Habitat use across the tidal cycle by black-headed gulls breeding in the Wadden Sea

Movements of birds foraging in intertidal areas are often strongly linked to the tidal cycle, as water levels determine where and when birds can forage. The strength of this link likely depends on the ability to forage in habitats other than intertidal areas and on constraints imposed by breeding duties. Few studies have focused on the use of intertidal areas by generalists, such as the black-headed gull Chroicocephalus ridibundus, that occupy a wide variety of habitats within and beside intertidal mudflats. We investigated to what degree black-headed gulls (1) use intertidal mudflats versus terrestrial habitats during different phases of the tidal cycle, (2) follow the tidal wave to exploit recently exposed mudflats and (3) whether these behaviours are influenced by central place foraging. For this, 11 black-headed gulls breeding on the Wadden Sea island Griend were tracked during two years using GPS loggers. When commuting to and from Griend, up to 75% of their time was spent in intertidal areas during low and incoming tide, which increased to 92% when not behaving as central place foragers. While their movements were strongly linked to the tidal cycle, they did not follow the tidal wave across the tidal basin during either period. Rather, individuals foraged either predominantly west or east of Griend during low and incoming tide and mostly remained visiting these areas when not behaving as central place foragers. As one of the most abundant species in the Wadden Sea, the extensive use of intertidal mudflats highlights the importance of black-headed gulls within the intertidal food web of the Wadden Sea.