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.
Figs. 45-46. Outlines of wings. × 1/2
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.
Fig. 47. Graph showing relative lengths of bones of the arm. 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. 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.