NO. I LOCOMOTOR MECHANISMS OF BIRDS HARTMAN IJ 



Tanagra to 3.98 in Habia, while the aspect ratio is only 1.47 in 

 Rhodinocichla but reaches 1.89 in Piranga rubra. The muscles of the 

 lower extremities are from 5.19 percent in Tanagra luteicapilla to 

 11.36 percent in Rhodinocichla. 



Fringillidae. — This almost worldwide group of birds, feeding 

 everywhere that seeds are to be found from the ground to the treetops, 

 shows a great range of locomotor equipment. Their hearts vary from 

 0.60 percent in Pezopetes to 1.57 percent in the male Saltator maximus, 

 and flight muscles show as great a range (19.5 to 29 percent). The 

 buoyancy index of the wings ranges between 3.09 and 3.72, the aspect 

 ratio between 1.42 to 1.88. The highest values for muscles of the 

 lower extremities are more than twice as high (13.2 percent) as the 

 lowest (5.43 percent). 



After this survey of the variations in the families, let us compare 

 the locomotor organs individually. 



HEARTS 



Since the heart limits the activities of a bird, it is proper to consider 

 it first. Whatever the size of a musculature, it cannot act for long 

 without an adequate circulation. Thus the tinamou, with its tremen- 

 dous breast muscles but very small heart, can make only short flights. 

 On the other hand, birds with only a moderate equipment of muscles 

 can fly for long periods if they possess large hearts. It is this relation- 

 ship between the heart and the locomotor muscles that we wish to deal 

 with primarily, but there are other factors which also must be con- 

 sidered, namely, sex, season, latitude, and altitude. 



Sex. — Whenever there is evidence of sex difference in the relative 

 weight of the heart, the sexes are separated for consideration. In the 

 following species the hearts are heavier in the male than in the female : 

 Gallus gallus, Phaethornis guy, Anthracothorax nigricollis, Chlorostil- 

 bon canivetii, Damophila julie, Amazilia edward, Amazilia tzacatl, 

 Elvira chionura, Lampornis castancoventris, Selasphorus scintilla, 

 Sittasomus griseicapillus, Cranioleuca erythrops, Anabacerthia striati- 

 collis, Mitrephanes phaeocercus, Thryothorus modestus, Habia rubica, 

 Habia fuscicauda, Saltator maximus, and Pipilo erythrophthalmus. 

 Heavier hearts in males may also hold for other species, but the 

 number of individuals available is not always sufficient to settle the 

 question. 



Seasonal variation. — A difference in relative heart size is noted in 



