Reduced foraging effort at the expanding range margin could facilitate poleward shift in a colonial seabird, the Northern gannet Morus bassanus

Abstract

**Abstract:** Species distributions are shifting poleward in response to climate change, but not all with the same pace or pattern. In particular, colonial predators may struggle to keep pace with prey because colony formation is rare. Despite its importance, little is known about the complex process of range shifting in colonial animals. Seabirds provide a useful model, as they are tied to land in the breeding season, but their marine prey is not. Existing colonies provide defence, mates and information, but also cause intense competition for food, which is reflected in high foraging effort as individuals must travel further and search for longer. As such, smaller colonies can expand more quickly than larger colonies, particularly if nearby foraging grounds attract recruits. The presence of nearby colonies also increases foraging effort due to competition, suggesting a possible benefit to being at range margins. Here we examine foraging trip duration and range, using data obtained from bird-borne loggers, to quantify effort across a large latitudinal gradient (48.15°N to 71.23°N) in the northern gannet Morus bassanus. This wide-ranging marine predator provides a rare example of a well-documented poleward-shifting seabird. We collated data representing 850 breeding gannets from 20 of the 54 currently occupied colonies. Trip duration and range both increased substantially with colony size. However, after controlling for colony size, trip duration and range also decreased significantly with latitude suggesting suitable conditions for gannets on the northern edge of their range but poor conditions at the southern edge. Lower foraging effort at the expanding range margin may allow distribution shift in colonial species in response to environmental change if the barriers to colony formation can be overcome. **Authors:** Bethany Clark¹, Freydís Vigfúsdóttir², Sarah Wanless³, Keith Hamer⁴, Thomas Bodey⁵, Jez Blackburn⁶, Sam Cox⁷, Stefan Garthe⁸, David Grémillet⁹, Amélie Lescroël¹⁰, William Montevecchi¹¹, Pascal Provost¹², Ewan Wakefield¹³, Stephen Votier⁵ ¹BirdLife International, ²University of Iceland, ³Centre for Ecology and Hydrology, ⁴University of Leeds, ⁵University of Exeter, ⁶British Trust for Ornithology, ⁷Centre d'Etude Biologique de Chizé, ⁸Kiel University, ⁹University of Montpellier, ¹⁰Point Blue, ¹¹Memorial University of Newfoundland, ¹²Ligue pour la Protection des Oiseaux, ¹³University of Glasgow