SKELETON

A critical a.n.a.lysis of the skeletons provides evidence that aids the student in estimating which differences are merely the result of habits developed in relatively recent geological time as opposed to those which owe their existence to more ancient heritage. Stresses caused by the action of different sets of muscles can apparently stimulate changes in bones to meet new needs, and the evidence from genetics is that such mutations in wild birds are minute and c.u.mulative, rather than of large degree and of sudden appearance. Once adaptive mutations have occurred, if genetic isolation from one source or another accompanies it, a new population different from the parental stock may become established. Study of the skeleton of any species of living bird may indicate those characters identifiable as modifications fitting it to a particular environment. If no distinguishing characters are discovered that may be attributed to environmental factors, such a species can be spoken of as generalized; the inference then is that such a species is not modified for a single, particular ecological niche.

Some parts of the skeleton, obviously, are more adaptable or plastic than others. The beak seems to be the most adaptable part. Probably this results from its frequent use; it is the part of the bird to capture the food. The long bones, meeting the environment as legs which serve as landing mechanisms or as locomotory appendages, and as wings which provide considerable locomotion for most birds, probably come next in order as regards plasticity. In these parts, then, one may look for the most change in birds, which, within relatively recent geologic times, have been modified to fit a particular set of conditions. From the beak and long bones of a species in which habits are unknown, one can infer the habits and habitat from a comparison with the skeletal features of species of known habits.

_Skull._--The skulls in all three subfamilies have essentially the same general appearance and structure, the most marked differences being, as would be expected, in the bills and a.s.sociated bones.

The most specialized bill is to be found in _Dulus_; its bill is decurved, and the a.s.sociated bones are correspondingly changed for support of the bill. For example, the palatines and "vomer" are much wider, the palatines are more concave from below and have longer posterior processes than the corresponding bones in _Bombycilla_.

Moreover, the "vomer" in _Dulus_ and in _Phainoptila_ is larger and heavier than in _Bombycilla_, and the quadrate and pterygoid bones are relatively large for support of the beak. The palatines, however, are weak in _Phainoptila_. In the Ptilogonatinae, with the exception of _Phainoptila_, the wings of the palatines flare more than in _Bombycilla_, but not to the extent that they do in _Dulus_, nor does the palatine bone present a concave appearance in the Ptilogonatinae.

The premaxilla is a relatively weak bone in _Bombycilla_ and _Phainopepla_, stronger in _Ptilogonys_, and is notably heavy in _Phainoptila_ and _Dulus_, and in these latter two genera shows a sharply-ridged tomium. The maxillae connect to somewhat widened nasal and naso-lateral processes in all the genera, and the premaxillae narrow abruptly from this point forward. In the family, _Phainopepla_ and _Phainoptila_ show the least flaring in this region.

[Ill.u.s.tration: Figs. 1-7. Skulls in lateral view of five genera of Bombycillidae. Natural size.

1. _Phainoptila m. melanoxantha_, s.e.x?, MNH no. 26493, 15 mi.

SE Cartago, Costa Rica.

2. _Ptilogonys caudatus_, male, MNH no. 24492, 15 mi. SE Cartago, Costa Rica.

3. _Phainopepla nitens_, male, MNH no. 24752, Pima Co., Arizona.

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

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

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

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

[Ill.u.s.tration: Figs. 8-14. Skulls in ventral view of five genera of Bombycillidae. Natural size.

8. _Phainoptila m. melanoxantha_, s.e.x?, MNH no. 26492, 15 mi.

SE Cartago, Costa Rica.

9. _Ptilogonys caudatus_, male, MNH no. 24492, 15 mi. SE Cartago, Costa Rica.

10. _Phainopepla nitens_, male, MNH no. 24754, Pima Co., Arizona.

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

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

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

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

[Ill.u.s.tration: Figs. 15-21. Skulls in dorsal view of five genera of Bombycillidae. Natural size.

15. _Phainoptila m. melanoxantha_, s.e.x?, MNH no. 26493, 15 mi.

SE Cartago, Costa Rica.

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_, s.e.x?, USNM no. 223895, Bozeman, Montana.]

This flaring, immediately lateral to the antorbital plate, is common to all Bombycillids and const.i.tutes 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 pa.s.serine 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 pa.s.serines, since it is found in the generalized Corvids and in nestling t.u.r.dids. 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 pa.s.serine 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.

[Ill.u.s.tration: Figs. 22-28. Humeri of five genera of Bombycillidae.

Natural size.

22. _Phainoptila m. melanoxantha_, s.e.x?, 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.