Offshore foraging of Mediterranean Gulls Larus melanocephalus in Portugal during the winter

Movements and behaviour of Mediterranean Gulls Larus melanocephalus were observed along the Atlantic coast of central Portugal in December 2001. The observations suggest that at least part of the Mediterranean Gull population wintering in central Portugal feeds extensively at sea and possibly during the night. In the morning, Mediterranean Gulls flew in straight lines towards the coast, where they settled on water to roost in flocks of several tens of birds. Because there were no gulls near the coast at sunrise, it is assumed that these arrivals were Mediterranean Gulls that had spent the night out at sea. With an estimated ‘ground speed’ corrected for wind speed, it was estimated that the birds could have covered several tens of kilometres offshore between sunrise and the peak of coastal arrivals. One late afternoon, there were no roosting birds at the coast, but foraging birds were observed which were apparently successful. The question is raised whether the foraging activity is solely restricted to the hours around sunrise and sunset, or whether it is truly nocturnal. Changes in prey availability resulting from vertical migrations in the water column may have influenced diurnal patterns in foraging activity of the gulls. Alternatively, the possible utilisation of discards cannot be ruled out, since night-active purse seine vessels normally discharge discards early morning. However, outside the breeding season, the Mediterranean Gull is so far only known to attend trawlers during the day.