6 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 144 



we have had to assume muscle weight proportional to body weight, 

 which is true only in the most general terms. 



MUSCLE WEIGHT 



In figure 13 we show the weight of the large pectoral muscle plotted 

 against total weight for birds. The large pectoral muscle powers the 

 downbeat of the wings, and so is the prime source of energy for flight. 

 We see that for the entire procession of birds, from a tiny kinglet to a 

 mute swan, the large pectoral averages 15.5 percent of the body weight 

 with very little "scatter" on either side of the mean. 



In figure 14 the weight of the large pectoral muscle is plotted against 

 the weight of the wing. Here the scatter is considerably greater and the 

 wing weight increases with the 1.1 power of the muscle weight. Body 

 weight, on the other hand, increases with the first power of muscle 

 weight. The rationale here is based on the data presented in figure 9. 

 We recall that wing weight increases more rapidly than body weight, 

 and since muscle weight is directly proportional to body weight it 

 must also increase more rapidly than the weight of the muscle. 



Figure 15 shows the weight of the small pectoral muscle (which 

 powers the upbeat) plotted against body weight. Here we find the 

 same proportional relationship that existed for the large pectoral mus- 

 cle, but a far greater scatter from the mean. In general the gallinaceous 

 birds have relatively large small pectorals ; for soaring birds and birds 

 of prey the small pectoral is a much lower percentage of body weight. 

 The explanation is not readily apparent. Gallinaceous birds are rela- 

 tively poor fliers, but it is hard to say why this should be associated 

 with a relatively large small pectoral. 



In figure 16 the weights of the two pectoral muscles are plotted 

 against each other. We see that on the average the large pectoral has 

 10 times the weight of the small pectoral. The scatter from the mean is 

 considerable, owing of course to the variability in relative weight of the 

 small pectoral muscle. 



The relative muscle weights provide the best available evidence for 

 the presumption that for ordinary birds power for flight is provided 

 wholly by the downbeat of the wings. If we make the reasonable as- 

 sumption that power output is proportional to the weight of the muscle 

 we see that the small pectoral can provide no more than 10 percent of 

 the power required for flight. Since power must be expended merely to 

 lift the wings, the contribution of the small pectoral muscle to flight 

 may well be considerably less than this percentage. 



For hummingbirds the situation is quite different. Large and small 

 pectorals account for 25 to 30 percent of total weight as compared with 



