Part 9

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.

29. Phainoptila m. melanoxantha, sex?, MNH no. 26493, 15 mi. SE Cartago, Costa Rica.

30. Ptilogonys caudatus, male, MNH no. 24492, 15 mi. SE Cartago, Costa Rica.

31. Phainopepla nitens, male, MNH no. 24754, Pima Co., Arizona.

32. Ptilogonys cinereus, female, Louisiana State University no. 297, Xilitla Region, San Luis Potosi, Mexico.

33. Dulus dominicus, female, USNM no. 292652, Don Don, Haiti.

34. Bombycilla cedrorum, male, MNH no. 15331, Bexar Co., Texas.

35. Bombycilla garrula, sex?, USNM no. 223895, Bozeman, Montana.]

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.

36. Phainoptila m. melanoxantha, sex?, MNH no. 26493, 15 mi. SE Cartago, Costa Rica.

37. Ptilogonys caudatus, male, MNH no. 24492, 15 mi. SE Cartago, Costa Rica.

38. Phainoptila nitens, male, MNH no. 24754, Pima Co., Arizona.

39. Ptilogonys cinereus, female, Louisiana State University no. 297, Xilitla Region, San Luis Potosi, Mexico.

40. Dulus dominicus, female, USNM no. 292652, Don Don, Haiti.

41. Bombycilla cedrorum, male, MNH no. 15331, Bexar Co., Texas.

42. Bombycilla garrula, sex?, USNM no. 223895, Bozeman, Montana.]

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.

Table 4. Lengths of Leg Bones in cm.

=========================+=======+=============+================= Species | Femur | Tibiotarsus | Tarsometatarsus -------------------------+-------+-------------+----------------- Ptilogonys caudatus | 2.04 | 3.10 | 1.94 Ptilogonys cinereus | 1.89 | 2.90 | 1.77 Phainopepla nitens | 1.76 | 2.78 | 1.72 Phainoptila melanoxantha | 2.43 | 3.77 | 2.58 Dulus dominicus | 2.09 | 3.34 | 2.09 Bombycilla garrula | 2.32 | 3.46 | 1.99 Bombycilla cedrorum | 1.92 | 2.95 | 1.64 -------------------------+-------+-------------+-----------------

Table 5. Leg-trunk Ratios (in percent)

====================+=======+=============+=================+======= Species | Femur | Tibiotarsus | Tarsometatarsus | Total --------------------+-------+-------------+-----------------+------- Ptilogonys caudatus | 73 | 110 | 69 | 252 Ptilogonys cinereus | 71 | 109 | 66 | 246 Phainopepla nitens | 69 | 106 | 65 | 240 Phainoptila | 74 | 115 | 60 | 249 melanoxantha | | | | Dulus dominicus | 73 | 119 | 73 | 265 Bombycilla garrula | 68 | 101 | 59 | 228 Bombycilla cedrorum | 63 | 96 | 53 | 212 --------------------+-------+-------------+-----------------+-------