266 BULLETIN 17 6, UNITED STATES NATIONAL MUSEUM 



the 36 swifts and comprised TO to 99 percent of the contents, with 

 numbers per stomach varying from 15 to 90 individuals. Mayflies 

 (Ephemeridae) and caddiceflies (Trichoptera) were also taken at 

 times and formed 5 to 35 percent of the content." 



Behavior. — Mr. Rathbun (1925) writes admiringly of the powers 

 of flight exhibited by the northern black swift : 



In all its flight actions this Swift shows a power and an easy grace that 

 win our admiration. It seems to live upon the wing, and to restrict its flight 

 most of the time to a considerable elevation, the height being seemingly in- 

 fluenced by the character of the weather. It is generally the case that during 

 the continuance of a low atmospheric pressure the Swifts will not fly very 

 high, but when this condition ceases they then ascend. At the time of high 

 pressure the Swifts are often at a great height and it is not uncommon to see 

 them gliding at the very limit of vision. At such times so high are some of 

 them that even with the aid of field glasses they show but faintly against the 

 sky. It would be mere surmise as to the height that they attain, several 

 thousand feet certainly, and as some have even disappeared from view, when 

 the glasses were in use, one has no knowledge of the height to which they go. 

 On a bright summer day to see these dark birds circling far above is always 

 captivating. Should the sky happen to have clouds some of which are white 

 and shining, the Swifts as they wheel across their glistening surfaces, are 

 plainly outlined, but seem to fade insensibly from view when in turn they cross 

 the open spaces of the sky. At such times it is the constant shifting view with 

 the seeming change in distance of the gliding Swifts that adds to our interest 

 when watching them. * * * 



But this bird has also the power of very rapid flight. Infrequently it hap- 

 pens near the close of day that some will be seen hastening to their mountain 

 retreats, at such times being widely scattered and flying rather low. With 

 strong and rapid wing beats an almost direct course is followed, and but a 

 few seconds elapse from the time one is first seen until after passing it fades 

 from sight in the distance. And when thus observed in full flight, the power 

 shown for fast flying never fails to impress the beholder. 



The position of the Black Swift's wings as it glides or circles is dihedrally 

 down. We have never seen anj^ variation from this, and in this respect a 

 contrast is shown by our Chaetura, whose wings are often highly elevated when 

 sailing short distances or on entering their nesting places or roosting resorts. 

 This wing position of the Black Swift seems worthy of mention, although it 

 may be possible that it varies at times under conditions with which we are not 

 familiar. 



He mentions in his notes an occasion on which the swifts were 

 flying low; he was crossing Lake Washington in a row boat on a 

 rainy day in June: "While crossing, we noticed very many of these 

 birds flying about at quite a low height above the water. Two were 

 taken, and then we proceeded on our way. It was not long before a 

 heavy rain began to fall and this hastened our return ; but, when we 

 reached the spot where first the swifts had been observed, suddenly 

 on all sides of the boat were numbers of the birds hawking about 

 quite close to the water's surface, some flying past not more than 

 fifteen or twenty feet away." 



