Ornithophily
Ornithophily or bird pollination is the pollination of flowering plants by birds. This sometimes (but not always) coevolutionary association is derived from insect pollination (entomophily) and is particularly well developed in some parts of the world, especially in the tropics, Southern Africa, and on some island chains.[1] 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 and long bills, that are either capable of hovering flight or light enough to perch on the flower structures.
Plant adaptations
Plant adaptations for ornithophily can be grouped primarily into those that attract and facilitate pollen transfer by birds, and those that exclude other groups, primarily insects,[4] protecting against 'theft' of nectar and pollen.[5] The ovules of bird flowers also tend to have adaptations that protect them from damage during vigorous foraging by hard bird bills.[6]
One of the general adaptation patterns is the red flower color for many plant species. The pollinator, birds, are
The flowers of generalist bird-pollinated plant species differ from those pollinated by specialized birds, such as hummingbirds or sunbirds by lacking long corolla tubes and having brush-like, exserted stamens.[10] Most bird pollinated flowers are red and have a lot of nectar. They also tend to be unscented.[11] 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.[12][13] 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.[14]
Different plants have also developed specific adaptations for bird pollination. Many plants of the family
The rat's tail babiana (
Plants need to protect against nectar and pollen being taken by non-pollinators.[22] Such animals are sometimes classified as thieves, which simply remove resources without pollinating, and robbers, which damage the flower to access resources. Flowers specialized for pollination by long-billed birds may be especially vulnerable to theft.[23] For example, some bees and birds that cannot reach down the long tubes of bird pollinated flowers simply pierce the flower at the base to obtain nectar, without pollinating.[24]
Bird adaptations
The main families of specialized nectar feeding birds that are involved in ornithophily are the hummingbirds (
Hummingbirds have the ability to digest sucrose unlike many passerines that prefer hexoses (fructose and glucose). Starlings and their relatives will completely avoid sucrose.[25] 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.[26] 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.[27][28] 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.[29]
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.[30]
Patterns in the evolution of ornithophily
About 7000 neotropical plant species are hummingbird-pollinated[31] in contrast to about 129 species of North American plants that have evolved ornithophilous associations.[32] Nearly a fourth of the 900 species of the genus Salvia are bird-pollinated in Central and South America and a few also occur in South Africa.[33] Tropical China and the adjacent Indochinese countries harbor relatively few bird-pollinated flowers, among them is Rhodoleia championii, a member of the family Hamamelidaceae, 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).[34]
Hummingbirds rely on nectar for energy, and ornithophilous flowers need hummingbirds’ assistance with pollination in order to reproduce. While the birds are feeding, pollen sticks to their beaks, which will rub off on the next flower they visit, pollinating it.[35] Over time, the co-dependence on one another causes the co-evolution of pollination syndromes.[36] For example, different species of hummingbirds have differently shaped beaks, presumably to allow them to drink nectar from the flowers around them.[37] It is widely believed that short-billed hummingbirds drink from wider flowers with short petals, and hummingbirds with longer bills have close relationships with flowers with long, narrow corollas. Most of the time, long-billed species have access to both short and long flowers, but they often avoid short flowers to avoid competition.[38] Further, hummingbirds with curved bills will forage at straight-petaled flowers, but straight-billed birds are less likely to visit curved flowers.[39] Ornithophilous flowers pollinated by hummingbirds often have reproductive structures that are vertically oriented. This creates a favorable upright body position for hummingbirds during feeding, one that allows them to sufficiently flap their wings for hovering.[40] Hummingbirds prefer to visit larger and taller floral displays, and it has been proven and confirmed through many studies that birds prefer flowers with red or pink petals over other colors.[41]
Bird pollination is considered as a costly strategy for plants and it evolves only where there are particular benefits for the plant.[42] High altitude ecosystems that lack insect pollinators, those in dry regions or isolated islands tend to favour the evolution of ornithophily, most by specialized nectarivorous birds, such as hummingbirds or sunbirds.[5] Plants pollinated by generalist birds are most diverse in tropical and subtropical lowlands with a pronounced climatic seasonality. These plants are mostly large, woody species that produce a large number of open flowers at the same time in contrast to the mostly small shrubs and herbs that are pollinated by specialized nectarivorous birds. Since generalist bird-pollinated plants are mostly self-incompatible they needed to adapt to pollinators that mostly provide outcrossing, such as generalist birds. These birds mostly feed on arthropods, fruits or seeds even if much nectar is available and therefore move a lot through the forest. By this activity they often move between nectar-providing plants and provide outcrossing.[43] Generalist bird-pollinated plants even evolved deterring mechanisms against specialized nectarivorous birds and bees since these groups tend to establish feeding territories within one tree and thus most conduct self-pollination.
On islands however, generalist bird pollination did not evolve to avoid self-pollination but adapted to a reliable pollinator since bees and butterflies are rare just as on montane forests.[43]
Migration and flowering synchrony
Time of flowering is often used to mark the start of spring in temperate climate zones. Recently, studies have consistently found that plants respond to increasing temperatures by flowering earlier.
Other associations
Several mite species (mainly in the genera
See also
- Zoophily (pollination by vertebrates)
References
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