Albert Bertolero

The Biology of the Little Tern in the Ebro Delta (Northwestern Mediterranean)

Abstract Aspects of the biology of the Little Tern (Sterna albifrons) were studied at the Ebro Delta (northwestern Mediterranean) during 1996-1997, including colony site selection, clutch size and egg size. Using a single visit to avoid disturbances, average number of eggs was 2.62 eggs per nest, and was significantly larger in 1996 than in 1997, when the modal clutch size was only two eggs. Mean egg volume was 8.88 cm3 (SD ± 0.57), and mean egg size in a clutch (for both two- and three-egg clutches) was also significantly larger in 1996 than in 1997. Furthermore, average egg volume in three-egg clutches (clutches that were probably completed) was significantly different among the four main breeding areas within the delta. Clutch size was not significantly different among breeding areas in either year. Little Terns bred associated with other colonial Charadriiforms more than in other Mediterranean breeding areas. The terns associated with all the commonest colonial species of shorebirds, but never associated with the Yellow-legged Gull (Larus michahellis), Audouin’s Gull (L. audouinii) or Black-headed Gull (L. ridibundus). The number of breeding pairs decreased from 625 in 1961 to 332 in 2003, this decline occurring mainly since the early 1990s. Mean population declined significantly by 2% per year for the whole period. During the last decade, the number of breeding areas has increased from four to nine, despite the decrease in breeding numbers (rate of decrease 7.1% per year). Nothing is known about the factors driving this decrease, but changes are probably linked to processes in other colonies at a metapopulation level.

Breeding traits of Hermann's tortoise Testudo hermanni hermanni in two western populations

Over a period of two years we used radiology to investigate breeding traits in females of two western populations of the threatened Hermanns tortoise Testudo hermanni hermanni. The main purpose of the study was to see if reproductive traits - clutch size, clutch frequency, annual egg production and egg width - varied between populations in Corsica (France) and the Ebro Delta (Spain), and if these traits were affected by female body size and amounts of spring rainfall. All the breeding traits analysed were greater in the Corsican population than in the Ebro Delta population. These differences were also significant when we used female body size as a covariable. In both populations the amount of spring rainfall affected breeding traits but not clutch size. Nevertheless, only in the Corsican population did clutch frequency, annual egg production and egg width increase significantly in the year with a wet spring. Contrary to the predictions of Optimal Propagule Size theory, we found that egg size and clutch size increased with female size, and that both variables were independent. On the other hand, clutch frequency was not related to female body size. Thus, in the Hermanns tortoise, as clutch size did not change between years and clutch frequency was independent of female size, all females are able to increase their reproductive output in years with favourable conditions by increasing their clutch frequency. Nevertheless, other factors besides female size and rainfall may influence in a highly complex way variability in breeding traits among populations.

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 Audouins 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.

Colonisation in social species: the importance of breeding experience for dispersal in overcoming information barriers

Studying colonisation is crucial to understand metapopulations, evolutionary ecology and species resilience to global change. Unfortunately, few empirical data are available because field monitoring that includes empty patches at large spatiotemporal scales is required. We examine the colonisation dynamics of a long-lived seabird over 34 years in the western Mediterranean by comparing population and individual data from both source colony and the newly-formed colonies. Since social information is not available, we hypothesize that colonisation should follow particular dispersal dynamics and personal information must be crucial in decision making. We test if adverse breeding conditions trigger colonisation events, if personal information plays a role in colonisation and if colonisers experience greater fitness. Our results show a temporal mismatch between colonisation events and both density-dependence and perturbations at the source colony, probably because colonisers needed a longer prospecting period to compensate for the lack of public information. Colonisers were mostly experienced individuals gaining higher breeding success in the new colony. Our results highlight the demographic value that experienced individuals can have on metapopulation dynamics of social long-lived organisms.

Living on the edge: Demography of the Slender-billed gull in the Western Mediterranean

Small and peripheral populations are typically vulnerable to local extinction processes but important for the metapopulation dynamics of species. The Slender-billed gull (Chroicocephalus genei) is a long-lived species breeding in unstable ephemeral coastal habitats. Their Western Mediterranean populations are relatively small and represent the edge of their global geographical distribution. At a local scale, using long-term data (14 years) on annual breeding success and capture-resights of marked individuals, we estimated and compared the vital rates and evaluated the connectivity of two Spanish populations (Ebro Delta and Doñana) varying in their local environmental conditions. At a metapopulation scale, we analyzed 22 years of data on breeding numbers to predict their future prospects by means of population demographic models. Local survival and breeding success of gulls from the Ebro Delta was lower than those from Doñana, which is likely the result of higher permanent emigration and/or winter mortality in the former. Gulls from the Ebro Delta wintered mostly in Mediterranean areas whereas those from Doñana did so in Atlantic coasts, where food availability is higher. Whereas adult local survival was constant, juvenile local survival showed temporal parallel variations between colonies, probably related to natal dispersal to other breeding colonies. Our results suggested that dispersal was higher at the Ebro Delta and gulls emigrating from their natal colonies settled preferentially in close patches. We found large fluctuations in breeding numbers among local populations probably related to the fact that the Slender-billed gull is a species adapted to unstable and unpredictable habitats with high abilities to disperse between suitable patches depending on environmental stochastic conditions during breeding.

Variation in Immune Parameters and Disease Prevalence among Lesser Black-Backed Gulls (Larus fuscus sp.) with Different Migratory Strategies

The ability to control infections is a key trait for migrants that must be balanced against other costly features of the migratory life. In this study we explored the links between migration and disease ecology by examining natural variation in parasite exposure and immunity in several populations of Lesser Black-backed Gulls (Larus fuscus) with different migratory strategies. We found higher activity of natural antibodies in long distance migrants from the nominate subspecies L.f.fuscus. Circulating levels of IgY showed large variation at the population level, while immune parameters associated with antimicrobial activity showed extensive variation at the individual level irrespective of population or migratory strategy. Pathogen prevalence showed large geographical variation. However, the seroprevalence of one of the gull-specific subtypes of avian influenza (H16) was associated to the migratory strategy, with lower prevalence among the long-distance migrants, suggesting that migration may play a role in disease dynamics of certain pathogens at the population level.

Predator arrival elicits differential dispersal, change in age structure and reproductive performance in a prey population

Predators are an important ecological and evolutionary force shaping prey population dynamics. Ecologists have extensively assessed the lethal effects of invasive predators on prey populations. However, the role of non-lethal effects, such as physiological stress or behavioural responses like dispersal, has been comparatively overlooked and their potential population effects remain obscure. Over the last 23 years, we developed a mark-recapture program for the Audouin’s gull and an intensive carnivore monitoring program to assess how the appearance and invasion of the study site by carnivores affects population dynamics. We evaluate changes in turnover of discrete breeding patches within the colony, age structure and breeding performance. Once carnivores entered the colony, the number of occupied patches increased, indicating a higher patch turnover. Breeders responded by moving to areas less accessible to carnivores. More importantly, the presence of carnivores caused differential (and density-independent) breeding dispersal: experienced, better-performing breeders were more likely to leave the colony than younger breeders. This differential dispersal modified the age structure and reduced the reproductive performance of the population. Our results confirm the importance experience in the study of populations. The role of differential dispersal for animal population dynamics might be more important than previously thought, especially under scenarios of global change.

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. Audouins 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 Audouins 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.