Very few organisms consume nectar exclusively over their whole life cycle, either supplementing it with other sources, particularly insects (thus overlapping with insectivores) or only consuming it exclusively for a set period. Many species are nectar robbers or nectar thieves, performing no pollination services to a plant while still consuming nectar. Nectar-feeding is widespread among birds, but no species consumes nectar exclusively. Most combine it with insects for a mixed diet. Of particular interest are four lineages of specialized nectar consuming birds in the New World: the Hummingbirds (Trochilidae) and three members of the Tanager (Thraupidae) family; Bananaquits, Flowerpiercers and Honeycreepers. These groups have adapted to permit a nectar-central diet, showing higher activity of digestive enzymes which break down sugars, higher rates of absorption of sugars, and altered kidney function. Birds need the enzyme sucrase in their bodies, in order to digest the sucrose of nectar. And most simply don’t have enough. Scientists think birds that can readily digest sugar, like warblers, have an adaptive advantage. When they fly to the tropics for the colder months, they can tap into sources of sugar that other birds just can’t handle. That sweet tooth, it turns out, is important to their survival.