Pollination refers to the transfer of pollen to the stigma of the flower. Pollination may occur through the actions of pollinators or by the wind. Types of pollinators, such as bees, birds, bats and butterflies, differ in the morphology of their feeding structures and their nutritional requirements. The morphology, color, odor, and reward (nectar and/or pollen) of flowers have evolved to attract a particular type of pollinator. Often, one can guess the type of pollinator that visits a flower based on the characteristics of that flower.