VAUX'S SWIFT 299 



leaf hoppers (as determined by gullet examination of the young), was placed 

 far back In the open mouth of each young one. * « * 



After the parent bird had fed one of the young, it caught sight of me and 

 dropped to a lower level in the chimney where it alighted out of sight. I moved 

 closer and placed my head directly over the opening to get a better view. As I 

 did so, I heard the rapid beating of wings and, thinking the bird was coming 

 out, I instinctively jerked my head to one side to avoid being hit. It did not 

 appear, so I looked in a second time and again I heard wing beats. This time I 

 kept my position, and after my eyes had become adjusted to the darkness, I 

 observed its stunt several times. The bird would let go its hold on the wall, and, 

 by rapidly beating its wings, suspend itself in the middle of the chimney and at 

 the same time produce the br-r-r-r-ing sound. Apparently the sound was pro- 

 duced by the beating of the wings themselves, for I could not observe them 

 touching the sides of the chimney. During these performances the young were 

 quiet. I interpreted this behavior as a means employed to intimidate the 

 intruder, much as does the hissing of the chickadee or the swooping dive of the 

 Red-tailed Hawk. 



Pliuniages. — As far as I can learn from the rather scanty material 

 examined, the sequence of plumages and molts in Vaux's swift is about 

 the same as in the closely related eastern chimney swift. The spiny 

 quills of the nestling develop into a juvenal, or first winter, plumage 

 that is much like that of the adult. The narrow whitish edgings on 

 the scapulars of the young bird soon wear away ; and adults will aver- 

 age paler on the throat and under parts. 



I have seen adults molting the contour feathers and primaries in 

 August, but the molt of the primaries must be very gradual in a bird 

 that spends so much of its life on the wing. Mr. Rathbun tells me 

 that this swift has a complete molt during summer. 



Food. — Almost nothing has been published on the food of Vaux's 

 swift, beyond the statement by Mr. Davis (1937) that the food fed to 

 the young consisted of insects, largely leafhoppers. Probably its food 

 consists wholly of small flying insects, such as mosquitoes, gnats, 

 various flies, and perhaps small beetles. On dull, damp days much 

 of its food is gathered at low levels, but on clear hot days, when the 

 insects fly high, it ascends to great heights in pursuit. I can find no 

 record of stomach contents. 



Behavior. — Mr. Rathbun says in his notes that "Vaux's swift flies at 

 all heights, at times just above the surface of the ground, and again 

 it will be seen high against the sky. Its flight need not be mistaken 

 for any swallow. It is fast, lacks a certain smoothness, and is apt 

 to fly more directly, circle less; also, it has at times somewhat of a 

 darting movement, erratic as it were, which brings to mind the 

 actions of a bat. In fact, this bird is always in a hurry." 



Ralph Hoffmann (1927) notes a marked difference in flight between 

 this swift and the swallows: "A close observation of the tail shows 

 that it never displays a forked tip; it either ends in a point like a 

 cigar or is spread like a fan when the bird makes a sudden turn. 



