future research is needed to relate isotopic signatures and Hg values in mantle feathers to trophic ecology in wintering areas. Results for Hg indicate that the excretory role played by primary feathers precludes their use as indicators of trophic ecology.

MERCURY AND STABLE ISOTOPES IN FEATHERS OF AUDOUIN'S GULLS AS INDICATORS OF FEEDING HABITS AND MIGRATORY CONNECTIVITY

To better understand migratory connectivity between breeding and nonbreeding populations, we analyzed mercury (Hg) and stable isotope signatures of nitrogen (δ15N), carbon (δ13C), and sulfur (δ34S) in Audouin's Gulls (Larus audouinii) breeding in two different colonies, the Ebro Delta (northeastern Spain) and the Chafarinas Islands (southwestern Mediterranean). Although abundant information is available on the biology and trophic ecology of this gull's breeding populations, little is known about migration patterns, distribution in winter, or conditions faced during the nonbreeding period. Analyses were carried out on first primary feathers, grown during the summer while gulls are on the breeding grounds, and mantle feathers, grown during the winter. Different isotopic signatures (δ15N, δ13C, and δ34S) in summer (primary) feathers from each area agree with the observed differences in diet between the two colonies. In winter (mantle) feathers, isotopic signatures did not differ, consistent with a common wintering ground and common diet, although the lack of isotopic basemaps in marine systems precludes assignment to a geographical area of reference. Future research is needed to relate isotopic signatures and Hg values in mantle feathers to trophic ecology in wintering areas. Results for Hg indicate that the excretory role played by primary feathers precludes their use as indicators of trophic ecology.

Testing habitat copying in breeding habitat selection in a species adapted to variable environments

The habitat copying hypothesis states that animals use the reproductive performance of conspecifics to evaluate habitat quality and choose their future breeding site. We used data from Audouin's Gull Larus audouinii (1992–2003), a species adapted to unpredictable environments, to analyse subcolony (as patch) choice within a colony (small spatial scale). We also assessed the suitability of alternative hypotheses to the habitat copying hypothesis. The probability of subcolonies being reoccupied annually increased with their size (as number of nests), which suggests the existence of group adherence effects. Subcolony growth rate was related to its average reproductive success (or patch reproductive success) in the previous year: the higher the reproductive success in a colony, the higher the probability of growth the following year. However, this last result was obtained without considering the effect of colony size on the response variable because colony size is related to it. Therefore, results suggest at the population level that in this system habitat copying might either be one of the strategies used by the species in selecting its breeding habitat, or one of the possible strategies operating alone. The other strategies are group adherence mechanisms, and also the effect of conspecific attraction. At the individual level we failed to find evidence of habitat copying and only the previous success of an individual affected its fidelity to a subcolony. The importance of the lack of environmental predictability in the system is discussed, as predictability is a prerequisite of habitat copying.

Assessment of hypotheses about dispersal in a long‐lived seabird using multistate capture–recapture models

Summary Dispersal contributes to spatio‐temporal variation in population size and is a key process in studies of life history evolution and studies with conservation implications. However, dispersal is still one of the major gaps in our knowledge of ecological dynamics. The very large literature on metapopulation dynamics lacks empirical bases on dispersal and relevant behavioural parameters. We used multistate capture–recapture models (data from 1988 to 2001) to address hypotheses about movement probability and habitat selection within a system of two breeding colonies in Audouin's gulls (Larus audouinii), an endemic species breeding in the Mediterranean and considered as threatened. Movement probability varied over time, and differed greatly between the colonies. We did not find evidence of an influence of colony size or density of predators on movement probability. In dispersers, our results did not support the hypotheses that movement probability between year t and t+ 1 was influenced by mean breeding success in the colony of origin (i.e. an indicator of habitat quality) or the destination colony in year t or t+ 1, or by the ratio of breeding success in these colonies in year t or t + 1 (i.e. quality gradient). Overall, movement probability was higher from the smaller colony to the larger, and from the colony with lower breeding success in year t to the more productive one. This provides slight support for two nonexclusive hypotheses about habitat selection (conspecific attraction and conspecific success attraction). Movement probability from the smaller, less productive colony was very high in some years, suggesting that the dynamics of both colonies were strongly influenced by adult dispersal. However, in absolute numbers, more individuals moved from the larger, more productive colony to the smaller one. This suggests that the system may function as a source–sink system. Use of multistate models to re‐assess local survival showed that survival was lower in the less productive colony with higher emigration probability. This may reflect genuine differences in mortality between colonies, or more probably differences in permanent emigration from the study area.

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.

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.