Intercolony variations in movement patterns and foraging behaviors among herring gulls (Larus argentatus) breeding in the eastern Wadden Sea

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. δ¹³C and δ¹⁵N 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.

Keywords: GPS tracking; Larus argentatus; foraging strategy; pellet; stable isotope analysis.

Figure 1

Adult herring gull with prey. Photograph credit: Sven Sturm

Figure 2

Study area—Oland, Langeness, Amrum. Inserted map: location of the study area within northern Europe

Figure 3

Foraging trips of tracked Larus argentatus (n = 37) breeding at three different colonies in the German North Sea (2012–2015)

Figure 4

Carbon isotope ratios in red blood cells of herring gulls from breeding colonies on Amrum, Langeness, and Oland. Boxes represent 50% of the data between the first (25%) and the third (75%) quartile. Upper and lower whiskers represent maximum and minimum, respectively. Horizontal line marks the median. Level of significance: ***<0.001; **<0.01; *<0.05; n.s.: not significant

Figure 5

Nitrogen isotope ratios in red blood cells of herring gulls from breeding colonies on Amrum, Langeness, and Oland. For explanation of box plot, see Figure 4. Level of significance: ***<0.001; **<0.01; *<0.05; n.s.: not significant