The Distribution between Feeding Sites of Herring Gulls Breeding at Walney Island, U.K.

(1) The major foods of herring gulls breeding at Walney were earthworms and terrestrial invertebrates obtained mainly on pasture fields within an hour of sunrise, domestic waste obtained mainly on refuse tips from 08.30 to 16.30 h on weekdays and 08.30 to 11.30 h on Saturday mornings, Mytilus edulis and Carcinus maenas obtained mainly when the height of water was below 1.9 m below O.D., and Asterias rubens obtained mainly when the height of water was below 3.1 m below O.D. (2) The minor foods included Cancerpagurus, Macoma balthica, marine fish, fish offal discarded by fishermen, eggs and chicks obtained by cannibalism, sheep and cattle food obtained from fields, and waste food scraps obtained from litter bins, streets and gardens. (3) Earthworm availability was in part determined by the wetness of the ground. (4) More gulls (herring and lesser black-backed) were seen at refuse tips which received more domestic waste. At refuse tips further from the colony fewer of the adult gulls present were herring gulls, and more were lesser black-backed gulls. (5) More gulls were seen at Walney refuse tip if more domestic waste had been unloaded in the previous 1 5 h. (6) The number of herring gulls feeding at mussel skears fluctuated with the tidal cycle. Herring gulls ate mainly 1st year mussels >3 mm which were ground into small fragments in the gizzard, the residue being evacuated in the faeces. (7) The ideal free distribution predicts the distribution of gulls between refuse tips but leaves as a paradox the low energy returns obtained by herring gulls feeding at other sites when the tip was open.