Pygostyle.—This part of the skeletal system is variable in the species dealt with, not so much in size as in complexity. It reflects, of course, the character of the caudal muscles and their size, as well as the length of the rectrices and the corresponding force necessary to hold these feathers upright and in a useful position. Firm attachment is important even in flight, because the tail is used as a rudder, and in the Ptilogonatinae as a brake. The pygostyle is most modified in this subfamily.

In lateral aspect, the pygostyles of the species of the Ptilogonatinae are similar. The crest of the bone is flattened dorsally, and has a broad anterior surface that is thin and bladelike. This is widest in Ptilogonys caudatus, and narrowest in Phainoptila, in which genus, however, the entire bone is of small size. The centrum is widest in Ptilogonys caudatus, and is progressively narrower in P. cinereus, Phainopepla, and Phainoptila. Greater width provides a larger area of attachment for the larger rectrices and also more area for insertion of the lateralis caudae muscle, the size of which varies more than that of the other caudal muscles in the different species of the Bombycillidae.

In proportionate size (see Table 7), the pygostyle of Bombycilla is the smallest in the family. The dorsal spinous portion is acutely pointed instead of flattened as in the Ptilogonatinae. In Dulus, the spinous portion is extremely thin, and shows a decided curve dorsad from the centrum, and there is no flattened area anterior to the spinous portion as is seen in Ptilogonys.

The centrum in cross section varies considerably. In Bombycilla the walls are indented, with definite terminal knobs; both knobs and indentations are more pronounced in B. garrula than in cedrorum, however. The spinous portion is enlarged in both species, and the rest of the neck region is constricted (Figs. 29-35).

The centrum of Dulus in posterior aspect presents the appearance of a simple shield; little of the indentation seen in Bombycilla is present. The spinous portion is plain, with no constriction nor terminal enlargement in the neck. The centrum in Phainopepla is similar to that in Dulus, but has a small expansion at the base of the spine, the entire centrum being wider in proportion to its over-all size than in any of the other species mentioned previously. The centrum in Ptilogonys shows great width, and the spine is in a large expanded tip as in Bombycilla. The lateral edges of the centrum in P. cinereus are "winged" and in two separate halves; whereas the centrum of P. caudatus is fairly plain, its specialization being reflected primarily in breadth and flatness. In cross section of the centrum, Phainoptila is similar to Phainopepla, although, in the former, the bone is smaller in proportion to the size of the animal, and the lateral wings are more angular than in Phainopepla.

In specialization for muscle attachment, the centra of the pygostyles of the Ptilogonatinae have more area for muscle attachment than do the centra in the Bombycillinae and Dulinae; the centrum is wide, the spinous portion is long, and the bone is flattened anteriorly. The most generalized pygostyle is in Phainoptila, and that of Dulus differs only slightly. In Bombycilla the pygostyle is proportionately small, but is complex in shape; there is seemingly not the need for greatly expanded areas since the caudal muscles are less specialized in this genus.

Sternum.—The sternum in Bombycillids is typically passerine in general shape and in having a long and deep carina or sternal crest. The caudal process of the bone is broad, with the terminal ends flattened, forming dorsally a graceful V-shaped outline, whereas the outline of the posterior end of the sternum is broad and convex.

In lateral aspect, the carina is deeper in Bombycilla than in other genera of the family, and is deepest in B. garrula. In this species, the manubrium is more extended and comparatively larger than in the other species of the family. The anterior edge of the keel forms the sharpest angle in B. cedrorum. In Dulus, the keel is moderately deep, the manubrium short, and there is a distinct indented curve between the manubrium and the anterior angle of the keel.

In ventral aspect the lateral processes of the sternum tend to flare outwards in adult Ptilogonatines on almost the same plane as the rest of the bone, whereas in Bombycilla and Dulus the same process is closer to the body of the sternum. In Bombycilla the xiphoid process is more dorsal in position than in other species in the family, and in Dulus an upward curve is very noticeable. The process in these two genera is narrower than in the Ptilogonatinae, and lacks the heavy distal terminal enlargement which is apparent in Ptilogonys.

Relative Lengths of Bones.—In instances where the animals being compared are obviously different in over-all size, it is useful to express the size of a given part in relation to some other part of the same individual organism if the aim is to obtain clues as to differences in functions of the parts being compared. Differences in actual lengths of corresponding bones in two kinds of animals often, of course, reflect only the difference in over-all size of the animals. Consequently, the relative size of the part is expressed as a percentage in this paper. In computing a percentage it is well, of course, to select some relatively stable part of the animal to use as a denominator in the mathematical expression that yields the percentage. The thoracic region of the vertebral column is thought to be such a part. For example, the length of the humerus divided by the length of the thoracic region yields, in Phainopepla and Ptilogonys, respective percentages of .84 and .85. These are roughly the same, whereas the actual lengths of the humeri are 2.21 and 2.39 cm.

Leg-trunk Percentages.—Table 4 shows the relative lengths of the legs and of the separate bones in the legs of the different species of the Bombycillids. Table 5 shows corresponding lengths for other passerine birds. The total length of the leg was computed by adding the figures obtained for the lengths of the femur, tibiotarsus and tarsometatarsus. The lengths of the toes were disregarded. Length of leg was recorded in this same way by Richardson (1942:333), who thought that only in swimming and running birds do the toes contribute to the functional length of the hind limb.

Table 4 shows that of the birds compared in this paper, Dulus has the longest legs. In order of decreasing length the others are the Ptilogonatinae, and finally the Bombycillinae, which have the shortest legs of all. In Waxwings the length of the legs, expressed as percentages of the body-lengths, are identical with those birds that are similar in habits, that is to say, birds which do not use the hind limb except in perching. It can be noted by reference to Table 5 that Tachycineta and Myadestes fall into this category. This shortness of limb is obviously adaptive, and each of the segments of the limb has been correspondingly shortened, with no element reduced at the expense of the other two. The short leg can be more easily folded against the body while the bird is in flight, than can a long leg which is more unwieldy. It may be noted from tables 4 and 5 that birds which spend much time on the ground, or that hop a great deal in the underbrush, have longer legs than do birds which spend much time in flight. Two birds with noticeably long legs are Hylocichla mustelina, a typical ground dweller, and Parus atricapillus, which hops about in the trees and underbrush.

Insofar as the lengths of the legs show, Dulus and Phainoptila are the most generalized of the Bombycillidae, since the relative length of leg is approximately the same as that of more generalized birds such as warblers, crows and thrushes of similar locomotory habits. In other words, Dulus and Phainoptila have remained unspecialized, in contrast to the waxwings in which adaptive changes fitting them for a perching habit have taken place. Ptilogonys and Phainopepla are intermediate in length of leg between Phainoptila and Bombycilla, and Ptilogonys and Phainopepla have progressed from life on the ground toward the perching habit. Bombycilla cedrorum is more specialized than is B. garrula in shortness of leg, and the reduction is comparable, as is noted above, to that in the legs of Tachycineta.

In birds which have the legs much modified for walking or for hopping in the brush, such as Polioptila and Eremophila, it is noteworthy that the distal segment, the tarsometatarsus, is the longest, whereas in birds such as Myiarchus and Tachycineta, that do not utilize the limbs in this manner, the tibiotarsus, the middle segment, is the longest. Mammals much modified for walking or hopping likewise have the proximal segment, the femur, short, and the distal segment long (Howell, 1944). The waxwings have all of the segments short; these birds are modified for strong and sustained flight. Their hind limbs are used principally for landing devices and for perching. No one element of the leg has been shortened much, if any, more than any other.

Arm-trunk Percentages.—Tables 1 and 2 show the total length of the arm, and lengths of the separate arm elements, relative to the trunk. Table 3 gives the corresponding lengths for birds other than the Bombycillidae. Total length of arm was obtained by adding together the lengths of the humerus, ulna, and manus, and by dividing the figure thus obtained by the length of the trunk as was done for leg lengths in tables 4 and 5. The method of adding together the component parts does not give the entire length of the wing, since the length of the feathers, which add effectively to the total length, as well as do the lengths of the small carpal elements, is lacking.

It may be noted that Phainoptila and Bombycilla have the shortest arm in the family Bombycillidae. The humerus, radius and ulna are comparable to the same elements in thrushes and the catbird, and it is only the extremely short manus in Phainoptila that affects the total. The manus in Phainoptila is comparatively smaller than in any other genus of the family Bombycillidae, and this indicates poor flight power. Bombycilla has a total length corresponding closely to that in warblers, but the lengths of the distal elements correspond closely to those in the catbird and thrushes. Of the three segments, the humerus is, relatively, the most shortened. Next in order of increasing length of arm is Dulus; measurements for it are roughly the same as those of Myadestes. The wing bones of the Ptilogonatinae, other than Phainoptila, are the longest in this series, and they most nearly resemble the same bones in flycatchers, Parids, and gnatcatchers.

It is notable that, in general, birds with long and narrow wings appear to have relatively the shortest humeri, with the distal bones, especially the manus, variable in length and seemingly correlated with the manner of feather attachment. Those birds with rounded and short wings have the longest humeri. In swallows, for example, the humerus is short, whereas the other arm bones are long, and the manus is unusually large and heavy. A short humerus gives better lever action in the flight stroke than a long humerus does.

Caudal Muscles.—The muscles of the caudal area that were used for comparison were the levator caudae and the lateralis caudae. These muscles are used by the living bird to maintain the position of the pygostyle and therefore the rectrices; these muscles are especially important to those birds that utilize the tail as a rudder in flight and as a brake. As may be seen by reference to Table 11, the two muscles are largest in proportion to body weight in the Ptilogonatinae, in which subfamily the species have long rectrices and must have correspondingly well-developed muscles in order to utilize the rectrices to best advantage in flight. The lateralis caudae differs more according to species than does the levator caudae, showing that rudder action of the tail is of primary importance in the adaptation for capturing insects. It will be remembered that the pygostyle in this subfamily has a flattened lateral surface for attachment of the levator caudae muscle, and it is therefore to be expected that this muscle will be larger in the Ptilogonatinae than it is in either the Bombycillinae or the Dulinae. The levator coccygis, together with the two muscles mentioned above, is responsible for elevation of the tail. The levator coccygis is less altered in different species of the family than is the lateralis caudae. It may be noted that the caudal muscles of Dulus and Bombycilla constitute a smaller percentage of the total weight of the bird than in any of the genera in the subfamily Ptilogonatinae.

Pectoral Muscles.—The pectoral set of muscles varies but little in the family; flight power is seemingly not dependent upon size of either the pectoralis major or pectoralis minor. The data indicate that the insertion on the humerus, with consequent changes in the relative length of that bone, is more significant in type of flight and over-all flight power than is the actual size of the muscle mass. The deltoid muscle, for example, is smaller in Bombycilla than in members of the other two subfamilies. The humerus in Bombycilla is shortened, and the muscle therefore does not need to be large to accomplish the same powerful stroke that would be accomplished by a longer humerus and a larger, more powerful deltoid muscle. In the case of the deltoid, the shortening of the humerus and the more complex arrangement of the points of insertion have obviated the necessity of enlarging the muscle.

Leg Musculature.—The muscles of the thigh are noticeably larger in birds that have long leg bones. (See Table 12 for size of muscles.) On the tibiotarsus, the peroneus and gastrocnemius muscles were measured. When expressed as a percentage of the weight of the bird, the peroneus has much the same relative weight in all but one of the species, whereas the gastrocnemius varies much. The peroneus is proportionately large only in Phainoptila, in which genus all the leg muscles are well developed, but the gastrocnemius is larger in all the Ptilogonatinae and in Dulus than it is in the specialized Bombycilla, in which it has probably been reduced as the leg bones and other muscles have been reduced.

The volume of the muscles of the hind limb changes more readily in response to saltation and running than do the muscles of the forelimb to flying.

The digestive tract is relatively uniform in all genera of the family; there are only slight differences between the species. The degree of compactness of the visceral mass varies, Phainoptila and Ptilogonys caudatus having the folds of the digestive tract loosely arranged, whereas Ptilogonys cinereus and Phainopepla have folds which adhere more tightly to the ventriculus and liver. In Dulus and Bombycilla, as compared with the Ptilogonatinae, the visceral mass (primarily liver and ventriculus) is situated more posteriorly in the body cavity, and is more compact, and the intestine is more tightly coiled.

The coiling of the intestine, if its degree of compactness is disregarded, is nearly identical in the birds of the family; there are four major loops between the ventriculus and the anus. The length of this section of the tract is, however, somewhat variable, as can be seen by reference to Table 13, in which the actual and relative lengths of the intestine are given. It may be seen that in Bombycilla and in Phainopepla, the tracts are much shortened. This is notable, since these are frugivorous birds, and in many frugivorous birds, the tract is lengthened for better extraction of edible portions of the food. Possibly the action of the digestive juices is correspondingly more rapid in Bombycilla and Phainopepla, thereby permitting the necessary nutriment to be extracted by a short digestive tract.

In a migratory bird, or one that depends on flight power to find food and escape capture by predators, as in the case of the waxwings, the compacted and shortened visceral mass would seem to be advantageous, because of the consequent reduction in weight. I consider the longer intestine to be the ancestral condition, and that the intestine has become shorter to meet new environmental conditions.