The Comparative Functional Morphology of the Pelvic Appendage in Three Genera of Cuculidae

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Abstract
The osteological and myological features of 3 genera of Cuculidae with markedly different locomotor habits are compared. Geococcyx and Crotophaga possess 14 cervical vertebrae each, whereas 3 forms of Coccyzus possess only 13 cervical vertebrae each. Length of cranium is less variable than either length of synsacrum or length of the dorsal vertebral region. Ratios of depth of carina to length of sternum reflect differences in flying ability among the 3 genera. There is an increase in the absolute length of both the wing and the leg in the series Coccyzus, Crotophaga. Geococcyx. as well as an increase in the relative length of the several segments of the pelvic appendage; and, conversely, there is a decrease in the relative length of each segment of the pectoral appendage, except the humerus of Geococcyx, in the same series. The leg muscle formula for Coccyzus in AXYAm. Of the 43 pelvic muscles described for birds it lacks the following: Mm. iliotrochantericus medius, gluteus medius et minimus, piriformis pars iliofemoralis, adductor digiti IV, and extensor brevis digiti III. The leg muscle formula for Crotophaga and Geococcyx is ABXYAm. Excepting M. piriformis, these genera lack the same muscles as does Coccyzus. Because of bilateral action of leg muscles, a hopping bird (Coccyzus) is able passively to maintain balance of the axis in the transverse plane (to prevent horizontal roll), and in the frontal plane (to prevent lateral swing), but only actively in the sagittal plane. Lack of balance in the sagittal plane results in an alternately upward and downward pitch of the axis during leg retraction. Unilateral action of muscle in a running bird necessitates active balance in all three planes. The progressive cranial and lateral shift in the series Coccyzus to Geococcyz of the origin of several pelvic muscles increases the vertical component of their pull and thereby decreases their tendency towards roll and swing and possibily towards pitch of the axis. This shift is effected partly by the craniolateral migration of the origin of M. semitendinosus, and partly by an increase in size of pars iliofemoralis simultaneously with a decrease in size of pars caudofemoralis of the piriformis muscle. This tendency towards roll of the body axis in Geococcyx is further reduced by: (1) the increased size of the pars iliofemoralis and the reduction in strength of pars caudofemoralis of the piriformis muscle; (2) the caudal migration of the origin of the iliotrochantericus posticus muscle; and (3) the replacement by fleshy fibers of the central tendinous portion of the belly of the iliotibialis muscle. It is believed that the differences in locomotor habits exhibited by the 3 genera most satisfactorily may be explained by reference to the progressive increase in leg length from Coccyzus to Geococcyx. The increase in relative size and the changes in proportions of certain leg muscles facilitate but are not requisite for terrestrial locomotion and alternate leg action. The innate behavior pattern may play a role in determining the locomotor pattern distinctive of the 3 genera. On the basis of leg muscle formula (AXYAm), the genus Coccyzus does not fit into the subfamily Phaenicophaeinae (ABXYAm), and on the basis of the number of cervical vertebrae (13), this genus does not fit into any of the 6 subfamilies currently proposed.