Hummingbirds are very distinctive in their form and behavior, the evolution of which is tightly connected to the evolution of their primary source of energy - floral nectar. About forty million years ago, the practical use of this dense fuel, available only in widely-dispersed, insect-sized aliquots - it was originally intended for insect pollinators - presented a severe test to the avian bauplan. This selective pressure forced broad changes in form and function, affecting anatomical structures ranging from the feeding apparatus to the locomotor system. We describe here how these pressures shaped a bird that flies like a bird into one that flies like a fly.