Table 10. Relative Length and Width of Skull (in percent)
| Species | Length of Skull | Width of 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 | ||
Fig. 43. Part of skeleton of Bombycilla cedrorum showing method of measuring the length of the trunk. Natural size.
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.
Fig. 44. Graph showing relative lengths of bones of the leg. The percentage values are shown on the axis of the ordinates.
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.