The foraging behaviour and population dynamics of the northern gannet
The northern gannet Morus bassanus population has been increasing in size over the last century. Larger gannet colonies in the British Isles are growing more slowly per capita than the smaller ones. For the first time, this study provides empirical evidence consistent with the hypothesis that seabird populations are regulated through density dependent competition for food (a theory proposed four decades ago). On average, gannets from larger colonies have to travel further to forage than birds from smaller colonies, leading to reduced provisioning rates at larger colonies. Differences were found in the foraging behaviour of male and female gannets. Females dived deeper and spent more time resting on the sea surface than males. Females also tended to forage in a specific location whereas males were much more variable. This was the first demonstration of sex-specific foraging behaviour in a monomorphic seabird. Foraging location and depth may be driven by sex-specific dietary needs, differences in foraging efficiency or competitive ability. State dependent foraging decisions were examined. Typically, gannets alternate foraging at sea with periods attending the chick at the nest. However, sometimes the attending parent will leave the chick alone. This decision is influenced by the prolonged absence of the partner and a low arrival mass of the adult, prior to its attendance shift. Unattended foraging trips were shorter in duration and therefore closer to the colony than attended trips. Since leaving the chick unattended is risky, there is a trade-off between offspring predation and short-term food requirements. When aspects of the foraging behaviour of gannets were compared between a large and small colony, both striking contrasts and similarities were evident. At the larger colony there were no sex differences in trip duration or the propensity to leave the chick alone whilst at the smaller colony males made significantly shorter trips than females and left the chick alone much more than females. However at both colonies unattended trips were shorter than attended trips. These results suggest interactions between density dependent competition and sex-specific individual behaviour. Identifying important areas for foraging is a major challenge for marine conservation. This study has demonstrated that colony size, sex and condition all affect the foraging decisions of the northern gannet.