RAYNER, J. M. V. *; MAYBURY, W. J.; COULDRICK, L. B.: Aerodynamic control by the avian tail

The bird tail has been credited as a means of controlling stability and manoeuvrability in flight and as a supplementary lifting organ, but others have emphasized its role in sexual selection and display. Theoretical models of tail aerodynamics based on slender wing theory have not hitherto been tested. Flow visualization and force measurements with mounted European starling Sturnus vulgaris reveal the geometry of vortices around the tail, and the effect of the tail on airflow around the body. Lift from the spread starling tail at low speeds is strongly influenced by the presence of the body and does not correspond to predictions of slender wing theory. A furled tail has an important effect on body aerodynamics; it acts as a splitter plate, controlling the onset of separation on the anterior part of the body, and reducing parasite drag significantly. Flow control and body drag reduction is hypothesized to be the major role of the tail in normal forward flight, although the tail plays varied roles in different species. It probably evolved from a balancing, and heavy, bony tail in bipedal theropods.