Comparative Anatomy of Musophagidae

Abstract

Introduction

 The 20 species which comprise the avian order Musophagidae' (commonly called turacos) have a number of physical and anatomical characteristics that set them apart from many other birds. While uniformity among the 20 species is not complete, certain generalizations can be made. One of these is that the sexes are visually indistinguishable in all of the species save C. Ieucogaster, in which the males have a black beak and the females a green beak. Unfortunately, most of the literature regarding the anatomy of these birds was developed more than 40 years ago, leaving many questions unan-

swered and many generalizations suspect in light of new information about these species' ecology including behavior and diet.

Feathers

Probably the most distinguishing feature of these birds are two unique pigments deposited in their feather keratin. One, turacoverdin, is a green pigment found in the rami in all species of Tauraco and Musopbaga, and in Corytbaeola cristata. The other, turacin,

provides the red colored feathers in species of the first two genera. Both pigments contain copper and spectral data demonstrates that the former is likely an oxidized version of the latter. (Dyck, 1992) In fact, the two pigments are intermingled within individual feathers in the breast patches and crests of some species and turacoverdin occurs only in the presence of turacin.

Other species outside the

Musophagidae order have turacoverdin pigment, including Itbaginis (pheasant) and Rollo/us (partridge), both members of the Galliformes. An additional interesting note is that both pigments are soluble in a weak base - which may have led to the myth that wild birds lose

feather color when exposed to rain.

Another uncommon feature of these birds is that the feathers of the head and breast of most species are deficient in barbules so that they seem hairy or have a "down-like texture." (Moreau, 1938; Moreau, 1958) These feathers make up the characteristic erectile "crests" found on all but three of the species. In addition, the contour feathers have an aftershaft. (Sibley & Ahlquist, 1990)

The wing to tail ratios are uniform across the species, with the wing length at four-fifths of the tail length. The wings are rounded with the secondaries usually a bit shorter than the primaries. Also, some of the primaries are slotted, probably related to the short, slow flight patterns used by these birds. (Moreau, 1958) None of the species have eyelashes hut all of them do have a tufted, hilobed uropygial gland. (Lowe, 1943)

Feet

These birds are defined as having semi-zygodactyl feet with the fourth toe being reversible and not permanently directed backwards. A typical resting position will find the outer toe at approximate right angles to the main axis of the foot, but it can be moved further back or directed forward, depending upon the bird's perching needs. Additionally, the claws are short and abruptly curved. (Moreau, 1938) Some researchers have reported that nestlings have a short (approximately lmm) wing-claw hut this seems to vary from species to species and even among individuals within a species, and because of this it cannot be said with certainty that it is a defining anatomical feature. (Moreau, 1958; Fry, et al., 1988)

Beak&Head

All turacos have short strong beaks with curved culmen. In some species the culmen is ridged ( T bannermam, leucolophus, and macrorhynchus) and in others curves back into a frontal plate (M. rossae and violacea). The nostrils are located on the beak but vary in shape and position. Some species have slit-shaped nares while others have circles; in many species the nostrils are covered with feathers. (Moreau, 1958)

Internally, a single unifying factor is that all species lack a vomer. (Sibley & Ahlquist, 1990) Additionally, the palatine processes arise from the more proximal end of the maxillary bone and the distal ends of the uncinate bone end in a sharp curve where they meet the outer rim of the palatine. (Lowe, 1943) Moreover, the quadrate bone in turacos is pneumatized, one of the facts that earlier taxonomists relied upon to distinguish these birds from cuckoos that have a solid quadrate. Finally, the tongue is short and thick and of a triangular shape, consistent with the tongue shape found in other frugivorous birds.

Digestive System

Turacos have no crop but do have an exceptionally large and well developed proventriculus, with walls twice as thick as those of the gizzard. The gizzard, in turn, has only a thin muscular structure with no horny cuticle or koilin lining its interior. Additionally, turacos lack a caeca. These anatomical features are consistent with the fact that turacos are primarily frugivores, though they have been known to eat insects and snails, particularly when they are feeding young in the nest. (Fry, et al., 1988) The major exception to this diet is seen in the Great Blue Turaco C. cristata which ingests a significant number of leaves, algae, and rootless floating plants as part of its diet. (Sun, et al., 1997)

Since so little has been published regarding the digestive systems of the turacos it is difficult to make accurate generalizations regarding this portion of their anatomy. Two studies regarding intestinal parasites have been undertaken, primarily for the purpose of determining the phylogenetic relationship of these birds to other species, but these studies do not describe the anatomy in any detail. (Clay, 1947; Bennett, 1993)

As an aside, with the recent confirmation by Sun and his associates that a significant portion of the diet of C. cristata is leaves and related plant matter, it would be interesting to pursue the comparative anatomy of the digestive system of the turacos generally and the Great Blue in particular.

As stated below, recent research points to a possible evolutionary relationship between turacos and the hoatzin. The hoatzin is noted for being an herbivore that has a crop, which acts as a glandular muscular stomach used to grind up tough leaves. Thus, a reexamination of the digestive anatomy of the Great Blue might be in order to see if its foregut differs from other turacos in conformity with its variant diet. Likewise, the dietary research raises the additional question of whether the Great Blue actually lacks a ceca - since that organ is often involved in the fermentation-digestion of plant material.