Nectarivorous birds are those that rely predominantly on the products of flowering trees and shrubs for food. Australian avian nectarivores include honeyeaters of the family Meliphagidae and lorikeets and swift parrots (Lathamus discolor) of the family Psittacidae. Nectar is a sugar-rich, liquid food source that provides abundant amounts of energy for birds that are able to harvest it. However, it contains very low levels of amino acids, vitamins, and trace minerals necessary for avian maintenance, growth, and reproduction. Therefore, birds classified as nectarivores need to forage for other food resources. Manna, honeydew, and lerp are common food sources for a variety of honeyeaters and nectarivorous parrots; these foods are high in carbohydrates and low in protein, similar to nectar. Pollen protoplasm is composed of highly digestible protein and contains a diverse amino acid profile; however, only 3 Australian species of psittacine birds have been recorded engaging in active pollen harvesting. Insects are thought to be the main source of protein for nectarivores. Nectarivorous birds have developed a variety of morphologic and physiologic adaptations. Morphologic adaptations to nectarivory include changes in body size, plumage, beak and tongue structure, and the alimentary organs. The physiology of nectarivory is still poorly understood, but there are indications that adaptations may include lowered metabolic rates, lowered protein requirements, and changes in digestive and renal physiology. Considerable work is needed to illuminate the specific nutritional requirements of nectarivores for maintenance, growth, and reproduction.