title: A passerine spreads its tail to facilitate a rapid recovery of its body posture during hovering.
abstract: We demonstrate experimentally that a passerine exploits tail spreading to intercept the downward flow induced by its wings to facilitate the recovery of its posture. The periodic spreading of its tail by the White-eye bird exhibits a phase correlation with both wingstroke motion and body oscillation during hovering flight. During a downstroke, a White-eye's body undergoes a remarkable pitch-down motion, with the tail undergoing an upward swing. This pitch-down motion becomes appropriately suppressed at the end of the downstroke; the bird's body posture then recovers gradually to its original status. Employing digital particle-image velocimetry, we show that the strong downward flow induced by downstroking the wings serves as an external jet flow impinging upon the tail, providing a depressing force on the tail to counteract the pitch-down motion of the bird's body. Spreading of the tail enhances a rapid recovery of the body posture because increased forces are experienced. The maximum force experienced by a spread tail is approximately 2.6 times that of a non-spread tail.
authors.affiliation: Department of Mechanical Engineering, National Taiwan University, No. 1, Section 4, Roosevelt Road, Taipei 10617, Taiwan, Republic of China.
authors.forename: Jian-Yuan
authors.initials: JY
authors.lastname: Su
authors.forename: Shang-Chieh
authors.initials: SC
authors.lastname: Ting
authors.forename: Yu-Hung
authors.initials: YH
authors.lastname: Chang
authors.forename: Jing-Tang
authors.initials: JT
authors.lastname: Yang
mesh_terms: D000818:Animals; D011187:Posture / Q000502:physiology*; D046370:Sparrows / Q000502:physiology*; D013623:Tail / Q000502:physiology*; D014921:Wings, Animal / Q000502:physiology