Table 3. Arm-trunk Ratios (in percent)

SpeciesHumerusRadiusUlnaManusTotal
Corvus brachyrynchos90101111106307
Dendroica audubonii68829077237
Setophaga ruticilla69829175235
Myadestes townsendi71849681248
Sialia sialis72849886256
Hylocichla mustelina75819280247
Parus atricapillus859010681272
Tachycineta thalassina7195107128306
Myiarchus crinitus8310511592290
Dumetella carolinensis76758978243
Polioptila caerulea859310571261
Eremophila alpestris919911095296
Muscivora forficata85111120108313

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 Luís 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.