Spring in Seattle and an Anna's hummingbird mystery

I’m with a few other birders, walking slowly, when we see an Anna’s hummingbird doing something that surprises me. She’s hovering near a sprawling tangle of Himalayan blackberries along the water’s edge. As we watch, she moves methodically from one blackberry leaf to the next, probing the edges of the undersides with her slender beak. Hummingbirds are well-known for feeding on flower nectar… but the blackberries are nowhere near flowering yet. What is she doing?