Foraging, feeding and time‐activity niches of eight species of breeding seabirds in the coastal wetlands of the Adriatic Sea

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Abstract
We studied the three main components of trophic niches in the richest assemblages of breeding gulls and terns in Italy (8 species) that typify the communities of the large deltas of Mediterranean Europe. Each species feeds mainly on fish, except for the gull‐billed tern that feeds essentially on terrestrial vertebrates. Other prey contributed slightly to the energy balance, although small invertebrates prevailed numerically in the diets of the Mediterranean and black‐headed gulls. The seabirds cumulatively consumed a variety of fish, in frequencies that closely reflected the latter's abundance. Selection of fish species by each seabird were strongly determined by the bird's foraging habitat, and not by any preference for particular prey. The size of the prey was related to the size of the seabird. The diurnal rhythms of feeding activity were uniform, and the time component did not contribute to niche segregation. Dietary patterns suggest that within the overall space of their niches, the seabirds select trophic resources mainly through preferences for specific types of foraging habitats with their associated prey types. In the selected habitats, various prey types are exploited in relation to availability, and prey is selected on the basis of size. Although it is well known that interspecific competition is not proportional to the overlap in resource use, in our community of seabirds evidence that competition promotes niche segregation comes from the observation that overlap in the foraging and feeding niches are clearly complementary, and that the foraging niches and the preferred prey size seem to be regularly rather than randomly spaced. We suggest that the seabirds segregate their trophic niches by selecting different macro‐habitats, while their overlap in food is high, thus confirming earlier reports that habitat is more important than food in the segregation of trophic niches.