The twig or bird snakes of the genus Thelotornis are a group of back-fanged snakes in the family Colubridae. All species have slender and elongated profiles, long tails, narrow heads, and pointed snouts. The eyes of each species have horizontal pupils, shaped like keyholes, which give twig snakes binocular vision. Twig snakes are greyish-brown with faint light and dark markings. When threatened, they inflate their throats to display bold black markings between the scales.
The twig snake is one of the several back-fanged colubrids whose bite is highly venomous and potentially fatal. The venom is hemotoxic, and although its effects are very slow, and bites are rare, no antivenom has been developed and several fatalities (such as Robert Mertens) have occurred.
The African twig snakes are distinctive in appearance and unlikely on that continent to be mistaken for any other snake, if indeed the observer notices them. Preying on lizards, frogs, and sometimes birds, they conceal themselves in trees, but often at a low enough level to be able to also strike at terrestrial prey, which they may swallow upwards after killing. Their cryptic coloration and apparent ability to freeze or sway gently, as chameleons do, like a twig on a tree (hence the name), makes them hard to spot. Indeed, they may be more abundant in areas than is immediately obvious.
Thelotornis is characterised by a depressed and flat head, keyhole-shaped pupils, and in T. kirtlandii, a projecting canthus rostralis which forms a shallow loreal groove on each side of the head. This allows a certain amount of binocular vision to the snake. In appearance, the head at least is unlikely to be mistaken for any other African snake. Other characteristics include a very long tail and large back fangs. The iris in T. capensis and T. kirtlandii is yellow, and presumably therefore also in T. usambaricus. The subspecies of T. capensis, T. c. mossambicanus is sometimes considered a distinct species.
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