I was watching male bumblebees around the cherry tree a few days ago when I saw some honeybees around. On close inspection, I noticed they were visiting extrafloral nectaries, little red swellings in the petiole of cherry leaves, which secrete nectar, usually taken by ants. The bee would visit several leaves in succession, but being high in the tree, it was tricky to get good shots. I noticed an early bumblebee, Bombus pratorum, doing the same. So, even when not in bloom, cherry trees are producing nectar used by bees and bumblebees. Many other plants have these nectar glands, including willows, cherry laurel and plums.