Part 12

Osteologically, the various species of the Bombycillidae are remarkably similar. Small variations do exist, but these are primarily differences in relative size. The modifications of the beak enable palm-chats to feed on parts of plants, and the beak of Phainoptila shows some similarity in this respect. Rounded wings, which cause a bird to fly by means of short, relatively weak strokes, are correlated with a comparatively long humerus, whereas long and pointed wings, which enable a bird to fly with more powerful strokes of the wing, are correlated with a relatively short humerus. There is a positive correlation between a short humerus and a long external condyle, and between a long humerus and the absence or smallness of the external condyle.

In the Bombycillidae short bones of the leg are adaptive, and long bones of the leg are the generalized condition. Although all passerine birds were differentiated relatively late in geologic time, long hind limbs still could have been present in the immediate ancestors of passerine birds. As adaptive radiation took place in the class Aves, some birds, the Bombycillidae included, became more and more adapted for an arboreal, and eventually an aerial habitat, with consequent loss of saltatorial and running ability.

Birds, like mammals, have a short femur, the most proximal element in the leg, if the species is adapted to run fast. If the species is not adapted to run fast, birds, unlike mammals, have the tibiotarsus longer than any of the other elements; in mammals that are not adapted to run fast, the femur and tibia are approximately the same length. In non-running birds as compared with running birds, the leg element distal to the tibiotarsus, and the one proximal to it, are considerably shortened. In waxwings, all three elements of the hind limb are shortened, indicating that the reduction in length has been, evolutionarily speaking, a rapid process, in order to reduce the limbs to a convenient size as soon as possible.

The shape of the pygostyle varies in the Bombycillidae, but the simple shieldlike bone of Phainoptila is judged to resemble closely the ancestral type. In Ptilogonys there is a tall dorsal spine, coupled with a wide and heavy centrum and flattened lateral areas, for support of the long rectrices. In Bombycilla the bone is small with knobs on the centrum that have been developed for muscle attachment.

The muscles were carefully dissected in each genus and in most of the species. The same homologous muscles are present in all species. Significant differences were found only in the relative size of certain muscles. No satisfactorily accurate method of measuring these differences was found. Consequently, less use was made of the results of the dissections than was originally planned.

The set of pectoral muscles varies but slightly in relative mass, and the variation is not considered significant. The deltoid muscle was selected for measurement since its point of insertion is unusually variable, while the mass of the muscle varies little. We can conclude that the extent of the area of insertion of the tendon of a muscle can determine that muscle's relative efficiency, while the muscle itself remains the same in bulk.

The muscles of the hind limb are notably larger in species that have long legs, and a good index of the hopping ability may be gained by study of certain of these muscles. In the Bombycillidae, and in those Ptilogonatinae that do not use the hind limbs for hopping, the bones are shortened, and the associated muscles are correspondingly smaller.

The gross anatomy of the digestive tract is practically identical in the members of the family. The variability noted is mainly in the degree of compactness of the visceral mass in Bombycilla and in Phainopepla. Also there is a tendency for the Bombycillinae and the Dulinae to have the mass situated more posteriorly than it is in the Ptilogonatinae. Moreover, Bombycilla has a shorter intestine than do the other genera. All of this indicates that the waxwings (Bombycillinae) have the center of gravity situated more advantageously for flight than do the birds of the two other subfamilies.

SUMMARY

1. The silky flycatchers, waxwings, and palm-chats are included in the family Bombycillidae; the Ptilogonatidae and Dulidae are reduced to subfamily rank.

2. The coloration of the birds of each subfamily is different because the ecological needs are different.

3. Waxwings were at one time regularly migratory, but are now nomadic, since they are adapted to live in northern latitudes for the entire year.

4. The corresponding bones in different members of the family closely resemble one another, and the differences which do exist are the results of responses within relatively recent times to changes in habits.

5. In the Bombycillidae a rounded wing is judged to be the primitive condition. As the wing becomes more pointed, the humerus becomes shorter and its external condyle longer.

6. The hind limbs are short in birds that depend most on flight power, but are longer and the distal elements are disproportionately longer in birds that depend on saltation or on running.

7. The pygostyle varies in shape and size between genera and even between some species.

8. The pectoral muscles differ in size only slightly in the different members of the family, but the insertions are more extensive for these muscles in birds that fly a great deal.

9. The muscles of the hind limb vary in mass, but not in kind, in the members of the family Bombycillidae.

10. In the Bombycillidae that depend on flight power, rather than on saltation or on running power, there is a tendency for the digestive tract to become shorter and for the whole visceral mass to become more compact.

BIBLIOGRAPHY

ANDERSON, E. M.

1915. Nesting of the Bohemian Waxwing in northern British Columbia. Condor, 17(4):145-148, 1915.

ANDERSON, M. P.

1907. A collecting trip in Korea. Condor, 9(5):146-147, 1907.

ANDERSON, R. M.

1909. Nesting of the Bohemian Waxwing (Bombycilla garrulus). Auk, 26(1):10-12, 1909.

ARMSTRONG, E. A.

1942. Bird display. Cambridge Univ. Press, xvi + 381 pp., 22 plates, 1942.