Table 12. Weights of Muscles (These percentages expressed in terms of weights of the body)
| Species | P. major | P. minor | Deltoid | Thigh | Peroneus | [Gastrocnemius] |
| Ptilogonys caudatus | 2.42g. | .29g. | .55g. | .43g. | .15g. | |
| 4.94% | .59% | 1.12% | .88% | .31% | .96% | |
| Ptilogonys cinereus | 2.19g. | .28g. | .53g. | .30g. | .08g. | |
| 5.57% | .71% | 1.35% | .71% | .21% | 1.02% | |
| Phainopepla nitens | 1.30g. | .20g. | .30g. | .28g. | .10g. | |
| 4.99% | .77% | 1.15% | 1.12% | .40% | 1.42% | |
| Phainoptila melanoxantha | 3.93g. | .44g. | .92g. | 1.09g. | .48g. | |
| 6.18% | .69% | 1.45% | 1.61% | .75% | 2.97% | |
| Dulus dominicus | 2.09g. | .22g. | .50g. | .73g. | .18g. | |
| 4.81% | .50% | 1.15% | 1.68% | .41% | 1.01% | |
| Bombycilla garrula | 3.85g. | .45g. | .55g. | .50g. | .15g. | |
| 5.31% | .62% | .76% | .69% | .18% | .59% | |
| Bombycilla cedrorum | 2.58g. | .35g. | .50g. | .37g. | .10g. | |
| 5.00% | .68% | .97% | .73% | .19% | .83% | |
Pectoral Muscles.—The pectoral set of muscles varies but little in the family; flight power is seemingly not dependent upon size of either the pectoralis major or pectoralis minor. The data indicate that the insertion on the humerus, with consequent changes in the relative length of that bone, is more significant in type of flight and over-all flight power than is the actual size of the muscle mass. The deltoid muscle, for example, is smaller in Bombycilla than in members of the other two subfamilies. The humerus in Bombycilla is shortened, and the muscle therefore does not need to be large to accomplish the same powerful stroke that would be accomplished by a longer humerus and a larger, more powerful deltoid muscle. In the case of the deltoid, the shortening of the humerus and the more complex arrangement of the points of insertion have obviated the necessity of enlarging the muscle.
Leg Musculature.—The muscles of the thigh are noticeably larger in birds that have long leg bones. (See [Table 12] for size of muscles.) On the tibiotarsus, the peroneus and gastrocnemius muscles were measured. When expressed as a percentage of the weight of the bird, the peroneus has much the same relative weight in all but one of the species, whereas the gastrocnemius varies much. The peroneus is proportionately large only in Phainoptila, in which genus all the leg muscles are well developed, but the gastrocnemius is larger in all the Ptilogonatinae and in Dulus than it is in the specialized Bombycilla, in which it has probably been reduced as the leg bones and other muscles have been reduced.
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