RECORDS. 341 



birds of prey habitually interlock their primary feathers in 

 flight. 



It was found that when hawks are examined immediateh* 

 after they have been killed, there usually appear deep depres- 

 sions in the edge of the posterior webs of the emarginate primary 

 feathers, where the feathers have been in contact, which are 

 caused by the interlocking of the primaries. 



The measurements consisted in determining the width of 

 these depressions at short intervals of time immediately after 

 the death of the hawks. It was found that the depressions 

 gradually disappeared, both in cases where the feathers were 

 found locked and were then unlocked, and in cases where the 

 feathers were found unlocked. Data were thus obtained from 

 w^hich well-defined curves were constructed, showing the re- 

 covery of the web of the feathers after the pressure caused by 

 the interlocking feathers was relieved. A number of life-size 

 photographs were taken of the primary feathers immediately 

 after the hawks were killed and the photographs of the depres- 

 sions in the feathers when measured by a Repsold measuring 

 machine, gave curves which agreed very well with those ob- 

 tained by direct measurement. Similar curves were obtained 

 by artificially interlocking the primaries for several hours and 

 then measuring the recovery of the web of the feathers with a 

 micrometer microscope. It was found that artificial locking of 

 the feathers for ten minutes produced very slight or no depres- 

 sions and locking them for several hours produced depressions 

 only about one half as deep as those found when the hawks 

 were killed. In the latter case they were from 2 to 3.5 milli- 

 meters deep, and required from one to five hours to be reduced 

 to twenty per cent, of the original depth, the rate of change of 

 the depth of depression being most rapid at first. 



It was concluded from the measurements and photographs 

 that the primary feathers found with the depressions in the web 

 had been interlocked several hours or more, previous to the 

 death of the hawks, which were killed while sailing in a strong 

 wind, and that the theory of interlocking of the primaries of the 

 wing in flight had been conclusively confirmed. 



