Part 10

Table 6. Leg-trunk Ratios (in percent)

=======================+=======+=============+=================+====== Species | Femur | Tibiotarsus | Tarsometatarsus | Total -----------------------+-------+-------------+-----------------+------ Corvus brachyrynchos | 71 | 120 | 77 | 268 Corvus corax | 73 | 139 | 78 | 290 Dendroica audubonii | 62 | 109 | 81 | 252 Setophaga ruticilla | 66 | 127 | 94 | 287 Myadestes townsendi | 61 | 99 | 60 | 220 Sialia sialis | 66 | 111 | 72 | 249 Hylocichla mustelina | 75 | 133 | 97 | 305 Parus atricapillus | 78 | 138 | 99 | 315 Tachycineta thalassina | 61 | 97 | 56 | 214 Myiarchus crinitus | 68 | 106 | 74 | 248 Dumetella carolinensis | 73 | 136 | 94 | 303 Polioptila caerulea | 75 | 144 | 113 | 332 Eremophila alpestris | 73 | 113 | 115 | 301 Muscivora forficata | 62 | 98 | 61 | 221 -----------------------+-------+-------------+-----------------+------

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

=========================+========+=======+=========+========= | | | Length, | Width, Species | Length | Width | percent | percent -------------------------+--------+-------+---------+--------- Ptilogonys caudatus | 9.8 | 3.9 | 45 | 18 Ptilogonys cinereus | 8.8 | 4.1 | 41 | 19 Phainopepla nitens | 8.4 | 3.9 | 41 | 19 Phainoptila melanoxantha | 8.5 | 3.5 | 35 | 14 Dulus dominicus | 8.5 | 2.9 | 38 | 13 Bombycilla garrula | 7.0 | 3.5 | 31 | 15 Bombycilla cedrorum | 7.1 | 2.9 | 35 | 14 -------------------------+--------+-------+---------+---------

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

=========================+=========+======== Species | Sternum | Carina -------------------------+---------+-------- Ptilogonys caudatus | 85 | 28 Ptilogonys cinereus | 91 | 32 Phainopepla nitens | 81 | 26 Phainoptila melanoxantha | 76 | 25 Dulus dominicus | 107 | 28 Bombycilla garrula | 88 | 33 Bombycilla cedrorum | 82 | 31 -------------------------+---------+--------

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

=========================+========+=======+=========+========= | Length | Width | Length | Width Species | of | of | of | of | Skull | Skull | Sternum | Sternum -------------------------+--------+-------+---------+--------- Ptilogonys caudatus | 34.9 | 15.6 | 23.9 | 7.8 Ptilogonys cinereus | 33.4 | 14.7 | 24.3 | 8.5 Phainopepla nitens | 33.3 | 15.1 | 21.3 | 6.9 Phainoptila melanoxantha | 39.7 | 16.0 | 24.8 | 8.2 Dulus dominicus | 36.4 | 16.6 | 30.5 | 8.0 Bombycilla garrula | 37.0 | 16.8 | 30.0 | 11.2 Bombycilla cedrorum | 34.0 | 15.5 | 25.3 | 9.6 -------------------------+--------+-------+---------+---------

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 (in percent)

=========================+========+======= | Length | Width Species | of | of | Skull | Skull -------------------------+--------+------- Ptilogonys caudatus | 160 | 72 Ptilogonys cinereus | 158 | 69 Phainopepla nitens | 162 | 73 Phainoptila melanoxantha | 161 | 65 Dulus dominicus | 164 | 75 Bombycilla garrula | 164 | 74 Bombycilla cedrorum | 162 | 74 -------------------------+--------+-------

Leg-trunk Percentages.--Table 4 shows the relative lengths of the legs and of the separate bones in the legs of the different species of the Bombycillids. Table 5 shows corresponding lengths for other passerine birds. The total length of the leg was computed by adding the figures obtained for the lengths of the femur, tibiotarsus and tarsometatarsus. The lengths of the toes were disregarded. Length of leg was recorded in this same way by Richardson (1942:333), who thought that only in swimming and running birds do the toes contribute to the functional length of the hind limb.

Table 4 shows that of the birds compared in this paper, Dulus has the longest legs. In order of decreasing length the others are the Ptilogonatinae, and finally the Bombycillinae, which have the shortest legs of all. In Waxwings the length of the legs, expressed as percentages of the body-lengths, are identical with those birds that are similar in habits, that is to say, birds which do not use the hind limb except in perching. It can be noted by reference to Table 5 that Tachycineta and Myadestes fall into this category. 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. Two birds with noticeably long legs are Hylocichla mustelina, a typical ground dweller, and Parus atricapillus, which hops about in the trees and underbrush.

Insofar as the lengths of the legs show, Dulus and Phainoptila are the most generalized of the Bombycillidae, since the relative length of leg is approximately the same as that of more generalized birds such as warblers, crows and thrushes of similar locomotory habits. In other words, Dulus and Phainoptila have remained unspecialized, in contrast to the waxwings in which adaptive changes fitting them for a perching habit have taken place. Ptilogonys and Phainopepla are intermediate in length of leg between Phainoptila and Bombycilla, and Ptilogonys and Phainopepla have progressed from life on the ground toward the perching habit. Bombycilla cedrorum is more specialized than is B. garrula in shortness of leg, and the reduction is comparable, as is noted above, to that in the legs of Tachycineta.

In birds which have the legs much modified for walking or for hopping in the brush, such as Polioptila and Eremophila, it is noteworthy that the distal segment, the tarsometatarsus, is the longest, whereas in birds such as Myiarchus and Tachycineta, that do not utilize the limbs in this manner, the tibiotarsus, the middle segment, is the longest. Mammals much modified for walking or hopping likewise have the proximal segment, the femur, short, and the distal segment long (Howell, 1944). The waxwings have all of the segments short; these birds are modified for strong and sustained flight. Their hind limbs are used principally for landing devices and for perching. No one element of the leg has been shortened much, if any, more than any other.

A. Bombycilla cedrorum; B. Bombycilla garrula; C. Dulus dominicus; D. Phainoptila melanoxantha; E. Phainopepla nitens; F. Ptilogonys cinereus; G. Ptilogonys caudatus. a. femur; b. tibiotarsus; c. tarsometatarsus; d. total.]

Arm-trunk Percentages.--Tables 1 and 2 show the total length of the arm, and lengths of the separate arm elements, relative to the trunk. Table 3 gives the corresponding lengths for birds other than the Bombycillidae. Total length of arm was obtained by adding together the lengths of the humerus, ulna, and manus, and by dividing the figure thus obtained by the length of the trunk as was done for leg lengths in tables 4 and 5. The method of adding together the component parts does not give the entire length of the wing, since the length of the feathers, which add effectively to the total length, as well as do the lengths of the small carpal elements, is lacking.

45. Ptilogonys caudatus, showing relation of outline of wing to bones of arm.

46. Bombycilla cedrorum, showing relation of outline of wing to bones of arm.]

It may be noted that Phainoptila and Bombycilla have the shortest arm in the family Bombycillidae. The humerus, radius and ulna are comparable to the same elements in thrushes and the catbird, and it is only the extremely short manus in Phainoptila that affects the total. The manus in Phainoptila is comparatively smaller than in any other genus of the family Bombycillidae, and this indicates poor flight power. Bombycilla has a total length corresponding closely to that in warblers, but the lengths of the distal elements correspond closely to those in the catbird and thrushes. Of the three segments, the humerus is, relatively, the most shortened. Next in order of increasing length of arm is Dulus; measurements for it are roughly the same as those of Myadestes. The wing bones of the Ptilogonatinae, other than Phainoptila, are the longest in this series, and they most nearly resemble the same bones in flycatchers, Parids, and gnatcatchers.

A. Bombycilla cedrorum; B. Bombycilla garrula; C. Dulus dominicus; D. Phainoptila melanoxantha; E. Phainopepla nitens; F. Ptilogonys cinereus; G. Ptilogonys caudatus. a. humerus; b. radius; c. ulna; d. manus; e. total.]

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 (1942). 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.

Caudal Muscles.--The muscles of the caudal area that were used for comparison were the levator caudae and the lateralis caudae. 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. The lateralis caudae differs more according to species than does the levator caudae, showing that rudder action of the tail is of primary importance in the adaptation for capturing insects. It will be remembered that the pygostyle in this subfamily has a flattened lateral surface for attachment of the levator caudae muscle, and it is therefore to be expected that this muscle will be larger in the Ptilogonatinae than it is in either the Bombycillinae or the Dulinae. The levator coccygis, together with the two muscles mentioned above, is responsible for elevation of the tail. The levator coccygis is less altered in different species of the family than is the lateralis caudae. It may be noted that the caudal muscles of Dulus and Bombycilla constitute a smaller percentage of the total weight of the bird than in any of the genera in the subfamily Ptilogonatinae.

a. Levator coccygis; b. Levator caudae; c. Lateralis caudae; d. Lateralis coccygis; e. oil gland; f. dorsal tip of pygostyle.]

Table 11. Caudal Muscles (Actual and Relative Weights)