Numbers, behaviour and origin of Mediterranean Gulls (Larus Melanocephalus) wintering along the west coast of southern Portugal

Several thousands of Mediterranean Gulls Larus melanocephalus were observed in the winters of 2004/2005 and 2005/2006 at two sites of the West coast of Southern Portugal. Along the Lisbon coast (Guincho-Cascais-Estoril-Parede, west of Lisbon) a maximum of 6,000 birds was observed and along the Alentejo coast at least 1,000 birds were present in the Mira river mouth, near Vila Nova de Milfontes. With an unknown, but probably considerable number of birds wintering along the Algarve coast, the total number of birds wintering in Portugal south of Cabo da Roca is estimated at a minimum of 7,000-8,000 birds. This estimate is six times higher than the previous estimate in the nineties of 1,200 wintering birds. This increase is likely explained by the increase of the breeding population in Western and Central Europe. Behavioural observations suggest that along the Lisbon coast the birds mainly feed offshore. A substantial proportion of the birds (up to 4,000 birds) fed on the outflow of an offshore sewage exit west of Cascais, but birds also foraged on natural prey (along front lines). Whether fishery discards is an important food source far offshore, like it is in the Mediterranean, is unknown. Sightings of colour ringed birds indicate that birds came from Belgium, the Netherlands and France with a small proportion coming from other countries (ranging from North-western Europe to as far east as the Ukraine). A low re-sighting rate of individual colour ringed birds suggests that a very high number of ringed birds wintering in Portugal remained unnoticed so far. While the total number of Mediterranean Gulls demonstrates that the Portuguese coast south of Cabo da Roca is the most important known wintering site along the European and North-African Atlantic coast, the first data gathered from colour ring readings highlight the importance of this area as a wintering ground for the Mediterranean Gulls that breed in North-western Europe.