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Ornithophily or bird pollination is the pollination of flowering plants by birds. This coevolutionary association is derived from insect pollination (entomophily) and is particularly well developed in some parts of the world, especially in the tropics and on some island chains. The association involves several distinctive plant adaptations forming a "pollination syndrome". The plants typically have colourful, often red, flowers with long tubular structures holding ample nectar and orientations of the stamen and stigma that ensure contact with the pollinator. Birds involved in ornithophily tend to be specialist nectarivores with brushy tongues, long bills, capable of hovering flight or are light enough to perch on the flower structures.
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Plant adaptations
Bird pollination is considered as a costly strategy for plants and it evolves only where there are particular benefits for the plant. High altitude ecosystems that lack insect pollinators, those in dry regions or isolated islands tend to favour the evolution of ornithophily in plants.
Plants adaptations can be grouped into mechanisms that attract birds, those that exclude insects, protect against nectar theft and pollination mechanisms in the strict sense. The ovules of bird flowers also tend to have adaptations that protect them from damage.
Most bird pollinated flowers are red and have a lot of nectar. They also tend to be unscented. Flowers with generalist pollinators tend to have dilute nectar but those that have specialist pollinators such as hummingbirds or sunbirds tend to have more concentrated nectar. The nectar of ornithophilous flowers vary in the sugar composition, with hexoses being high in passerine pollinated species while those that are insect pollinated tend to be sucrose rich. Hummingbird pollinated flowers however tend to be sucrose rich. Many plants of the family Loranthaceae have explosive flowers that shower pollen on a bird that forages near it. They are associated mainly with flowerpeckers in the Dicaeidae family. In Australia, some species of Banksia have flowers that open in response to bird actions thereby reducing the wastage of pollen. In tropical dry forests in southern India, ornithophilous flowers were found to bloom mainly in the hot dry season. As many as 129 species of North American plants have evolved ornithophilous associations. Nearly a fourth of the 900 species of the genus Salvia are bird pollinated in the South African region. Tropical China and the adjacent Indochinese countries harbor relatively few bird-pollinated flowers, among them is Rhodoleia championii, a member of the Hamamelidaceae family, which at any one site can be visited and pollinated by up to seven species of nectar-foraging birds, including Japanese white-eyes (Zosterops japonicus, Zosteropidae) and fork-tailed sunbirds (Aethopyga christinae, Nectariniidae).
The rat's tail babiana (Babiana ringens) is a species of plant that produces a strong stalk within the inflorescence that serves as a perch for the malachite sunbird as it visits the flower. Heliconias have special sticky threads that help in the adhesion of pollen to smooth structures such as the bill of a hummingbird. Some African orchids of the genus Disa have pollinaria that stick to the feet of visiting sunbirds.
Plants need to protect against nectar being taken by non-pollinators. These agents are classified into nectar robbers, which may destroy the flower, for example cut the flower at the base to obtain nectar and nectar thieves that obtain nectar without pollinating the flower.
Bird adaptations
The main families of specialized nectar feeding birds that are involved in ornithophily are the hummingbirds (Trochilidae), sunbirds (Nectariniidae), and the honey-eaters (Meliphagidae). Other important bird groups include those in the families the Icteridae, the honeycreepers (Thraupidae, Drepanidae), white-eyes (Zosteropidae) and the South African sugar-birds (Promeropidae). Birds may obtain nectar either by perching or by hovering with the latter mainly found in the hummingbirds and sunbirds. Within the hummingbirds, two kinds of foraging are noted with territorial "hermit" hummingbirds and the non-hermits which forage longer distances
Hummingbirds have the ability to digest sucrose unlike many passerines that prefer hexoses (fructose and glucose). Starlings and their relatives will completely avoid sucrose. Nectar feeding birds typically have a mechanism to quickly excrete excess water. They may have to drink four to five times their body mass of liquid during the day to obtain enough energy. Hummingbirds are capable of excreting nitrogenous wastes as ammonia since they can afford more water loss than birds that feed on low-moisture food sources. Hummingbirds and sunbirds also have special anatomical and physiological adaptations that allow them to quickly excrete excess water. Hummingbirds are also able to turn off their kidney function at night.
In some birds such as white-eyes, the pollen dusted by the plants on the forehead of the birds may increase the wear of these feathers leading to increased moulting and replacement.
Other associations
Several mite species (mainly in the genera Proctolaelaps, Tropicoseius and Rhinoseius, family Ascidae) have evolved a phoretic mode of life, climbing into the nostrils of hummingbirds that visit flowers and hitching a ride to other flowers where they can feed on the nectar. Hummingbird flower mites favour plants in the families of Heliconiaceae, Costaceae, Zingiberaceae, Amaryllidaceae, Rubiaceae, Apocynaceae, Bromeliaceae, Gesneriaceae, Lobeliaceae and Ericaceae, members of which are associated with hummingbirds.