Abstract
Herring gulls (Larus argentatus) are opportunistic predators that prefer to forage in the intertidal zone, but an increasing degree of terrestrial foraging has recently been observed. We therefore aimed to analyze the factors influencing foraging behavior and diet composition in the German Wadden Sea. Gulls from three breeding colonies on islands at different distances from the mainland were equipped with GPS data loggers during the incubation seasons in 2012–2015. Logger data were analyzed for 37 individuals, including 1,115 foraging trips. Herring gulls breeding on the island furthest from the mainland had shorter trips (mean total distance = 12.3 km; mean maximum distance = 4.2 km) and preferred to feed on the tidal flats close to the colony, mainly feeding on common cockles (Cerastoderma edule) and shore crabs (Carcinus maenas). In contrast, herring gulls breeding close to the mainland carried out trips with a mean total distance of 26.7 km (mean maximum distance = 9.2 km). These gulls fed on the neobiotic razor clams (Ensis leei) in the intertidal zone, and a larger proportion of time was spent in distant terrestrial habitats on the mainland, feeding on earthworms. δ 13C and δ 15N values were higher at the colony furthest from the mainland and confirmed a geographical gradient in foraging strategy. Analyses of logger data, pellets, and stable isotopes revealed that herring gulls preferred to forage in intertidal habitats close to the breeding colony, but shifted to terrestrial habitats on the mainland as the tide rose and during the daytime. Reduced prey availability in the vicinity of the breeding colony might force herring gulls to switch to feed on razor clams in the intertidal zone or to use distant terrestrial habitats. Herring gulls may thus act as an indicator for the state of the intertidal system close to their breeding colony.
Highlights
Spatial movements and the foraging ecology of seabirds provide important information on food availability and potential habitat changes
Herring gulls in our study showed a wide range of foraging behaviors, from intertidal to combined intertidal–terrestrial strategies. This switch was driven by the distance between the colony and the mainland, the level of the tide, the time of day, and the food availability near the breeding colony
Behavioral observations, and analyses of pellet and logger data demonstrated that herring gulls utilized the nonindigenous species Ensis leei as new prey source
Summary
Spatial movements and the foraging ecology of seabirds provide important information on food availability and potential habitat changes. Breeding herring gulls at the coast showed tide-dependent foraging patterns (Mendel et al, 2008; Sibly & McCleery, 1983). Similar observations have been made for lesser black-backed gulls (Larus fuscus), previously known as a predominantly marine species, but which is increasingly adopting a dual foraging strategy utilizing both marine and terrestrial habitats (Isaksson, Evans, Shamoun-Baranes, & Akesson, 2016), possibly as a result of food depletion at sea (Garthe et al, 2016; Votier et al, 2004). A multi-colony study of lesser black-backed gulls based on individual movement patterns revealed that foraging behaviors differed between neighboring colonies (Corman, Mendel, Voigt, & Garthe, 2016)
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