FEEDING SPECIALIZATION AND PREFERENCE IN HERRING-GULLS

(1) Previous claims that herring gulls have preferences for particular foods or that they specilize in them have been based on food remains at the nest and repeated observations of tagged individuals. The complex temporal pattern of food availability and the constraints of incubation make such observations hard to interpret. (2) We devise a ratio index of feeding specialization for herring gulls using a strict definition of food availability which takes into account the necessity for one member of a pair to guard the nest at all times during incubation. We define preference in relation to choices between simultaneously available foods. (3) Twelve pairs of herring gulls (Larus argentatus Pont.) were tagged with radios and their whereabouts and behaviour away from the large breeding colony at Walney Island, Cumbria, U.K. were continuously monitored for a total of 997 h during the incubation seasons of 1976 and 1977. (4) The ratio specialization index showed that most individuals specialized in one or more of the available foods; the variety of foods taken is not solely dependent on availability. (5) Most individuals did not use at least one food that was available to them. Where a food was available at alternative sites individuals were usually seen exclusively at one of them. (6) Individuals repeated earlier choices between simultaneously available foods on eighteen out of nineteen occasions. Individuals had characteristic preference orderings between foods. (7) The time budget of individuals was correlated with their food specialization indices for tips and terrestrial invertebrates but not for intertidal foods.