Rafting behaviour in the Northern Gannet Morus bassanus: evidence for social information transfer

When departing from or returning to their breeding colonies, many seabirds land on the water adjacent to the colony, often forming large aggregations numbering many hundreds of birds. This behaviour is known as ‘rafting.' Despite its prevalence, rafting remains poorly understood for most species. One general hypothesis is that rafting allows birds to avoid predators or preen away from the colony and is therefore influenced by environmental or abiotic factors. However, some studies have suggested that rafting facilitates the transfer of social information about foraging locations by allowing birds to orientate to the bearings of conspecifics returning from prey patches at sea. We investigated rafting in a large pelagic seabird, the Northern Gannet Morus bassanus, to address two hypotheses: (1) that rafts are a source of social information on the direction of recent foraging grounds and (2) that rafting is influenced by local abiotic conditions. At a large colony in Newfoundland, Canada, we recorded the return and departure directions of gannets, as well as raft size and orientation throughout the day. We also recorded wind strength and precipitation. Consistent with the social information hypothesis, we found that gannets departing from the raft tended to depart in the direction of returning foragers, whereas those departing from the colony did not. We found no correlation between the orientation of the raft and the bearings of returning foragers but did find that rafts were largest in the mornings when, we suggest, the need for social information might be greatest. Raft size, however, was not influenced by the abiotic factors we measured. We therefore conclude that, while abiotic influences do not significantly affect the rafting behaviour of gannets (at least within the range of abiotic conditions for which observations were possible), rafting may serve a social information gathering purpose for gannets.