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It measures 13 centimeters long and weighs 7 grams, with a 3 centimeter beak. It is robust, with a long and curved beak, with a light yellow mandible and a black jaw. Upperparts tan green; darkened post-ocular stripe and narrow white superciliary stripe; rusty lower parts, being more intense in the throat and upper chest. Brown rectrices with a wide black subterminal stripe and a white tip, the central ones being green and not elongated. The female is lighter ventrally and has a more curved bill than the male. Perched, or even hovering in flight, it can be seen insistently waving its tail back and forth, living up to its main popular name. The juvenile of the species was erroneously described as a separate species, Threnetes grzimeki.