FLIGHT 261 



Before analyzing these modes of aerial locomotion, it is desirable 

 to i*efer to some of the conditions under which birds are placed, since 

 these must be taken into account if it be desired to understand the 

 problems of flight. What, in the first place, is known about the 

 relation between the weight of birds and the area of their wings, 

 and how do birds differ from one another in this respect % This 

 subject has been carefully studied by MiillenhofF^ and others. It 

 has been found that the relation of the wing- or rather sail-area to 

 the weight of the bird varies greatly. As might be expected, the 

 greater the sail-area, the more powerful, other things being equal, 

 is the flight. 



Another matter in which birds differ greatly is the strength of 

 the muscles which move the wings. It may be assumed that the 

 strength of these muscles corresponds with their weight. The 

 relation between the weight of the pectoral muscles and that of the 

 AA^hole bird has been investigated among others by Legal and 

 Reichel,^ who found that the pectoral muscles weigh on an average 

 about one-sixth of the whole bird ; but that in different types of 

 birds there may be considerable difference in this respect. For 

 instance, in a House-Pigeon the proportion was ^_ per cent, while /= 

 in a Herring-Gull it was only 16 per cent. Some birds therefore 

 have much more powerful wing-muscles than others. 



The shape of the wing, moreover, varies considerably also in 

 different birds. Some, like the Swallow, have long and narrow 

 wings, while others, like the Quail, have short and broad wings. 

 The wings of some soaring birds, as Eagles and Vultures, are 

 rounded at the points, and the primary feathers are separated from 

 one another at their tips, giving a notched appearance to the end 

 of the extended wings. A typically flying bird, such as a Falcon, on 

 the other hand, has pointed wings with little separation of the tips 

 of the primary feathers. These differences correspond to differences 

 in the power and mode of flight. 



Birds, as well as all other animals that fly, may be divided into 

 categories according (a) to the ratio between the sail-area and the 

 weight, (^) to the strength of the pectoral muscles, and (c) to the 

 shape of the wings. Of these categories or " types " Miillenhoff 



^ "Die Grosse der Flugflachen." Archiv fur die gesammte Physiologie 

 (Pfliiger's), xxxv. (1885) pp. 407 et seqq. Miillenlioff follows Harting, Legal and 

 Reichel, Marey, and others in estimating tlie ratio between the sail-area of Birds 

 and their weight by the formula A-/P^ = a, in which A is the area (in square 

 centimetres) of the out - stretched wings and tail as well as of the body, 

 spread out on a flat surface, while P is the weight of the bird (in gi-ammes). 

 The values found for u- in different birds range from 2 85 (Golden-Eye) to 6735 

 (Barn-Owl). 



- Verhandlungen der Schlesischen Gesellschaft fur vaterldnd-Cultur. Breslau : 

 1882. 



