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.

Birds are drawn to nectar due to its high energy content and easy consumption. Nectar, produced by flowering plants to attract pollinators, is a sweet liquid with simple sugars like glucose and sucrose. The birds’ efficient digestive system helps them extract the maximum energy from drinking nectar, providing them with a quick burst of energy.

Since nectar is predominantly sugar, it requires a particular enzyme called sucrase to dissolve in the organism and be used for energy. Unfortunately, some birds don’t have the needed enzymes and can’t consume nectar. Birds that drink nectar have a higher survival rate. They’re more adaptable and persist in regions with no other food source.