Pollen couriers across the Mediterranean: the case of migrating warblers

At stopovers sites in the Sahara and on Mediterranean islands, European warblers may feed on nectar. As a consequence of this behaviour the pollen of exploited plants often sticks on the birds' bill or plumage. We analyzed pollen samples carried by migrating warblers staging at Ventotene Island (Italy), a stopover site located in the Central Mediterranean and reached by large numbers of birds after a direct flight from North Africa. We discuss the possible role of migrants as pollen vectors. A total of 38 plant taxa were recorded in the pollen samples collected on 147 migrants. Pollen of Brassica sp., Ferule communis, Citrus sp. and Eucalyptus sp. were most commonly found. The nectar of the first two plants is largely exploited by warblers on Ventotene and the presence of their pollen on plumage is principally due to nectar feeding in situ. On the contrary, Citrus sp. and Eucalyptus sp., which are scarce and absent on the island respectively, were mainly collected on dried and agglutinated samples and their presence is likely to be due to nectar feeding at other sites than Ventotene. The other 34 plant taxa were recorded with low frequencies. Overall, the data suggest that nectar feeding migrants could play a role as pollen vectors only for Citrus sp., Eucalyptus sp., F communis and Brassica sp. However, the other less common plant taxa could also be useful markers of migration routes. We found species-specific relationships between bird species and the plants used for nectar feeding: short-distance migrants (mainly Eurasian Blackcap Sylvia atricapilla and Subalpine Warbler S. cantillans) land on Ventotene when Brassica sp. flowers are more abundant and they mainly carry the pollen of this species. On the contrary, long-distance migrants (Garden Warbler S. born and Common Whitethroat S. communis) principally transport the pollen of F communis, whose flowering period takes place at the same time as their staging period on the island. The study underlines the tendency of warblers towards nectar feeding during migration