frugivorous insofar as available data show, although again there is
relatively little information available concerning them. Skutch (MS)
has found that the Guatemalan _Ptilogonys cinereus_ catches insects by
repeated sallies into the air from a perch, after the manner of
flycatchers. He notes also that the birds feed on berries of _Eurya
theoides_ and _Monnina xalapensis_. It is well known that
_Phainopepla_ catches insects when these are available, and its liking
for berries is so apparent that in parts of its range, it is known as
the "pepper bird," since it frequents pepper trees (_Schinus molle_)
and feeds on the small red berries. The preserved specimens of
_Ptilogonys_ and _Phainoptila_ available for this study contain only
berries in the digestive tract. _Dulus_ feeds mostly, if not wholly,
on plant food. According to Wetmore and Swales (1931:349), berries,
fruits, and parts of flowers are eaten.
SKELETON
A critical analysis 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
cumulative, 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 associated bones.
The most specialized bill is to be found in _Dulus_; its bill is
decurved, and the associated 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.
[Illustration: Figs. 1-7. Skulls in lateral view of five genera of
Bombycillidae. Natural size.
1. _Phainoptila m. melanoxantha_, sex?, 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_, sex?, USNM no. 223895, Bozeman, Montana.]
[Illustration: Figs. 8-14. Skulls in ventral view of five genera of
Bombycillidae. Natural size.
8. _Phainoptila m. melanoxantha_, sex?, 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_, sex?, USNM no. 223895, Bozeman, Montana.]
[Illustration: Figs. 15-21. Skulls in dorsal view of five genera of
Bombycillidae. Natural size.
15. _Phainoptila m. melanoxantha_, sex?, 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_, 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.
[Illustration: Figs. 22-28. Humeri of five genera of Bombycillidae.
Natural size.
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
-------------------------+---------+--------+------+-------+-------
_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.
[Illustration: Figs. 29-35. Pygostyles in posterior view of five
genera of Bombycillidae. x 2.
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_.
[Illustration: Figs. 36-42. Pygostyles in lateral view of five
genera of Bombycillidae. x 2.
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
--------------------+-------+-------------+-----------------+-------