Time for seafood? Factors affecting land vs. sea foraging choice in breeding Mediterranean gulls

Abstract

In most bird species, breeding individuals behave as central-place foragers during reproduction, commuting regularly between the breeding site and foraging areas to find food for both self-maintenance and offspring rearing. Such movements are constrained by the need to find enough resources around the breeding site in a short time and may be modulated by several factors, including resource availability, weather, and breeding phase. By GPS-tracking 20 individuals across > 1400 foraging excursions, we characterised the foraging behaviour of Mediterranean gulls Ichthyaetus melanocephalus breeding in a coastal area, where they can forage in both terrestrial and marine habitats. Moreover, we investigated whether intrinsic factors and weather conditions affected the choice of performing foraging excursions targeting marine habitats. Females foraged closer to nesting sites than males during egg incubation compared to chick-rearing, whereas the range of male foraging excursions was unaffected by breeding stage. Overall, foraging excursions involved mostly farmland areas (43% of locations), inland waters (23%) and open sea (16%). Use of marine habitats increased from egg incubation to chick-rearing and was more frequent in males than females. Marine habitats were exploited mainly under favourable weather conditions, with small waves and moderate wind speed. Mediterranean gulls thus showed individually flexible foraging behaviour, expanding their trophic niche by increasingly relying on marine habitats late in the breeding season and possibly enhancing the provisioning of highly nutritious food to their offspring.

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