M. Dale Arvey

“ Pectoral Muscles.—The pectoral set of muscles varies but little in the family; flight power is seemingly not dependent upon size of either the pectoralis major or pectoralis minor. The data indicate that the insertion on the humerus, with consequent changes in the relative length of that bone, is more significant in type of flight and over-all flight power than is the actual size of the muscle mass. The deltoid muscle, for example, is smaller in Bombycilla than in members of the other two subfamilies. ”

“ Any characteristic is of survival value to an organism if in any way the characteristic enhances the chances of survival up to the time when the organism can successfully raise even a few young to maturity. ”

“ The distribution of the family is such that the more primitive groups are in the south. These are the Ptilogonatinae in Central America and Mexico, and the isolated Dulinae in Haiti and the Dominican Republic. This distribution would support the view that the origin was in the south. However, the Holarctic Bombycillinae are so typically birds of northern latitudes that, were it not for such close relatives south of their range, it would appear logical to infer a northerly origin with a subsequent shifting of populations both southward and northward. ”

“ This shortness of limb is obviously adaptive, and each of the segments of the limb has been correspondingly shortened, with no element reduced at the expense of the other two. The short leg can be more easily folded against the body while the bird is in flight, than can a long leg which is more unwieldy. It may be noted from tables 4 and 5 that birds which spend much time on the ground, or that hop a great deal in the underbrush, have longer legs than do birds which spend much time in flight. ”

“ On the tibiotarsus, the peroneus and gastrocnemius muscles were measured. When expressed as a percentage of the weight of the bird, the peroneus has much the same relative weight in all but one of the species, whereas the gastrocnemius varies much. The peroneus is proportionately large only in Phainoptila, in which genus all the leg muscles are well developed, but the gastrocnemius is larger in all the Ptilogonatinae and in Dulus than it is in the specialized Bombycilla, in which it has probably been reduced as the leg bones and other muscles have been reduced. ”

“ In specialization for muscle attachment, the centra of the pygostyles of the Ptilogonatinae have more area for muscle attachment [Pg 505] 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. ”

“ Birds, like mammals, have a short femur, the most proximal element in the leg, if the species is adapted to run fast. If the species is not adapted to run fast, birds, unlike mammals, have the tibiotarsus longer than any of the other elements; in mammals that are not adapted to run fast, the femur and tibia are approximately the same length. In non-running birds as compared with running birds, the leg element distal to the tibiotarsus, and the one proximal to it, are considerably shortened. ”

“ Desertion of the nest is not uncommon in waxwings, despite the tolerance to other animals that is shown by the birds. A new nest may suddenly be begun before the first one is finished, and all the materials from the first nest may be removed, or the nest may be abandoned before it is completed. The eggs may be left at any time up to hatching, and the young may be deserted, especially in the earlier stages of development.
The very large and bulky communal nest of Dulus is not radically different from the nest of waxwings. ”

“ 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. ”

“ 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. ”

“ 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. ”

“ Coloration, which varies relatively rapidly in response to differing ecological conditions, has become more different in the species of Bombycillidae than is true in many other families of passerine birds. The explanation lies in early geographical isolation of the three subfamilies, with consequent radiation in three directions. Waxwings have become adapted by possessing a thick protective layer of feathers and drab coloration broken by ruptive marks. They still retain the streaked plumage, which is probably ancestral, in the juveniles ”

“ The extreme condition is that found in Dulus, in which the territory is not limited even to [Pg 490] the nest, but to the individual compartment of the community nest. Phainopepla, a less gregarious bird than Dulus and waxwings, has a much more definite territory, although individuals of Phainopepla are tolerant of others of the same species; no feeding territory is established, and small flocks of birds feed together at any time of the year. ”

“ Some amount of territoriality is characteristic of this subfamily and dimorphism in color—the plumage of the male is outstandingly conspicuous—possibly is of selective value to the race. In a tropical forest community, a duller pattern possibly would be more visible and thus would be selectively disadvantageous.
COURTSHIP
Waxwings are gregarious birds and individuals establish no well-defined territories as do many birds. The nest itself is the only defended territory, and as Crouch (1936) has shown, the Cedar Waxwing will nest in close proximity to others of the same species. ”

“ Dissections showed the same muscles to be present in all genera of the Bombycillidae. There are, nevertheless, differences in the size of the muscles in the various species, and these differences have been investigated primarily as a check on differences noted in the structure of the bones. Even slight differences in mass can be important functionally, but the difficulty in accurately measuring the mass prevents wholly reliable conclusions. ”

“ These muscles are used by the living bird to maintain the position of the pygostyle and therefore the rectrices; these muscles are especially important to those birds that utilize the tail as a rudder in flight and as a brake. As may be seen by reference to Table 11, the two muscles are largest in proportion to body weight in the Ptilogonatinae, in which subfamily the species have long rectrices and must have correspondingly well-developed muscles in order to utilize the rectrices to best advantage in flight. ”

“ Bombycilla japonica thus may be close to the ancestral condition in the waxwing stock in the development of the waxy appendage.
The rectrices of all three species of waxwings seldom possess the wax tips, unless the secondaries have the maximum number of tips. In these individuals, the pigment seems to "spill over" onto the tail feathers. Eight is the maximum number of tips found on the secondaries. Rectrices with wax tips are more frequently found in B. garrula, and only occasionally in B. cedrorum. ”

“ Rather, the adults "store" food in the form of berries in the expanded esophagus or crop, feeding them whole to the young. Digestion is an unusually rapid process, involving merely minutes for the passage of berries and cherries. This is correlated with a short intestinal tract, which is unusual for a frugivorous bird. Nice's (1940) experiments with Cedar Waxwings revealed that cherries would pass through the digestive tract in 20 minutes, blueberries in 28 minutes, and chokecherries in 40 minutes. ”

“ Nice (1941) experimented with a young captive Cedar Waxwing and found that it had a decided preference for red or blue berries, and that meal worms were utilized as food only when the birds became educated by other captive birds of other species as to the food value of the worms. Post (1916) indicates that the food given to the nestlings of Cedar Waxwings is entirely animal for the first three days, and that a mixed diet of berries and insects is subsequently offered.
In feeding of the young, regurgitation of partly digested food does not take place, according to Wheelock (1905) . ”

“ The muscles of the hind limb are notably larger in species that have long legs, and a good index of the hopping ability may be gained by study of certain of these muscles. In the Bombycillidae, and in those Ptilogonatinae that do not use the hind limbs for hopping, the bones are shortened, and the associated muscles are correspondingly smaller. ”

“ 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 [Pg 501] the deltoid muscle. (No measurements are tabulated for this condyle, as the percentage of error in measuring this small structure is great.) ”

“ 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. ”

“ These ruptive marks on an otherwise blending type of plumage might be important to waxwings, and probably are more effective when the birds remain motionless in either a well-lighted area or in one that is partly in shadow, rather than in one that is wholly in shadow.
The red wax tips on the secondaries of the flight feathers, and sometimes found on the ends of the rectrices in Bombycilla, are puzzling and no wholly convincing reason has been suggested for their occurrence. Two instances are known of yellow instead of red-colored wax tips in B. cedrorum (Farley, 1924) . ”

“ Since the fossil record does not permit us to say when in geological time the family originated, we must rely on anatomical evidence and the distributional evidence of present-day species to estimate when the family stock had diverged from some unknown group sufficiently to merit the status of a separate family. ”

“ Probably Northern Mexico was the place and probably the climate was temperate. It is reasonably certain, because of the distribution of the species of the family, that they originated in the Americas. In the absence of paleontological data (Bombycilla alone is reported, in essentially its modern form, from the late Pleistocene—Wetmore, 1940a) , the place and time of origin cannot certainly be determined. ”

“ It would be valuable to know more concerning the life histories of the other birds considered in this paper, since behavior is inherent, and probably can be cited as evidence of close relationship or the opposite. All that I have been able to learn is that Phainopepla has a nuptial flight in which the male chases the female, and that Dulus (Wetmore and Swales, 1931:347) seeks the company of others of its kind at all times, and that two birds, presumably paired, will sidle up to one another when they are perched.
NEST BUILDING
There are numerous papers concerning the nesting of waxwings. ”

“ Coloration of adult females: Head plain Smoke Gray, passing into White on frontals, malars, and chin; back, scapulars, wing coverts, and rump Hair Brown; upper tail coverts Dark Gull Gray; remiges and rectrices Black with faint Dusky Green gloss, edged with Gull Gray; chest Dark Grayish Brown lightening to Wood Brown on sides and flanks; abdomen White ”