Nectar consumption of warblers after long-distance flights during spring migration

The consumption of nectar by European passerines has been reported only occasionally. In this study, we investigated the occurrence and significance of nectar consumption of small passerine birds on spring migration after crossing the Mediterranean Sea. On Ventotene Island in the Tyrrhenian Sea, four migrating species of Sylvia warblers (Garden Warbler S. borin, Subalpine Warbler S. cantillans, Whitethroat S. communis, Blackcap S. atricapilla) regularly foraged on the two most common flowering species at that time of year, Brassica fruticulosa (Cruciferae) and giant fennel Ferula communis (Umbelliferae, while other species visited flowers only occasionally or not at all. Feeding behaviour, pollen traces on the head, and the examination of pollen and sugar remains in droppings indicated that nectar was the main target of the Sylvia warblers, rather than pollen or insects on the flowers. This was confirmed by food choice experiments indicating a clear preference by Garden Warblers and Whitethroats for nectar from artificial flowers over mealworms. Although conclusive experiments are not available, we hypothesize that nectar might be a diet easy to obtain and to absorb for birds after a long-distance flight in which they have incurred a depletion of energy stores and a reduction of the digestive tract.