Part 8

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

17. Phainopepla nitens, male, MNH no. 24752, Pima Co., Arizona.

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

19. Dulus dominions, female, USNM no. 292642, Don Don, Haiti.

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

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

This flaring, immediately lateral to the antorbital plate, is common to all Bombycillids and constitutes a major skeletal characteristic useful for recognition of the members of the family, since the swelling is easily discernible both externally and on the cleaned skulls. In Phainopepla there is much variability in this character; some specimens have a narrower antorbital bridge than others. Only one skeleton of Phainopepla n. nitens was available. The flaring in the skull of this specimen is identical with that in Ptilogonys. Among the skulls of P. n. lepida in the University of Kansas Museum of Natural History, is No. 19228, a juvenile, taken 5 miles south of Tucson, Arizona. In this specimen, the flaring in the antorbital region is clearly evident and equal in amount to that in skulls of P. n. nitens, but the bird had not attained full skeletal growth. However, the flaring of the antorbital region appears to be common in the nestlings of many species of passerine birds. Other specimens of the subspecies lepida show a varying amount of flaring, the least (in the series available) being in No. 24754, MNH, in which the proportion of the skull (length divided by width) closely corresponds to that in Phainoptila; the skull of No. 24754 is long and thin, and the base of the bill is only slightly swollen. The skull of Phainopepla nitens lepida is more generalized than that of Phainopepla n. nitens, having a longer and narrower bill like the generalized Phainoptila. In Phainopepla n. nitens and in members of the genus Ptilogonys, more flaring occurs in the antorbital region.

Phainoptila, as noted above, has no great amount of flaring in the antorbital region. When more specimens of Phainoptila are examined, the base of the bill probably will be found to flare more in some individuals than in others; this would be expected if we may judge by the data on Phainopepla. The premaxilla and maxilla of Phainoptila are similar to the same bones in Dulus, and there is a well-marked ridge on the tomium (possibly for cutting flower parts). In Phainoptila, the palatines are narrower than in any other genus of the family and abut the lacrimals. The entire skull appears to be modified along different lines from those of the skull of Dulus; the skull of Phainoptila seems to be modified for a frugivorous rather than an insectivorous diet. The skull of Phainoptila probably is more nearly similar to the ancestral skull than is that of any other living species in the family. The wide gape characteristic of some members of the family is undoubtedly a modification for aiding in the capture of insects, and Phainoptila has progressed less in this direction than have other species in the family.

The mandibles vary somewhat in the shape and proportionate size of the bones. The mandible is proportionately, as well as actually, highest in Dulus. The medial condyle varies to some extent, being slightly flattened mediad in Bombycilla, and less so in the other genera. The mandible of Bombycilla narrows to the symphysis much more gradually than it does in the other genera.

The antorbital plate is large and divides the orbital chamber from the nasal chamber. The small lacrimal bone anterior to the plate articulates with the maxilla and the premaxilla. Shufeldt (1889) states that the free lacrimal ossicle might be of some taxonomic importance in the passerines, since it is found in the generalized Corvids and in nestling Turdids. I find it well developed and identical, with a double articulation and free ends, in all the Bombycillids. There is no significant variability in the family, and this is more evidence of close taxonomic relationship between the members of the family.

The size of the crania is somewhat variable, although the differences seem to be primarily those of proportion. Ptilogonatinae have long crania, whereas the crania of the Bombycillinae and Dulinae are shorter but deeper. I regard the longer cranium as primitive, and it is longest in Phainoptila. In order of decreasing relative length of the cranium, Phainoptila is followed by Ptilogonys caudatus, P. cinereus, and Phainopepla. Bombycilla garrula has the deepest cranium in the family.

The measurements of the lengths and widths of the skulls are given in Table 9. The relative length of the bill and relative width of the skull are given in Table 10. These relative measurements are calculated by using the actual measurements in Table 9 as numerators, the length of the skull from the lacrimal bone to the posteriormost end of the skull being used as the denominator. The data indicate that Phainoptila has a slightly narrower cranium.

Humerus.--Certain families of passerine birds have a noticeable variation in the characteristics of the humerus; the bone varies in length, in diameter, and in the complexity of the processes at either end. In the Bombycillids, however, the amount of variation is relatively small, and the diaphysis of the bone is somewhat twisted, especially so in Dulus. The deltoid tuberosity is variable, being shorter but more elevated in Bombycilla than it is in the Ptilogonatinae and in the Dulinae. The tendon from the pectoralis major muscle, which inserts on this process, probably finds better insertion on a higher process than on a lower but longer one.

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

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

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

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, sex?, 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 pronounced 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 | Manus -------------------------+---------+--------+------+------- 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 | Manus | 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 | Manus | 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 thalassina | 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 -------------------------+---------+--------+------+-------+-------