Table 13. Digestive Tract: Actual Length, and Length Relative to Thoracic Length
| Species | Length in mm. | Relative length (in percent) |
| Ptilogonys caudatus | 134 | 476.9 |
| Ptilogonys cinereus | 111 | 415.6 |
| Phainopepla nitens | 94 | 357.5 |
| Phainoptila melanoxantha | 150 | 457.1 |
| Dulus dominicus | 130 | 451.0 |
| Bombycilla garrula | 102 | 298.2 |
| Bombycilla cedrorum | 95 | 309.5 |
Beddard (1898:30) states that caecae in the tract may be highly variable in a single family of birds. The Bombycillidae is no exception in this regard. At the junction of the cloaca and the large intestine, there are two small caecae, the function of which is unknown to me. The caecae are largest in the Ptilogonatinae, smaller in the Bombycillinae, and smallest in the Dulinae. There may be a correlation between large caecae and more insectivorous diet and small caecae and frugivorous diet; however, the data are not conclusive in this regard.
It is here postulated that the center of origin for the ancestral stock of the Bombycillidae was in a region of North America, which at the time concerned was temperate or possibly even semi-tropical in climate. 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.
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. The phyletic age of the family is probably great, however, as evidenced by the spotty distribution of the birds.
In the evolution of this family, population pressure possibly played the initial role in forcing members of the primitive, southern stock to seek habitable areas on the periphery of the range. Some birds also, being possessed of the "adventuresome spirit", aided the northerly movement, thus effecting an extension of the breeding ranges to the north. So far as is now known, this family did not seek living space in South America. By extending its range, a species might find more abundant food and nesting sites. This process of extending the range probably would be costly to the species concerned, because only those individuals best able to adapt themselves to the new environmental conditions would be able to survive long enough to reproduce their kind.
The return flight to the south could, in time, be dispensed with, except in the coldest weather or when the local berry- and fruit-crop failed. Birds such as waxwings are, of course, able to subsist on dried fruits and berries in the critical winter season when strictly insectivorous birds, not so catholic in their food habits, must return south. It appears that waxwings are descendants of migratory birds that have adjusted themselves to a life in the north; and they are judged not to have evolved from year-round residents of the north.
Even a short migratory journey in spring by part of a population of birds, while the other part remained in the original range, would quickly isolate one breeding population from the other, resulting in the formation of different genetic strains that lead to subspecies, species, and finally to genera and families. Any variation away from the ancestral, "sedentary" stock would become established more quickly because of such isolation at the breeding period. By the same token, the parental stock can, and no doubt does, become modified to suit its environment more perfectly, thus accelerating the tempo of this type of divergent evolution.
The original "split" of the Bombycillines is thought then to have been the result of migration on the part of some of the ancestral stock, with subsequent loss of regular migration because the need to return south was lost. Early in development, and before the migrational tendency was entirely lost, an isolated population, which later became sedentary, as it was an island population, diverged to give rise to the Dulinae. The Dulinae are a homogeneous group since on the islands now inhabited by the birds, they have not been isolated sufficiently long to produce even well-marked subspecies.
The present day Phainoptila is most nearly like the ancestral group, and the remainder of the Ptilogonatinae have diverged to fit conditions similar to those to which the Tyrannid flycatchers, which parallel them, are also fitted.
In comparatively recent geological time, two basic lines developed from the Bombycilline stock, the future B. garrula and B. cedrorum. Possibly garrula originally was isolated in Europe and Asia, and later came into contact with B. cedrorum, following the time at which the two species were genetically well differentiated. It appears certain that B. japonica was an offshoot of the Bombycilline stock at an early time, since it has characteristics that seem relatively unspecialized. It possibly was isolated in the Orient.
Structural affinities of Dulus and Bombycilla are more pronounced than are those of Dulus and Ptilogonys, for example. Many of the structural features of Dulus parallel those of Phainoptila, and it seems likely that the Dulinae were separated early in the history of the family, perhaps as an isolated offshoot of the early migratory Bombycillinae.
Nomenclature, as used by a taxonomist, should of course indicate affinities as well as apply a name, and the rank of the family should be applied to a structural unit based on common anatomical characters that are more fundamental than, in my opinion, are those used by Ridgway (1904) in proposing family status for the silky flycatchers and the palm-chats. The characters in the diagnosis (page 478) of the family Bombycillidae are common features regarded as warranting a single family unit for the waxwings, silky flycatchers, and palm-chats. The differences in morphology used by previous workers to characterize each of these groups: (1) the silky flycatchers; (2) waxwings and; (3) palm-chats are regarded as more properly characters of only subfamily rank.
The existing coloration of the species of the Bombycillidae appears to have been acquired relatively late, geologically speaking. The three subfamilies responded to ecological stimuli in three different ways, and the resulting color patterns are unlike in the three groups. Dulinae to this day have a color pattern that is most like the ancestral color pattern, and this is recapitulated in the juvenal plumage of the Bombycillinae before they attain their adult plumage.
Consideration of the geographic distribution of the species of the family indicates that the center of origin of the family Bombycillidae was south of the present range of the waxwings (subfamily Bombycillinae). Waxwings probably are the descendants of a migratory population that diverged from the primitive population at an early time in the history of the family. Owing to their adaptations to survive in the north, waxwings no longer return south in the autumn. Palm-chats (subfamily Dulinae) are descendants of an isolated population of the family stock that developed communal living habits as one specialization. Silky Flycatchers (subfamily Ptilogonatinae) became modified to catch insects, and have specializations that roughly parallel those of the Tyrannid flycatchers.
Osteologically, the various species of the Bombycillidae are remarkably similar. Small variations do exist, but these are primarily differences in relative size. The modifications of the beak enable palm-chats to feed on parts of plants, and the beak of Phainoptila shows some similarity in this respect. Rounded wings, which cause a bird to fly by means of short, relatively weak strokes, are correlated with a comparatively long humerus, whereas long and pointed wings, which enable a bird to fly with more powerful strokes of the wing, are correlated with a relatively short humerus. There is a positive correlation between a short humerus and a long external condyle, and between a long humerus and the absence or smallness of the external condyle.
In the Bombycillidae short bones of the leg are adaptive, and long bones of the leg are the generalized condition. Although all passerine birds were differentiated relatively late in geologic time, long hind limbs still could have been present in the immediate ancestors of passerine birds. As adaptive radiation took place in the class Aves, some birds, the Bombycillidae included, became more and more adapted for an arboreal, and eventually an aerial habitat, with consequent loss of saltatorial and running ability.
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. In waxwings, all three elements of the hind limb are shortened, indicating that the reduction in length has been, evolutionarily speaking, a rapid process, in order to reduce the limbs to a convenient size as soon as possible.
The shape of the pygostyle varies in the Bombycillidae, but the simple shieldlike bone of Phainoptila is judged to resemble closely the ancestral type. In Ptilogonys there is a tall dorsal spine, coupled with a wide and heavy centrum and flattened lateral areas, for support of the long rectrices. In Bombycilla the bone is small with knobs on the centrum that have been developed for muscle attachment.
The muscles were carefully dissected in each genus and in most of the species. The same homologous muscles are present in all species. Significant differences were found only in the relative size of certain muscles. No satisfactorily accurate method of measuring these differences was found. Consequently, less use was made of the results of the dissections than was originally planned.
The set of pectoral muscles varies but slightly in relative mass, and the variation is not considered significant. The deltoid muscle was selected for measurement since its point of insertion is unusually variable, while the mass of the muscle varies little. We can conclude that the extent of the area of insertion of the tendon of a muscle can determine that muscle's relative efficiency, while the muscle itself remains the same in bulk.
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.
The gross anatomy of the digestive tract is practically identical in the members of the family. The variability noted is mainly in the degree of compactness of the visceral mass in Bombycilla and in Phainopepla. Also there is a tendency for the Bombycillinae and the Dulinae to have the mass situated more posteriorly than it is in the Ptilogonatinae. Moreover, Bombycilla has a shorter intestine than do the other genera. All of this indicates that the waxwings (Bombycillinae) have the center of gravity situated more advantageously for flight than do the birds of the two other subfamilies.
| 1. | The silky flycatchers, waxwings, and palm-chats are included in the family Bombycillidae; the Ptilogonatidae and Dulidae are reduced to subfamily rank. |
| 2. | The coloration of the birds of each subfamily is different because the ecological needs are different. |
| 3. | Waxwings were at one time regularly migratory, but are now nomadic, since they are adapted to live in northern latitudes for the entire year. |
| 4. | The corresponding bones in different members of the family closely resemble one another, and the differences which do exist are the results of responses within relatively recent times to changes in habits. |
| 5. | In the Bombycillidae a rounded wing is judged to be the primitive condition. As the wing becomes more pointed, the humerus becomes shorter and its external condyle longer. |
| 6. | The hind limbs are short in birds that depend most on flight power, but are longer and the distal elements are disproportionately longer in birds that depend on saltation or on running. |
| 7. | The pygostyle varies in shape and size between genera and even between some species. |
| 8. | The pectoral muscles differ in size only slightly in the different members of the family, but the insertions are more extensive for these muscles in birds that fly a great deal. |
| 9. | The muscles of the hind limb vary in mass, but not in kind, in the members of the family Bombycillidae. |
| 10. | In the Bombycillidae that depend on flight power, rather than on saltation or on running power, there is a tendency for the digestive tract to become shorter and for the whole visceral mass to become more compact. |
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Mention should be made here of an important paper by Jean Delacour and Dean Amadon (1949). The Relationships of Hypocolius (Ibis, 91:427-429, plates 19 and 20) which appeared after the present paper by Arvey was written. Delacour and Amadon stated that Hypocolius, a monotypic Persian genus, should be assigned to the Bombycillidae. Their conclusions (op. cit.:429) were as follows: "It might be advisable to set up three subfamilies in the Bombycillidae, one for Bombycilla, one for Hypocolius, and a third for the silky flycatchers, Ptilogonys, Phainopepla and Phainoptila. Further study may show that Dulus can be added as a fourth subfamily.
"Previously the Bombycillidae appeared to be an American group of which one genus (Bombycilla) had reached the Old World. Inclusion of Hypocolius in the family makes this theory uncertain. Without obvious affinities to other families, and consisting of a small number of scattered and rather divergent genera, the Bombycillidae would seem to be a declining group whose origin cannot safely be deduced from the distribution of the few existing species."