Accidental (biology)
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Vagrancy is a phenomenon in
biology Biology is the scientific study of life. It is a natural science with a broad scope but has several unifying themes that tie it together as a single, coherent field. For instance, all organisms are made up of cells that process hereditary i ...
whereby an individual animal (usually a
bird Birds are a group of warm-blooded vertebrates constituting the class Aves (), characterised by feathers, toothless beaked jaws, the laying of hard-shelled eggs, a high metabolic rate, a four-chambered heart, and a strong yet lightweig ...
) appears well outside its normal range; they are known as vagrants. The term accidental is sometimes also used. There are a number of poorly understood factors which might cause an animal to become a vagrant, including internal causes such as navigatory errors (endogenous vagrancy) and external causes such as severe weather (exogenous vagrancy). Vagrancy events may lead to colonisation and eventually to speciation.


Birds

In the Northern Hemisphere, adult birds (possibly inexperienced younger adults) of many species are known to continue past their normal breeding range during their spring migration and end up in areas further north (such birds are termed spring overshoots). In autumn, some young birds, instead of heading to their usual wintering grounds, take "incorrect" courses and migrate through areas which are not on their normal migration path. For example, Siberian passerines which normally winter in
Southeast Asia Southeast Asia, also spelled South East Asia and South-East Asia, and also known as Southeastern Asia, South-eastern Asia or SEA, is the geographical south-eastern region of Asia, consisting of the regions that are situated south of mainlan ...
are commonly found in
Northwest Europe Northwestern Europe, or Northwest Europe, is a loosely defined subregion of Europe, overlapping Northern and Western Europe. The region can be defined both geographically and ethnographically. Geographic definitions Geographically, Northw ...
, e.g. Arctic warblers in Britain. This is reverse migration, where the birds migrate in the opposite direction to that expected (say, flying north-west instead of south-east). The causes of this are unknown, but genetic mutation or other anomalies relating to the bird's magnetic sensibilities is suspected. Other birds are sent off course by storms, such as some North American birds blown across the Atlantic Ocean to Europe. Birds can also be blown out to sea, become physically exhausted, land on a ship and end up being carried to the ship's destination. While many vagrant birds do not survive, if sufficient numbers wander to a new area they can establish new populations. Many isolated oceanic islands are home to species that are descended from landbirds blown out to sea, Hawaiian honeycreepers and Darwin's finches being prominent examples.


Insects

Vagrancy in insects is recorded from many groups—it is particularly well-studied in butterflies and moths, and
dragonflies A dragonfly is a flying insect belonging to the infraorder Anisoptera below the order Odonata. About 3,000 extant species of true dragonfly are known. Most are tropical, with fewer species in temperate regions. Loss of wetland habitat threa ...
.


Mammals

In mammals, vagrancy has been recorded for bats, pinniped seals, whales, manatees, belugas, cougars, and more.


Reptiles

Vagrancy has been recorded for sea turtles, snakes (e.g. ''
Pelamis platura The yellow-bellied sea snake (''Hydrophis platurus'') is a venomous species of snake from the subfamily Hydrophiinae (the sea snakes) found in tropical oceanic waters around the world except for the Atlantic Ocean. For many years, it was plac ...
''), crocodilians, and probably also occurs in lizards. It therefore seems to be a fairly widespread phenomenon in reptiles. Saltwater crocodiles are especially prone to vagrancy, with individuals occasionally being recorded in odd places including Fiji, Iwo Jima, and even the Sea of Japan.


Plants

The term vagrant is also used of plants (e.g. Gleason and Cronquist, 1991), to refer to a plant that is growing far away from its species' usual range (especially north of its range) with the connotation of being a temporary population. In the context of lichens, a vagrant form or species occurs unattached to a substrate ("loose"), not necessarily outside its range. Another definition (de Lange & Molloy, 1995) defined vagrant species in New Zealand flora - although could also be applied for any given region. Their definition was, "taxa whose presence within the New Zealand botanical region is naturally transitory... those which have failed to establish themselves significantly beyond their point of introduction through reproductive failure or for quite specific ecological reasons.". One example was the presence of ''
Atriplex cinerea ''Atriplex cinerea'', commonly known as grey saltbush, coast saltbush, barilla or ''truganini'', is a plant species in the family Amaranthaceae. It occurs in sheltered coastal areas and around salt lakes in the Australian states of Western Aust ...
'' in New Zealand.


References

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Animal migration Biology terminology Ecology terminology Phytogeography