Primary Feathers in Flight. 165 



During the autumn I have often watched flocks of from twen- 

 ty-five to seventy-five Broad winged hawks coasting together 

 for a distance of upwards of half a mile without beating their 

 wings ; this maneuver, characteristic of their manner of 

 migration, was periodically repeated after a short interval of 

 soaring. 



VII. The Function of Interlocked Primaries in Flight. 



In coasting flight : The primary feathers, the most import- 

 ant feathers of the wing, are subjected to great strain and pres- 

 sure. The backward pressure on these feathers is particularly 

 great under the following conditions : (1) When a bird is sail- 

 ing or coasting through the air with set wings, fig. 15. (2) 

 When it is diving or twisting in pursuit of its quarry as in 

 birds of prey, fig. 16. In both cases some means of producing 

 rigidity in the end of the wing seems necessary. 



In these types of flight the interlocking of the primaries 

 would make the end of the wing very rigid, thereby not only 

 forming a strong surface to withstand the pressure of the air, 

 but when the primaries are interlocked no muscular force is 

 required to keep them partially extended. In addition, the 

 shape of the wing is curved by the process of interlocking, 

 figs. 18 and 19, in such a way that the lower rather than the 

 upper surface of the feathers bears the pressure of the air 

 when the bird is coasting or diving ; the effect of the interlock- 

 ing of the primaries appears to make a much more efficient 

 aeroplane of the wing than when the primary feathers are not 

 interlocked. 



JSfo special set of muscles is required for interlocking the 

 primaries. In coasting flight, if these feathers are extended 

 and then allowed to fallback, the pressure of the air forces the 

 end of each primary above the plane of the succeeding primary 

 and those that are emarginate become interlocked. These 

 feathers can usually be interlocked artificially by holding an 

 open wing in the hand and striking the air with it as in a 

 downward wing beat. 



In soaring flight {circling) : Whether the emarginate 

 primaries are interlocked in soaring flight or not is an 

 undecided question. 



Photographs and visual observation of Turkey buzzards and 

 other large birds show the primaries considerably separated 

 when these birds are soaring, but this is by no means evidence 

 that the feathers are not sufficiently overlapped to keep them 

 in place without muscular effort. 



When a bird is soaring in a light wind the air pressure acts 

 on the primaries almost entirely from beneath. The outer 

 primaries of large birds while soaring have been observed to 



