Sexual segregation in spatial and feeding ecology of seabirds

Abstract

Sexual Segregation (SS) is a phenomenon that occurs across a wide range of animal species, and that had been broadly categorized in spatial/habitat segregation and social segregation. SS of a given species is often related quantitatively to its Sexual Size Dimorphism (SSD), which frequently drives to sexual differences that can work as a mechanism to avoid competition between individuals of opposite sex. SS has been widely studied among terrestrial birds. The few existing studies on SS involving pelagic seabirds have focused on the breeding period, and the extent of SS in relation to environmental conditions and fisheries and its persistence during the non-breeding period remains poorly understood in this taxa. In this thesis, we aimed to understand the causes and consequences of SS in spatio-temporal distribution (inferred through geolocation and GPS-tracking data), migratory phenology (inferred through light-level data from geolocators), behaviour (inferred through immersion data from geolocators) and feeding ecology (inferred through Stable Isotope Analysis (SIA)) of three closely-related shearwaters: Scopoli’s, Cory’s and Cape Verde shearwaters (Calonectris diomedea, C. borealis, and C. edwardsii, respectively). To assess and understand potential sex differences within or between breeding (for Scopoli’s shearwater) and non-breeding periods (for the three Calonectris shearwaters), we adopted a multidisciplinary approach combining geolocation, immersion data, GPS-tracking, spatial modelling and SIA. Our findings pointed out that during the breeding period, females of Scopoli’s shearwater seem to be outcompeted by males and forced to increase their foraging effort, especially under unfavourable conditions. This result suggests unfavourable conditions induced an inter-sexual competition for limiting resources, and males forced females to forage further from the colony. Furthermore, males interacted with fishing vessels to a greater extent, profiting from discards more than females. Otherwise, during the non-breeding period, both sexes of the three Calonectris shearwaters shared the same non-breeding areas, suggesting competitive exclusion does not promote spatial segregation throughout the annual cycle. Sexual differences in the migratory phenology were subtle for the three species, with males consistently returning earlier to breeding colonies, and male Cory’s shearwaters remaining resident in a larger proportion than females, likely due to sex-specific reproductive roles at early stages of the breeding period. For both breeding and non-breeding periods, males consistently feed on higher trophic levels (inferred from higher δ15N) than females, suggesting that sexual differences in diet may persist year-round. For the breeding period, we found an increase of δ15N values correlated with fishing vessel attendance, indicating that sexual differences observed may mostly emerge from differential fishing vessel attendance and consumption of fishery discards between males and females. However, the influence of SSD in bill size in determining differences in diet remains unclear. Overall, we found consistent differences in foraging movements, migratory phenology, and resource use between sexes, indicating a robust SS during breeding and non-breeding periods in shearwater species. Furthermore, our results indicated that SS in foraging strategies of Scopoli’s shearwater may lead to unbalanced exposure of males and females to bycatch in the North-Western Mediterranean, which could reduce effective population size and compromise the population viability of the species. This thesis increase our knowledge about SS in Calonectris species, and show that species with slight SSD also present well-marked SS and that intrinsic and extrinsic factors can promote inter-sexual differences throughout the annual cycle. Furthermore, we highlight that SS in distribution and foraging behaviour of Scopoli’s shearwater, and probably in other seabird species, should be taken into consideration when implementing specific conservation plans in the marine environment.

La segregación sexual (SS) es un fenómeno habitual en animales, como consecuencia de diferencias entre sexos en la distribución espacio-temporal, el comportamiento o la ecología trófica. En aves marinas, el estudio de la SS se ha centrado principalmente en el período reproductivo. En cambio, su alcance e implicaciones en relación a las condiciones ambientales, así como su ocurrencia fuera del período reproductivo, han sido menos estudiadas. Nuestro objetivo fue comprender las causas y consecuencias de la SS en la distribución, fenología migratoria, comportamiento y ecología trófica de tres especies emparentadas: la pardela cenicienta mediterránea, cenicienta atlántica y de Cabo Verde (Calonectris diomedea, C. borealis y C. edwardsii, respectivamente). Para ello evaluamos las diferencias sexuales en el periodo reproductivo (en la pardela cenicienta mediterránea) y de invernada, y discutimos si dichas diferencias se extienden a lo largo del ciclo anual. Para el periodo reproductivo, nuestros resultados indicaron que las hembras de pardela cenicienta mediterránea son menos competitivas que los machos, viéndose obligadas a incrementar el esfuerzo de búsqueda de alimento, especialmente ante condiciones ambientales desfavorables. Además, los machos fueron más proclives a interaccionar con barcos pesqueros, haciendo mayor uso de descartes, pero exponiéndose a un mayor riesgo de captura accidental, lo que podría comprometer el tamaño efectivo de la población y su viabilidad en el Mediterráneo noroccidental. Fuera del periodo reproductivo, ambos sexos compartieron las zonas de invernada en las tres especies, indicando que la exclusión competitiva no causa, al menos a escala espacial, SS a lo largo del año. Ambos sexos se alimentaron en diferentes niveles tróficos, sugiriendo que la segregación trófica persiste a lo largo del año. Encontramos sutiles diferencias en el calendario migratorio, siendo los machos los primeros en retornar a las colonias, y un mayor porcentaje de machos que de hembras de la pardela cenicienta atlántica no migró, probablemente como estrategia ventajosa de cara a las etapas iniciales del periodo reproductivo. En general, encontramos evidencias de SS en los movimientos de búsqueda de alimento, fenología migratoria y ecología trófica, tanto durante el período reproductivo como en el período de invernada en las tres especies de pardela.