BIRDS OUTNUMBER INSECTS IN VISITING BRASSICA FLOWERS ON VENTOTENE ISLAND (CENTRAL MEDITERRANEAN)

European warblers migrating across the Mediterranean feed on nectar at stopover sites on islands, thus carrying pollen grains stuck to bills and plumage from one stopover to the next. Plants belonging to a single wild species of the Brassica oleracea group are largely used by songbirds for nectar feeding at Ventotene island and their pollen is commonly found on staging warblers. We studied the potential role of nectar feeding warblers in pollen dispersion of Brassica plants and compared it to that of insects. The study was carried out at Ventotene from the end of March to mid May during the years 2008 and 2009. Direct observations of bird and insect visits to plants showed that the frequency of bird visits was significantly higher than the frequency of flight-feeding insects, while no significant differences were found between the length of bird and insect visits. Plant caging with a 15 mm-mesh net in order to prevent bird visits, but not insect visits, resulted in a significant lowering of fruit production with respect to control plants. In summary, a mutualistic relationship could be hypothesised between songbirds and Brassica plants on Ventotene Island. Warbler species obtain a nutritional benefit through nectar consumption, and can transport pollen grains and potentially pollinate exploited plants across stopover sites, with possible implications for pollen carry-over from genetically engineered agricultural crop plants.