Terrestrial and Marine Foraging Strategies of an Opportunistic Seabird Species Breeding in the Wadden Sea

<div><p>Lesser black-backed gulls <i>Larus fuscus</i> are considered to be mainly pelagic. We assessed the importance of different landscape elements (open sea, tidal flats and inland) by comparing marine and terrestrial foraging behaviours in lesser black-backed gulls breeding along the coast of the southern North Sea. We attached GPS data loggers to eight incubating birds and collected information on diet and habitat use. The loggers recorded data for 10–19 days to allow flight-path reconstruction. Lesser black-backed gulls foraged in both offshore and inland areas, but rarely on tidal flats. Targets and directions were similar among all eight individuals. Foraging trips (n = 108) lasted 0.5–26.4 h (mean 8.7 h), and ranges varied from 3.0–79.9 km (mean 30.9 km). The total distance travelled per foraging trip ranged from 7.5–333.6 km (mean 97.9 km). Trips out to sea were significantly more variable in all parameters than inland trips. Presence in inland areas was closely associated with daylight, whereas trips to sea occurred at day and night, but mostly at night. The most common items in pellets were grass (48%), insects (38%), fish (28%), litter (26%) and earthworms (20%). There was a significant relationship between the carbon and nitrogen isotope signals in blood and the proportional time each individual spent foraging at sea/land. On land, gulls preferentially foraged on bare ground, with significantly higher use of potato fields and significantly less use of grassland. The flight patterns of lesser black-backed gulls at sea overlapped with fishing-vessel distribution, including small beam trawlers fishing for shrimps in coastal waters close to the colony and large beam-trawlers fishing for flatfish at greater distances. Our data show that individuals made intensive use of the anthropogenic landscape and seascape, indicating that lesser black-backed gulls are not a predominantly marine species during the incubation period.</p></div

δ¹³C and δ¹⁵N values from red blood cell and blood plasma in relation to the proportion of foraging spent at sea for the six lesser black-backed gulls for which information was available.

<p>One red blood cell and one blood plasma value are given for each individual. Bold lines: regression lines of linear model; dotted lines: 95% confidence interval.</p

Availability of terrestrial habitat types and their uses as feeding spots by lesser black-backed gulls tagged in the colony on Spiekeroog from 17 May to 4 June 2010.

<p>Availability of terrestrial habitat types and their uses as feeding spots by lesser black-backed gulls tagged in the colony on Spiekeroog from 17 May to 4 June 2010.</p

First passage time analysis of foraging behaviour of lesser black-backed gulls nesting on Spiekeroog in 2010.

<p>First passage time analysis of foraging behaviour of lesser black-backed gulls nesting on Spiekeroog in 2010.</p

Frequency distribution of (a) foraging-trip duration, (b) foraging range, and (c) total distance travelled for foraging trips targeting the sea (blue bars) and land (grey bars).

<p>Bars show the mean percentage over all eight individuals, and the vertical lines represent the standard error.</p

Flight tracks of eight lesser black-backed gulls breeding on Spiekeroog.

<p>Each colour represents all tracks from one individual. The study period was from 17 May to 4 June 2010. The location of the colony on the island of Spiekeroog in the south-eastern Wadden Sea is indicated by a triangle. Areas with water depth <5 m represent the maximum extension of tidal flats during low tide.</p

Flight tracks and zones of area-restricted search (ARS) for one lesser black-backed gull breeding on Spiekeroog in 2010.

<p>The location of the colony on the island of Spiekeroog is indicated by a triangle.</p

Relative uses of sea and land areas during foraging trips of lesser black-backed gulls breeding on Spiekeroog in 2010 over a 24-h cycle.

<p>Bars show the mean percentages of time spent at sea (blue bar) or on land (grey bar) for all eight individuals.</p