Part 11

============================================= Species | Levator | Lateralis ------------------------+---------+---------- Ptilogonys caudatus | .145g. | .022g. | .092% | .045% | | Ptilogonys cinereus | .030g. | .010g. | .076% | .026% | | Phainopepla nitens | .025g. | .008g. | .096% | .029% | | Phainoptila melanoxantha| .040g. | .015g. | .063% | .014% | | Dulus dominicus | .028g. | .006g. | .063% | .014% | | Bombycilla garrula | .034g. | .010g. | .048% | .014% | | Bombycilla cedrorum | .026g. | .008g. | .050% | .014% ---------------------------------------------

Table 12. Weights of Muscles (These percentages expressed in terms of weights of the body)

Key to Table A) Deltoid B) Thigh C) Peronus D) Gastrocnemius

==================================================================== Species |P. major|P. minor| A | B | C | D ---------------+--------+--------+--------+--------+-------+-------- Ptilogonys | 2.42g. | .29g. | .55g. | | | caudatus | 4.94% | .59 | 1.12% | .43g. | .15g. | | | | | .88% | .31% | .96% Ptilogonys | 2.19g. | .28g. | .53g. | | | cinereus | 5.57% | .71% | 1.35% | .30g. | .08g. | | | | .71% | .21% | 1.02% Phainopepla | 1.30g. | .20g. | .30g. | | | nitens | 4.99% | .77% | 1.15% | .28g. | .10g. | | | | | 1.12% | .40% | 1.42% Phainoptila | 3.93g. | .44g. | .92g. | | | melanoxantha | 6.18% | .69% | 1.45% | 1.09g. | .48g. | | | | | 1.61% | .75% | 2.97% Dulus | 2.09g. | .22g. | .50g. | | | dominicus | 4.81% | .50% | 1.15% | .73g. | .18g. | | | | | 1.68% | .41% | 1.01% Bombycilla | 3.85g. | .45g. | .55g. | | | garrula | 5.31% | .62% | .76% | .50g. | .15g. | | | | | .69% | .18% | .59% Bombycilla | 2.58g. | .35g. | .50g. | | | cedrorum | 5.00% | .68% | .97% | .37g. | .10g. | | | | | .73% | .19% | .83% ---------------+--------+--------+--------+--------+-------+--------

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. The humerus in Bombycilla is shortened, and the muscle therefore does not need to be large to accomplish the same powerful stroke that would be accomplished by a longer humerus and a larger, more powerful deltoid muscle. In the case of the deltoid, the shortening of the humerus and the more complex arrangement of the points of insertion have obviated the necessity of enlarging the muscle.

Leg Musculature.--The muscles of the thigh are noticeably larger in birds that have long leg bones. (See Table 12 for size of muscles.) 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.

The volume of the muscles of the hind limb changes more readily in response to saltation and running than do the muscles of the forelimb to flying.

DIGESTIVE TRACT

The digestive tract is relatively uniform in all genera of the family; there are only slight differences between the species. The degree of compactness of the visceral mass varies, Phainoptila and Ptilogonys caudatus having the folds of the digestive tract loosely arranged, whereas Ptilogonys cinereus and Phainopepla have folds which adhere more tightly to the ventriculus and liver. In Dulus and Bombycilla, as compared with the Ptilogonatinae, the visceral mass (primarily liver and ventriculus) is situated more posteriorly in the body cavity, and is more compact, and the intestine is more tightly coiled.

The coiling of the intestine, if its degree of compactness is disregarded, is nearly identical in the birds of the family; there are four major loops between the ventriculus and the anus. The length of this section of the tract is, however, somewhat variable, as can be seen by reference to Table 13, in which the actual and relative lengths of the intestine are given. It may be seen that in Bombycilla and in Phainopepla, the tracts are much shortened. This is notable, since these are frugivorous birds, and in many frugivorous birds, the tract is lengthened for better extraction of edible portions of the food. Possibly the action of the digestive juices is correspondingly more rapid in Bombycilla and Phainopepla, thereby permitting the necessary nutriment to be extracted by a short digestive tract.

In a migratory bird, or one that depends on flight power to find food and escape capture by predators, as in the case of the waxwings, the compacted and shortened visceral mass would seem to be advantageous, because of the consequent reduction in weight. I consider the longer intestine to be the ancestral condition, and that the intestine has become shorter to meet new environmental conditions.

Table 13. Digestive Tract: Actual Length, and Length Relative to Thoracic Length

=========================+========+============== | | Relative Species | Length | length | in mm. | (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.

ORIGIN OF THE SPECIES

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.

CONCLUSIONS

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.