MORPHOLOGICAL ASPECTS OF AVIAN TAIL MOVEMENTS - A FUNCTIONAL-APPROACHIN HIRUNDINES

It has been proposed that the aerodynamically optimal tail shape in bi rds should be triangular when spread and forked when closed. According to this aerodynamic model, any area distal to the point of maximum wi dth contributes drag, which is proportional to total tail area, but no t lift. One assumption of this model seems to be uniformity in structu res responsible for tail movements (i.e. tail muscles) among species. We tested for different muscular arrangements in species with differen t tail shapes. By dissecting tail muscles in three hirundine species ( Barn Swallow [Hirundo rustica], Common House-Martin [Delichon urbica], and Bank Swallow [Riparia riparia]), we demonstrate anatomical differ ences associated with deeply forked tails and speculate that such diff erences compensate for the increased cost of longer feathers. Therefor e, we propose that reliable assessment of the aerodynamic cost of tail s should be based on tail shape as well as internal anatomical structu res such as muscles.