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This section is aimed at explaining the bird and biology specific vocabulary you are likely to meet in other threads in Birdforum.
Tarsus: see Legs and Feet
Taxon, taxa: a taxonomic unit (in principle any taxonomic unit), most often used for either a subspecies or a species. For example, when discussing Kaempfer's Woodpecker calling it "the taxon obrieni" does not make a statement as to whether Kaempfer's Woodpecker is a full species or a subspecies.
Taxonomic order: a linear order of the species of birds of the world based on a phylogenetic analysis (see Phylogeny), and therefore also based on the evolutionary relationship among the birds. Bird families that are very old are placed first, more recent additions later. The commonly used order have for a long time been based on Voous 19771 but some recent DNA based studies have revolutionized the order: one example of the results are that Grebes and Flamingos are each others closest relatives.
Tertials: see Wings
Tibia: see Legs and Feet
Tomial Tooth: The notch on the upper mandible of falcons and some shrikes, located just behind the hook of the beak. It is used to break the neck of their prey. Also called 'the notch'.
Trapline, traplining: used to describe a route which takes a hummingbird to the same flowers (or flower groups) in the same order; each flower may be visited one to several times daily. This strategy are used more often in Hermits of genus Phaethornis but also in some other species of hummingbirds.
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Undulating: usually used about flight where the bird alternates upwards active flight with downwards gliding, so that the overall flight pattern reminds of a sinus curve. Especially common in e.g., Woodpeckers and several groups of smaller birds where the downwards glide may involve closed wings.
Vagrant: a bird that occurs outside of normal range, in a location where there has been very few previous occurrences.
Vent: see General Anatomy
Vermiculated: finely marked with wavy patterns, often only visible at close range. Most often used about patterns on birds (in feathers) but may also apply to eggs.
Vicariance: the situation when two closely related populations get separated, for example by ice during an ice age, by water such as a river or sea, a mountain range, etc. Some theories of speciation state that vicariance is a necessary step in producing two species from populations that were one species before.
Whiffling: the name given to the method of rapid descent that some waterfowl use. They twist their body and wings, first one way then the other, loosing lift and forward speed in the process to very quickly descend to the water surface. When large flocks of birds engage in this manouver the result can be astounding.
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Zygodactyl: Having two toes on each foot pointing forwards whilst the other two point backwards. The backwards pointing toes are the innermost and outermost toes. Woodpeckers and other arboreal species share this trait while Trogons have a Heterodactyl arrangement. see Legs and Feet
Zugstimmung: behavioral changes coming before and preparing the bird for its migration: for example leading to build-up of fat reserves that allow for long, unbroken migratory flights and also changing day/night pattern of activity (the actual change in body composition is known as Zugdisposition). The word is German but adopted into English language technical literature.
Zugunruhe: restlessness and activity displayed by a captive bird of a migratory species during the period it should have been actively migrating. Some scientific studies have used the Zugunruhe to successfully determine migratory direction in birds by capturing migrants, hold them in captivity and determine the direction they are trying to move in a specifically designed funnel. Zugunruhe is German but adopted into English language technical literature; also called migratory restlessness. (Sometimes this term is used more broadly, including the behavior here listed as Zugstimmung (see above)).