The Phylloscopus leaf warblers are hyperactive wee birds that specialise in feeding on aphids. Aphids are obligate plant sapsuckers who consume vast amounts of sugar-rich plant sap -so much so that they excrete large amounts of sugary liquid as a by-product. It follows that aphids are a high energy food source which may explain how Phylloscopus warblers are able to maintain high activity.

I had always assumed that these warblers used their bills like forceps to pick up insects however, on watching this bird carefully it was apparent that it was using its tongue to lap up the sap-sucking bugs. I can't find any reference to this in the literature.