DIET OF BLACK-HEADED GULLS LARUS-RIDIBUNDUS ON EMERGED INTERTIDAL AREAS IN THE TAGUS ESTUARY (PORTUGAL) - PREDATION OR GRAZING

The Black-headed Gull Larus ridibundus is the most abundant bird speci es in the Tagus estuary all year round, using intertidal areas as feed ing grounds. Their diet varied according to the season: during summer ragworms Nereis diversicolor were the main prey while during winter gu lls took siphons of the bivalve Scrobicularia plana, which represented more than 80% of the total ingested biomass. Average siphon feeding r ate ranged from 7.9 to 17.0 siphons min(-1), and the estimated size of siphons taken ranged from 6.5 to 32.0 mm. Other prey taken were Hydro bia ulvae and Carcinus maenas. The ingestion of bivalve siphons is a s pecial type of predation whose ecological implications are discussed b oth for energy fluxes through the estuarine ecosystem and for the prey population.