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

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

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.     

Colonization and population growth of Yellow-legged Gull Larus cachinnans in southeastern Poland: causes and influence on native species

The Yellow-legged Gull Larus cachinnans was first recorded in Poland in the 1980s. We analysed the probable factors responsible for its successful colonization of new areas. We also expected that such a large species should affect populations of other colonial waterbirds. We studied the breeding and feeding ecology in the largest inland colony of the Yellow-legged Gull in Poland, located in a sedimentation basin near Tarnów (southeastern Poland). The first breeding pair was recorded in 1992 and the population reached 177 pairs in 2001. The population growth rate in this colony, of about 58% per year, fits an exponential model. Nine localities with breeding pairs have been found recently in southern Poland and we now estimate the total population size to be 200–250 pairs. The large clutch size, and high hatching and breeding success in the Tarnów colony suggest that food was plentiful. Food items were frequently found at the nests. Fish, mainly Carp Cyprinus carpio , were the predominant food items delivered to chicks; however, there was more refuse brought to nests during the incubation stage. Immigration probably caused the growth of the colony studied, although our calculations have shown that natal productivity alone is sufficient to maintain this population. The study showed that the growing population of Yellow-legged Gull might cause considerable reduction in the population sizes of some of the native waterbird species.     

Female body condition and brood sex ratio in Yellow-legged Gulls Larus cachinnans

In the Yellow-legged Gull Larus cachinnans , males are the larger sex, and show more reproductive variance than females. We predicted that the proportion of male chicks in a brood should increase with female body condition. We investigated brood sex ratio by using DNA markers taken from samples of hatchlings or dead embryos, and female body condition using plasma cholesterol concentration as a reliable indicator. The brood sex ratio of females in good condition was male biased and the sex ratio of females in poor condition was female biased. This relationship was also significant in those nests where all the eggs laid were sexed. Thus, manipulation of embryo mortality cannot explain the biases reported in this study, suggesting that the sex ratio of the eggs was biased prior to laying. These results confirm that sex-ratio manipulation in gulls operates under natural conditions, and supports earlier experimental findings.     

Apparent survival rates of a long-lived partial migrant: the Yellow-legged Gull Larus michahellis

Capsule: In Yellow-legged Gull Larus michahellis populations from the eastern Adriatic Sea, there are considerable differences in the ratio of migratory birds and apparent survival among the colonies. Survival was dependent on age, colony and area of dispersal.

Aims: To identify the ratio of migratory birds within populations of Yellow-legged Gulls, and to compare apparent survival of gulls during migration in central and northern Europe and around the Adriatic Sea.

Methods: We analysed 15 years of resighting data of Yellow-legged Gulls using a MARK multi-state model to calculate survival rates. The effects of age, natal colony and area of dispersal were examined.

Results: Almost 60% of Adriatic Yellow-legged Gulls were migratory but the ratio varied among colonies (10.3–78.3%). Survival was dependent on age, colony and area of dispersal, with average values per group ranging between 0.599 (se 0.093) and 0.684 (se 0.084).

Conclusion: The ratio of migratory and dispersive Yellow-legged Gulls from different Adriatic colonies might be affected by both inheritance and food availability.     

Sex differences in growth rates of Yellow-legged Gull Larus michahellis chicks

Capsule: Chick growth rates was sex-dependent in an Atlantic Yellow-legged Gull Larus michahellis population.

Aims: To describe the growth rate of Yellow-legged Gull chicks and sex-associated variations, and obtain a discriminant function to sex them using morphological data.

Methods: Fifty-two Yellow-legged Gull chicks from a colony in northern Iberia were measured for body mass, head and bill length, tarsus length and three other bill-length associated variables, from the hatching date up to the age of 36 days. Birds were sexed using DNA analysis.

Results: Using logistic models, chick growth rates was observed to be similar between the sexes, while the asymptote was higher among males for the majority of the variables. Discriminant analyses showed that the variable head+bill was very reliable to predict the sex of >80% of chicks at an early developmental stage, and 100% of chicks if combined with tarsus length and two more bill-length associated variables at the age of 35 days.

Conclusions: This is the first study using a discriminant analysis to sex Yellow-legged Gull chicks, and also the first to describe the growth function for the species. The growth rate varied between sexes because males showed higher asymptote values within the growth function.     

Intermittent breeding in the Herring Gull Larus argentatus and the Lesser Black-backed Gull Larus fuscus

Thirty-three percent and 37% of adult Herring Gulls Larus argentatus and 34% and 40% of adult Lesser Black-backed Gulls Larus fuscus , all with some previous breeding experience, failed to breed in 1993 and 1994, respectively. About half of these intermittent breeders of both species failed to breed in both years. Common amongst the intermittent breeders were smaller individuals, birds with low survival chances and relatively unsuccessful breeders.     

Relationship between clutch size,egg volume and hatching success in a Yellow-legged Gull Larus michahellis colony in south-eastern Tunisia

This study aimed to test the hypothesis that clutch size covaries with egg volume and hatching success in the Yellow-legged Gull Larus michahellis. We determined clutch size and egg volume in a sample of 131 nests, and we used the data to check whether egg volume varied among nests according to clutch size, while taking into account the effects of egg laying order. We also estimated hatching success rate and investigated the relationship between hatching success and clutch size. Egg volume varied among clutches according to clutch size, with eggs being larger in three-egg clutches than in two-egg clutches. Moreover, three-egg clutches showed higher daily survival rates, and hence hatching success, than two-egg clutches. Overall, our results suggest that in the Yellow-legged Gull clutch size covaries with egg volume and hatching success, which could possibly reflect an age effect through different mechanisms. Indeed, older females could be hypothesised to exhibit greater breeding performance than younger females because of their higher experience in tapping energy resources for egg formation and defending nests from dangers. Moreover, due to their age, older females are likely to have lower residual reproductive potential and should invest more heavily in current breeding attempts.     

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

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

Trophic strategies of Yellow-legged Gull Larus michahellis on oceanic islands surrounded by deep waters

Capsule: The diet of Yellow-legged Gulls Larus michahellis on an oceanic island, surrounded by deep waters without a wide shelf, was mainly composed of terrestrial invertebrates.

Aims: To study the trophic ecology of Yellow-legged Gulls on an island surrounded by deep waters, to quantify the importance of terrestrial prey items and their availability, and to evaluate the relative importance of nutritional values of terrestrial and marine resources.

Methods: Diet was monitored for one year. We assessed the relative contribution of the main prey items and their macronutrients through the study of pellets and faeces.

Results: Terrestrial invertebrates were the most frequently consumed prey items (frequency of occurrence (FO) 67%), followed by marine fish (Osteichthyes FO 33%). Coleoptera and Orthoptera were the most consumed terrestrial invertebrates and provided a high nutritional value. The rate of consumption of terrestrial invertebrates varied in synchrony with the breeding season, being higher in spring, indicating their potential importance for reproduction.

Conclusion: It is unusual that terrestrial invertebrates constitute a large proportion of the diet of large gulls, but they seemed to cover their trophic energy requirements during the reproductive period.     

Opportunistic feeding responses of the Yellow-legged Gull Larus michahellis to accessibility of refuse dumps

Capsule The gulls adjust their diet composition and diversity according to refuse dump accessibility.

Aims To examine the influence of the accessibility of open-air refuse dumps on the pre-laying diet of the adult Yellow-legged Gull.

Methods We studied six colonies settled on six rocky islands off the southeastern coast of France. A comparative study of the diet of breeding adults from the six colonies was made through pellet analysis (a total of 848 pellets). We determined the main foraging habitat used (refuse dumps, terrestrial habitats, marine habitat) and the number of foraging habitats used simultaneously (one, two or three), from which we deduced the mean diet diversity.

Results Refuse dumps were consistently the main foraging habitat (evidence in 53–74% of pellets) for the six colonies, even when refuse dump accessibility was low. The majority of pellets contained materials from two simultaneous foraging habitats (evidence in 50–64% of pellets). We demonstrated the influence of a gradient of refuse dump accessibility in terms of adjustment of the pre-laying adult's diet. Indeed, high refuse dump accessibility leads to a poorly diversified diet dominated by refuse. In contrast, when refuse dump accessibility is low, Yellow-legged Gulls broaden their trophic niche, with an increased exploitation of alternative foraging habitats, such as terrestrial habitats.

Conclusion These results show the species' opportunistic feeding and high adaptability, two parameters which need to be known to foresee the consequences on population dynamics, feeding and predatory behaviour of a sudden and severe food shortage, for example due to closure of open-air refuse dumps.     

Assortative mating for carotenoid colouration but not size in the Yellow-legged Gull Larus michahellis

Capsule: Mediterranean Yellow-legged Gulls mate assortatively according to carotenoid-based colouration but not in relation to size.     

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

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

Pre-fledging survival in a Yellow-legged Gull Larus michahellis population in northern Iberia is mostly determined by hatching date

Capsule: Pre-fledging survival in a Yellow-legged Gull Larus michahellis population in northern Iberia is negatively correlated to hatching date.

Aims: To explore which factors have more importance in determining the pre-fledging daily survival rates in a Yellow-legged Gull population from northern Iberia. Specifically, we tested for the effect of hatching date and order, body size and condition and meteorological conditions on pre-fledging survival.

Methods: Cormack–Jolly–Seber models with mixtures were used to model daily survival rates.

Results: Daily survival rates were mostly negatively affected by hatching date.

Conclusions: Hatching date was the most important factor affecting survival of chicks during the pre-fledging period in a Yellow-legged Gull colony from northern Iberia.     

Maternal effects mediated by egg quality in the Yellow-legged Gull Larus michahellis in relation to laying order and embryo sex

Background

Maternal effects mediated by egg size and quality may profoundly affect offspring development and performance, and mothers may adjust egg traits according to environmental or social influences. In avian species, context-dependency of maternal effects may result in variation in egg composition, as well as in differential patterns of covariation among selected egg components, according to, for example, position in the laying sequence or offspring sex. We investigated variation in major classes of egg yolk components (carotenoids, vitamins and steroid hormones) in relation to egg size, position in the laying sequence and embryo sex in clutches of the Yellow-legged Gull (Larus michahellis). We also investigated their covariation, to highlight mutual adjustments, maternal constraints or trade-offs in egg allocation.

Results

Laying sequence-specific patterns of allocation emerged: concentration of carotenoids and vitamin E decreased, while concentrations of androgens increased. Vitamin A, estradiol and corticosterone did not show any change. There was no evidence of sex-specific allocation or covariation of yolk components. Concentrations of carotenoids and vitamins were positively correlated. Egg mass decreased along the laying sequence, and this decrease was negatively correlated with the mean concentrations of carotenoids in clutches, suggesting that nutritionally constrained females lay low quality clutches in terms of carotenoid content. Finally, clutches with smaller decline in antioxidants between first- and last-laid eggs had a larger increase in yolk corticosterone, suggesting that a smaller antioxidant depletion along the laying sequence may entail a cost for laying females in terms of increased stress levels.

Conclusions

Since some of the analyzed yolk components (e.g. testosterone and lutein) are known to exert sex-specific phenotypic effects on the progeny in this species, the lack of sex-specific egg allocation by mothers may either result from trade-offs between contrasting effects of different egg components on male and female offspring, or indicate that sex-specific traits are controlled primarily by mechanisms of sexual differentiation, including endogenous hormone production or metabolism of exogenous antioxidants, during embryonic development.     

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.