Daniel Oro

Influence of food availability on demography and local population dynamics in a long-lived seabird.

Few studies have addressed the effects of food availability as a proximate factor affecting local adult survival in long–lived organisms and their consequences at local population dynamics. We used capture–recapture analysis of resightings of 10 birth cohorts of ringed Audouins gulls, Larus audouinii, to estimate adult survival and dispersal (both emigration and immigration). For the first time, permanent emigration (the transient effect in capture–recapture analysis) was modelled for the whole population and not only for the newly marked birds. Gulls exploit to a large extent fishes discarded from trawlers, and a trawling moratorium established since 1991 has decreased food supply for the colony. This was used as a natural experiment of food availability to assess its effects on adult survival and emigration. These and other demographic parameters were used in a projection modelling to assess the probabilities of extinction of the colony under two scenarios of lower and higher food availability. Food availability (together with the age of individuals) influenced emigration probabilities, but not adult survival, which was estimated at 0.91 (s.e. = 0.02). When food was in shorter supply during the chick–rearing period, emigration was very high (ca. 65%) for younger breeders, although this rate decreased sharply with age. Probabilities of extinction were very high when food availability was low, and when environmental stochasticity was introduced, and only stochastic immigration from the outside seemed to prevent extinction. The results highlight the importance of dispersal processes in the population dynamics of long–lived organisms.

INFLUENCES OF FOOD AVAILABILITY AND PREDATION ON SURVIVAL OF KITTIWAKES

Life history theory predicts a trade-off between current reproduction and survival to maximize fitness. In long-lived seabirds, one might expect adults not to jeop- ardize survival by excessive reproductive investment when environmental conditions for breeding are adverse. Some environmental factors have reduced the fecundity of Black- legged Kittiwakes (Rissa tridactyla) breeding in Shetland, UK, a metapopulation that has declined substantially during the last two decades. Two hypotheses are proposed to explain this decrease: (1) reduced availability of sandeels (the main prey for kittiwakes), which has occurred since the late 1980s; and (2) increased predation by Great Skuas (Catharacta skua), the main predator of kittiwakes. Detailed data on food supply (sandeel stock size, by age groups), kittiwake body condition and breeding success, and diet and productivity of Great Skuas allowed us to assess the influence of each of these factors on adult kittiwake survival from 1986 through 1997. Capture-recapture models were constructed to examine correlations between local adult survival rate and environmental parameters. Body condition at the end of the breeding season and availability of 0-group (juveniles ? 1 yr old) sandeels positively influenced adult survival, whereas survival was lower when Great Skua pro- ductivity was high. Adult survival varied greatly during the study, from 0.53 + 0.04 to 0.98 ? 0.01 (mean + 1 SD), and was independent of sex. Both hypotheses were supported. Contrary to predictions of life history theory, kittiwakes did not refrain from breeding in years of poor food supply or high predator activity to buffer their survival at the cost of not rearing chicks. Although 0-group sandeel availability affected breeding success of kittiwakes, its influence on adult survival was also significant. The discovery that food supply affects adult survival rate has important implications for fisheries management to minimize impacts on seabirds.

Bias Associated with Diet Samples in Audouin's Gulls

We analyzed five different types of food samples from Audouins Gull (Larus audouinii), collected during the breeding seasons of 1994 and 1995 at its two main breeding colonies, the Ebro Delta and the Chafarinas Islands. These food samples included spontaneous regurgitates, dry boli containing partially digested food, food remains, pellets, and prey identified during direct observations of chick provisioning. We compared estimates of biomass, levels of taxonomic determination allowed by each kind of food sample, and the associated potential biases to assess which sampling method provides the best estimate of diet in gulls. Regurgitates allowed identification of most prey to species level and reliable biomass estimates, but their collection was time-consuming and invasive. Dry boli provided almost the same information as regurgitates at order level and were easy to collect. However, both underestimated soft-bodied prey and prey with large, hard parts. Food remains provided an estimate of diet composition that was highly biased towards prey with large distinctive hard parts. However, food remains were a good complement to dry boli, enhancing biomass estimates for food items that had a good relationship of weight and linear measurements of prey..hard parts. Direct observation allowed identification of prey only to upper taxonomic categories, and is useful when only a broad categorization of prey types is required. Pellets showed important biases towards fish with robust otoliths, and inaccurate conversion to biomass, but they can be useful for monitoring variations in the consumption of certain prey items. Several factors such as time spent collecting, sample availability, disturbance to animals, and the status of the species studied need to be considered when deciding on a method of diet assessment sampling.

Trophic niche width and overlap of two sympatric gulls in the southwestern mediterranean

Abstract The diets of two potential competitor species, Audouins Larus audouinii and yellow-legged gulls Larus cachinnans, were examined while they bred at the Chafarinas Islands during 1993, 1994, and 1995. Data were collected during two commercial fishing regimes: (1) trawling and purse seine fisheries, and (2) diurnal trawlers only. Since the food supply for the gulls in this area was heavily reliant on the activity of purse seine fisheries, these contrasting situations allowed us to analyze short-term effects, induced by daily changes in food supply, on niche width, dietary shift, and niche overlap between the two species. Overall, both species relied mainly on fish for food, especially Clupeiforms, in the case of Audouins gull irrespective of the fishing situation, and in the case of the yellow-legged gull, only when purse seine fishing was in operation. When purse seine boats did not operate (food shortage), yellow-legged gulls broadened their niche, consuming equal amounts of all the feeding resources, and they showed a dietary shift toward a greater consumption of prey from refuse tips. In contrast, Audouins gulls did not change their niche width, but showed a slight dietary shift away from the consumption of epipelagic fish, compensated by an increase in reliance on benthic-mesopelagic resources. Niche overlap was clearly higher on days when both fishing fleets operated, probably because a superabundant food resource facilitates high overlap without affecting coexistence between the two species. Since our study was developed on the basis of daily variations in food supply, and competition effects are to be expected on a longer-term basis, these changes can be seen as the outcome of the coexistence of two species in stable competitive equilibrium.

COLONIAL SEABIRD NESTING IN DENSE AND SMALL SUB-COLONIES: AN ADVANTAGE AGAINST AERIAL PREDATION'

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Is Laying a Large Egg Expensive? Female-Biased Cost of First Reproduction in a Petrel

ABSTRACT. Differential reproductive investment between sexes can lead to asymmetric costs of reproduction in birds. Long-lived procellariiform seabirds are single-egg layers with little sexual dimorphism and similar parental investment in incubation and chick rearing. However, sex-specific tasks exist at the beginning of the breeding season, including egg production by females (no courtship feeding by males in this group of species) and nest and mate guarding by males. Costs of reproduction could be evident during critical periods such as the first breeding attempt, because of inexperience in reproductive tasks or a higher proportion of low-quality individuals in young age classes, or both. Little is known about sex-specific costs of reproduction in monomorphic species, in which we expect costs to be similar. We investigated the effects of first reproduction on the subsequent survival of male and female European Storm-Petrels (Hydrobates pelagicus) and found that female survival (0.72; 95% confidence interval [CI]: 0.60–0.82) was lower than that of males (0.85; 95% CI: 0.76–0.89) after first reproduction. However, these differences were not observed in subsequent breeding attempts by experienced females (0.89; 95% CI: 0.86–0.91) and males (0.88; 95% CI: 0.86–0.91), probably because of an experience-related improvement in foraging efficiency, reproductive tasks, or predator avoidance. The effort invested by inexperienced females in the production of a large egg (≤25% of adult body mass) may explain our observed differences in survival.

Audouin’s gull breeders CR data

We acknowledge financial support through projects RESET (CGL2017-85210-P), IBISES (ref. CGL2013-42203-R) and MINOW (ref. H2020- 634495).