PROC. BIOL. SOC. WASH. 95(2), 1982, pp. 412-420 VARIATION OF A MUSCLE IN HUMMINGBIRDS AND SWIFTS AND ITS SYSTEMATIC IMPLICATIONS Richard L. Zusi and Gregory Dean Bentz Abstract. — Historically, features of the muscle tensor propatagiaHs pars brevis have been used to argue that swifts and hummingbirds comprise a single mono-phyletic order, the Apodiformes, and that this order is most closely related to the "pico-passeriforms." We describe variations of this muscle in swifts and hum-mingbirds as well as in other orders and conclude that the new evidence does not support these claims. The variations observed, however, do show morphological trends that help to clarify relationships within both swifts and hummingbirds. The phylogenetic relationships of hummingbirds and swifts to each other and to other avian orders are among the major unsolved ornithological problems in systematics. Subfamilial and generic relationships within swifts are fairly well understood (see Brooke 1970), but internal relationships of hummingbirds are still poorly known. Although we do not claim to have solved any of these problems, we have found that variation in a single muscle, M. tensor patagii brevis, bears on all of them. We report this variation and its systematic impHcations as a stimulus for further study of these problems, and as partial evidence toward their eventual solution. The tensor patagii brevis muscle (TPB) extends from the shoulder to the fore-arm in birds and apparently can serve either to flex the forearm or to support the prepatagial membrane of the extended wing. This muscle exhibits marked vari-ation among birds, especially in relation to its tendon or tendons of insertion. Garrod (1876) drew attention to this variation and to its taxonomic implications, and Fiirbringer (1888) devoted six plates and considerable discussion to this mus-cle. Since then it has been further described in major systematic and anatomical works (e.g. Buri 1900; Beddard 1898; George and Berger 1966). In this paper we describe in some detail the variation of TPB in swifts and hummingbirds, and we test Lowe's (1939:329) contentions that the fleshy beUy and tendon of insertion of TPB in hummingbirds is almost identical with that of the swift, that both are fundamentally passerine in design, and that the arrangement in non-passerines is quite different. Hummingbirds In hummingbirds the origin of TPB is consistently by a tendon from the head of the coracoid. The nearly parallel fibers of the short, wide belly pass distally and end on the surface of the extensor metacarpi radialis muscle (EMR) and on a short internal aponeurosis that fuses with the aponeurosis of origin of EMR. At this point of fusion the aponeurosis of TPB forms a tendon (the humeral tendon) that extends across the belly of EMR and inserts on the humerus. In humming-birds another aponeurosis or tendon (the distal tendon) passes distally from the
BioStor
