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.

Table 11. Caudal Muscles (Actual and Relative Weights)

SpeciesLevatorLateralis
Ptilogonys caudatus.145g..022g.
.092%.045%
Ptilogonys cinereus.030g..010g.
.076%.026%
Phainopepla nitens.025g..008g.
.096%.029%
Phainoptila melanoxantha.040g..015g.
.063%.014%
Dulus dominicus.028g..006g.
.063%.014%
Bombycilla garrula.034g..010g.
.048%.014%
Bombycilla cedrorum.026g..008g.
.050%.014%