Testing for sub-colony variation in seabird foraging behaviour: ecological and methodological consequences for understanding colonial living

Intraspecific interactions have important roles in shaping foraging behaviours. For colonial species such as seabirds, intense competition for prey around colonies may drive differences in foraging behaviour between age-classes and sexes or lead to individual specialisation. While much research has focussed on understanding these differences in foraging behaviour, few studies have investigated the possibility of sub-colony foraging asymmetries within colonies. Such knowledge could improve our understanding of the ecological processes associated with colonial living. It may also have important methodological implications in studies where the foraging behaviours recorded from individuals in a small number of sub-colonies are assumed to be representative of those from the colony as a whole. Here, we use GPS loggers and stable isotope analysis of red blood cells to test for differences in foraging behaviour among 7 sub-colonies of a large northern gannet Morus bassanus colony over 3 yr. We found no instances of statistically significant differences in foraging behaviour among sub-colonies. Although complimentary in situ observations found similarities among neighbours' departure directions, these results may be attributable to wind vectors. We therefore conclude that sub-colony foraging asymmetries are either limited or absent in northern gannets. However, given the current lack of knowledge across seabird species, we urge similar studies elsewhere.