The functions of the elongated tails of birds have been the subject of much discussion in recent years. It is clear that in some cases the tail represents a sexually selected ornament, playing a vital role in mate choice. However. what is becoming increasingly apparent is that the tail can also play vital aerodynamic roles during flight, and can thus be a result of natural selection. Tail length manipulation experiments carried out during this work have shown that elongated graduated tails have an aerodynamic role during gliding flight. A function in the maintenance of stability (ring necked pheasants (Phasianus colchicus)) and optimising gliding performance (black-billed magpie (Pica pica)). thus these elongated graduated tails have been shown to be a product of natural selection. The question of whether correlated evolution occurred between the advent of gliding flight and the graduated tail shape was investigated. It was found that graduated tails did not co-evolve with gliding flight. However, it was shown that graduated tails had correlated evolution with tail elongated. It has been suggested that the forces acting on a triangular tail can be predicted through the application of slender lifting surface theory and the tail being analogous to a delta wing. This would predict that the tail functioned as a consistent lift producing surface. This study considered whether the tail functioned as a lift producer or a control surface. Stereo video of Harris' hawks (Parabuteo unicinctus) was used to assess tail function of a triangular tail. However, the results did not provide conclusive evidence for either theory. In this thesis I show that elongated avian tails perform a number of naturally selected aerodynamic roles during flight.