This paper addresses the question of how, and under what ecological circumstances, bird pollination will be optimal for a plant, and which or how many of the available nectar-feeding bird species will be optimal pollen vectors.

Pollination by birds is energetically expensive for the plants, and should accur only when birds can mediate optimal patterns of pollen flow and seed set. Each nectar-feeding bird has potential advantages and disadvantages as a pollen vector, related to its size, morphology, and foraging behavior. Which available bird is the optimal pollinator depends on the plant's growth habit, spatial distribution, and breeding system. The various adaptations shown by plants favoring one pollinator over another all revolve around the secretion of nectar and the manner of presenting it to the birds. However, other aspects of plant morphology, physiology, ecology, or life cycle may affect the production and presentation of nectar, and influence plant-pollinator coevolution. Many question remain regarding the interrelations between pollination and the total biology of the plant; bird pollination systems may prove fruitful in yielding meaningful answers.