Phylogeny of the Waxwings and Allied Birds Part 5

25. _Ptilogonys cinereus_, female, Louisiana State University no. 297, Xilitla Region, San Luis Potosi, Mexico.

26. _Dulus dominicus_, female, USNM no. 292652, Don Don, Haiti.

27. _Bombycilla cedrorum_, male, MNH no. 15331, Bexar Co., Texas.

28. _Bombycilla garrula_, s.e.x?, USNM no. 223895, Bozeman, Montana.]

Distally, the two major condyles and the intercondylar groove or olecranon fossa that make efficient articulation with the ulnar process, are not variable. The external condyle, however, is significantly variable in the family. This condyle is longest and most p.r.o.nounced in birds in which the humerus is short in relation to the trunk, as for example in _Tachycineta_. In the Bombycillidae the condyle is smallest in _Phainoptila_, where it is a mere suggestion of a process. In the remainder of the Ptilogonatinae, the condyle is larger but rounded, and shows a double process in _Ptilogonys caudatus_, and a slightly pointed process in _P. cinereus_. The external condyle in _Dulus_ is not specialized, being low and rounded, but in _Bombycilla_, it is noticeably elongated, indicating a better attachment distally for the deltoid muscle. (No measurements are tabulated for this condyle, as the percentage of error in measuring this small structure is great.) Table 1 gives lengths of humeri, and Table 2 gives lengths of the humeri expressed as percentages of the length of the trunk, a standard measurement.

The area of insertion of the deltoid muscle is elongated in those birds with shortened humeri; these birds have also greater flight power than do birds with longer humeri and therefore a shorter external condyle.

Table 1. Lengths of Arm Bones in cm.

=========================+=========+========+======+======= Species | Humerus | Radius | Ulna | Ma.n.u.s -------------------------+---------+--------+------+------- Ptilogonys caudatus | 2.39 | 2.57 | 2.79 | 2.25 Ptilogonys cinereus | 2.24 | 2.48 | 2.78 | 2.38 Phainopepla nitens | 2.21 | 2.59 | 2.82 | 2.39 Phainoptila melanoxantha | 2.40 | 2.51 | 2.70 | 2.25 Dulus dominicus | 2.23 | 2.38 | 2.63 | 2.31 Bombycilla garrula | 2.35 | 2.58 | 2.88 | 2.67 Bombycilla cedrorum | 2.06 | 2.34 | 2.60 | 2.38 -------------------------+---------+--------+------+-------

Table 2. Arm-trunk Ratios (in percent)

=========================+=========+========+======+=======+======= Species | Humerus | Radius | Ulna | Ma.n.u.s | Total -------------------------+---------+--------+------+-------+------- Ptilogonys caudatus | 85 | 92 | 93 | 80 | 2.58 Ptilogonys cinereus | 84 | 90 | 103 | 89 | 2.76 Phainopepla nitens | 84 | 98 | 107 | 91 | 2.82 Phainoptila melanoxantha | 73 | 77 | 82 | 69 | 2.31 Dulus dominicus | 78 | 83 | 92 | 81 | 2.51 Bombycilla garrula | 69 | 75 | 87 | 78 | 2.34 Bombycilla cedrorum | 67 | 76 | 85 | 77 | 2.29 -------------------------+---------+--------+------+-------+-------

Table 3. Arm-trunk Ratios (in percent)

=========================+=========+========+======+=======+======= Species | Humerus | Radius | Ulna | Ma.n.u.s | Total -------------------------+---------+--------+------+-------+------- Corvus brachyrynchos | 90 | 101 | 111 | 106 | 307 Dendroica audubonii | 68 | 82 | 90 | 77 | 237 Setophaga ruticilla | 69 | 82 | 91 | 75 | 235 Myadestes townsendi | 71 | 84 | 96 | 81 | 248 Sialia sialis | 72 | 84 | 98 | 86 | 256 Hylocichla mustelina | 75 | 81 | 92 | 80 | 247 Parus atricapillus | 85 | 90 | 106 | 81 | 272 Tachycineta thala.s.sina | 71 | 95 | 107 | 128 | 306 Myiarchus crinitus | 83 | 105 | 115 | 92 | 290 Dumetella carolinensis | 76 | 75 | 89 | 78 | 243 Polioptila caerulea | 85 | 93 | 105 | 71 | 261 Eremophila alpestris | 91 | 99 | 110 | 95 | 296 Muscivora forficata | 85 | 111 | 120 | 108 | 313 -------------------------+---------+--------+------+-------+-------

_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.

[Ill.u.s.tration: Figs. 29-35. Pygostyles in posterior view of five genera of Bombycillidae. 2.

29. _Phainoptila m. melanoxantha_, s.e.x?, 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_, s.e.x?, 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 k.n.o.bs; both k.n.o.bs and indentations are more p.r.o.nounced 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_.

[Ill.u.s.tration: Figs. 36-42. Pygostyles in lateral view of five genera of Bombycillidae. 2.

36. _Phainoptila m. melanoxantha_, s.e.x?, 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_, s.e.x?, 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 pa.s.serine 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 --------------------+-------+-------------+-----------------+-------