EASTERN NIGHTHAWK 229 



same position, and his mouth never open." F. A. Hartman (1914) 

 writes, "It is very evident that the mouth plays no part, otherwise 

 tlie sound would be produced at other times [than on the dow^nward 

 <i;lide], * * * The bird threw its wings far to the front at the 

 end of his downward glide, so that the uppermost quill feathers were 

 pointed exactly in the direction of his glide. Going at such head- 

 long speed, these quill feathers when thrown edgewise to the air 

 vibrated strongly, causing the 'woof'." Alden H. Miller (1925) 

 while at Camp Lewis, Wash., during June and July succeeded in 

 attracting nighthawks within 10 feet of himself by merely waving 

 his hat in the air. He noticed that if the wings, during the down- 

 ward plunge, were held in the upturned V-shaped position, a normal 

 pose when soaring, no boom was heard, but when the wings were 

 bent downward near the end of the dive the boom sound was pro- 

 duced. The intensity of the sound, according to Mr. Miller, is more 

 or less proportional to the speed attained. The main explosive boom 

 seems to be preceded by a brief, lesser vibrating sound, which bursts 

 forth into the full bellow. Both parts seem to have a distinct 

 element of pitch, but the latter part is lower, with greater reso- 

 nance and depth of quality. 



The sound produced by the wings described above is a part of 

 the courtship performance and is usually produced near the nesting- 

 site. After the young no longer require the constant attention of 

 both parents, the male loses the glamour of romance and performs 

 less frequently and soon after ceases almost entirely until the court- 

 ship season of another year. However, birds migrating in August 

 sometimes "boom." 



Another note associated with the courtship season is a guttural 

 call uttered only when the male is at rest and in the presence of 

 the female. It may be described as an oft repeated auk^ auk, auk or 

 awk, aiok, awk. This note is produced by the syrinx, but as it is 

 uttered the bill is tightly closed and the gular membrane is tightly 

 distended each time the note is produced. The distension is caused 

 by the expulsion of air from the respiratory system. The distended 

 membrane, although feathered, acts as a resonator and modifies the 

 note, giving it a peculiar quality. Such a mechanism is present in 

 other birds. It is especially highly developed in the prairie chicken, 

 in which large lateral vocal sacs are present that give the "booming" 

 notes of this grouse great carrying power. 



The notes of the female nighthawk are simple calls uttered in 

 response to the those of the male or of the young. She may utter a 

 purring, pacifying note w^hen brooding the young, but often these 

 notes are so weak that they cannot be heard by the observer unless 

 he is stationed very near in a blind. 



