Read Ebook: Phylogeny of the Waxwings and Allied Birds by Arvey M Dale Martin Dale

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Table 5. Leg-trunk Ratios

Table 6. Leg-trunk Ratios

Table 7. Actual Length and Width in mm. of Pygostyle and Proportionate Length and Width of Pygostyle in percent of Lacrimal Length

Table 8. Length of Sternum and Depth of Carina expressed as percentages of the Length of the Trunk

Table 9. Skull and Sternum, Length and Width in mm.

The length of the trunk was taken as the distance from the anterior tip of the neural crest of the last cervical vertebra to the anterior edge of an acetabulum. The number of free thoracic vertebra was five in each specimen; consequently, there was no error from this source. In the cranium, a measurement was taken from the anterior edge of the lacrimal bone to the posteriormost end of the cranium, and the resultant figure was employed for a constant in cases in which small bones were compared.

Table 10. Relative Length and Width of Skull

It is notable that, in general, birds with long and narrow wings appear to have relatively the shortest humeri, with the distal bones, especially the manus, variable in length and seemingly correlated with the manner of feather attachment. Those birds with rounded and short wings have the longest humeri. In swallows, for example, the humerus is short, whereas the other arm bones are long, and the manus is unusually large and heavy. A short humerus gives better lever action in the flight stroke than a long humerus does.

MUSCULATURE

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. The method first used in the attempt to determine the mass of a given muscle was that of immersing the muscle in a liquid-filled graduated tube, and then measuring the amount of liquid displaced. This method, although adequate for large muscles, was subject to a great amount of error in the case of small muscles, and consequently was abandoned. The technique eventually used was that previously employed by Richardson . It consisted of dissecting out the muscle, placing it in embalming solution, leaving it there until a later period, and finally, weighing the muscle on scales, accurate to a milligram, after the muscle had been out of the liquid for a period of one minute. After being weighed, the muscle was measured by the displacement method in a graduated tube, as a check. The results indicate that, although the two methods give the same general results, weighing is accurate to one-hundredth of a gram, whereas the displacement method was accurate to only a tenth of a gram.

In determining the percentage of the weight of a muscle in relation to the total weight of the bird, the weight of the muscle was used as the numerator, and the weight of the preserved specimen was used as the denominator. Before weights were taken, all specimens were plucked in identical fashion.

Table 11. Caudal Muscles

Table 12. Weights of Muscles

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

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

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

Beddard 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 , 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.

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.

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.

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 in proposing family status for the silky flycatchers and the palm-chats. The characters in the diagnosis 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: the silky flycatchers; waxwings and; 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 . 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 are descendants of an isolated population of the family stock that developed communal living habits as one specialization. Silky Flycatchers became modified to catch insects, and have specializations that roughly parallel those of the Tyrannid flycatchers.

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 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.

SUMMARY

BIBLIOGRAPHY

ANDERSON, E. M.

ANDERSON, M. P.

ANDERSON, R. M.

ARMSTRONG, E. A.

BAIRD, S. F.

BEDDARD, F. E.

BERGTOLD, W. H.

BOULTON, R.

BURLEIGH, T. D.

BURT, W. H.

CARRIKER, M. A., JR.

CORY, C. B.

CROUCH, J. E.

ENGELS, W. L.

FARLEY, J. A.

FRANK, F.

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