l8 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. I43 



certain Florida birds collected in June as compared with those obtained 

 in the winter months, as shown in table 2 (p. 88). 



Latitude. — A difference in heart size is noted in a few species col- 

 lected in Florida compared with the same or closely related species 

 obtained in Panama, the hearts of Florida birds being larger. These 

 species are Butorides virescens, Coragyps atratus, Cathartes aura, and 

 Sturnella magna. 



Altitude. — Larger hearts are present in some species at higher alti- 

 tudes when compared with other species of the same family at lower 

 altitudes. Specimens of Crotophaga ani and Piaya cayana obtained at 

 4,500 feet have larger hearts than those of the same species obtained 

 at sea level (P<o.oi). 



In general the smaller birds have relatively larger hearts than large 

 birds. The lowest values appear for the tinamids, followed by the 

 strigids, nyctibiids, momotids, and bucconids, with others of lesser 

 deviations from the mean. The highest values emerge for the trochi- 

 lids, hirundinids, motacillids, and bombycillids. Comparison of the 

 heart weight with the habits of the bird leads one to the conclusion 

 that heart weight is directly related to the ability to sustain power 

 flight. 



The muscles of locomotion show even greater variation. I shall 

 consider first the muscles used in flight. As already indicated, we 

 weighed all the muscles involved except the small muscles along the 

 carpals, metacarpals, and phalanges. 



FLIGHT MUSCLES 



Sex, latitude, and altitude appear to have some influence in muscle 

 size in a few species. 



Sex. — In Cairina moschata (domestic) the female possesses flight 

 muscles that are larger than those of the male, while in Micrastur 

 ruficollis, Phaethornis guy, Amazilia amabilis, and Manacus vitellinus 

 flight muscles are larger in the male. In other species our data are 

 insufficient to determine the sex differences in the more inclusive flight 

 muscles, but in the more limited combination of pectoralis plus supra- 

 coracoideus, where we have more data, these muscles are larger in the 

 male than in the female of Pelecanus occidentalis, Micrastur ruficollis, 

 Amazilia amabilis, Amazilia edward, Amazilia tzacatl, Lampornis 

 castaneoventris, Selasphorus scintilla, Pharomachrus mocinno, Sit- 

 tasomus griseicapillus, Xiphorhynchus guttatus, Lepidocolaptes affinis, 

 Anabacerthia striaticollis, Myiarchus tuberculifer, Empidonax flaves- 

 cens, Mitrephanes phaeocercus, Thryothorus modestus, Tangara ic- 



