Contrasting diet quality of northern gannets Morus bassanus at two colonies

The diet of Northern Gannets Morus bassanus during chick rearing was determined at two colonies in the UK and Ireland that differed markedly in size (the Bass Rock with c. 40 000 breeding pairs, and Great Sallee with 2000 breeding pairs). Gannets from the Bass Rock took a significantly higher proportion of Mackerel Scomber scombrus than birds from Great Saltee. There was no significant difference between colonies in the proportion of sandeels (Ammodytidae), clupeids (Clupeidae) or gadoids (Gadidae), but birds from Great Saltee took a significantly higher proportion of other species than birds from the Bass Rock. These differences in diet between the colonies resulted in the average energy density of food loads being significantly higher at the Bass Rock than at Great Sallee. This pattern may reflect differences in the distribution and abundance of prey around the two colonies, but intrinsic factors may also be important, and these warrant further investigation.