Phylogeny of the Waxwings and Allied Birds

by M. Dale Arvey

Distally, the two major condyles and the intercondylar groove or olecranon fossa that make efficient articulation with the ulnar process, are not variable. The external condyle, however, is significantly variable in the family. This condyle is longest and most pronounced in birds in which the humerus is short in relation to the trunk, as for example in Tachycineta. In the Bombycillidae the condyle is smallest in Phainoptila, where it is a mere suggestion of a process. In the remainder of the Ptilogonatinae, the condyle is larger but rounded, and shows a double process in Ptilogonys caudatus, and a slightly pointed process in P. cinereus. The external condyle in Dulus is not specialized, being low and rounded, but in Bombycilla, it is noticeably elongated, indicating a better attachment distally for the deltoid muscle. (No measurements are tabulated for this condyle, as the percentage of error in measuring this small structure is great.) [Table 1] gives lengths of humeri, and [Table 2] gives lengths of the humeri expressed as percentages of the length of the trunk, a standard measurement.

The area of insertion of the deltoid muscle is elongated in those birds with shortened humeri; these birds have also greater flight power than do birds with longer humeri and therefore a shorter external condyle.

Table 1. Lengths of Arm Bones in cm.

Species	Humerus	Radius	Ulna	Manus
Ptilogonys caudatus	2.39	2.57	2.79	2.25
Ptilogonys cinereus	2.24	2.48	2.78	2.38
Phainopepla nitens	2.21	2.59	2.82	2.39
Phainoptila melanoxantha	2.40	2.51	2.70	2.25
Dulus dominicus	2.23	2.38	2.63	2.31
Bombycilla garrula	2.35	2.58	2.88	2.67
Bombycilla cedrorum	2.06	2.34	2.60	2.38

Table 2. Arm-trunk Ratios (in percent)

Species	Humerus	Radius	Ulna	Manus	Total
Ptilogonys caudatus	85	92	93	80	2.58
Ptilogonys cinereus	84	90	103	89	2.76
Phainopepla nitens	84	98	107	91	2.82
Phainoptila melanoxantha	73	77	82	69	2.31
Dulus dominicus	78	83	92	81	2.51
Bombycilla garrula	69	75	87	78	2.34
Bombycilla cedrorum	67	76	85	77	2.29