NOKTHERN- BLACK SWIFT 269 



Grinnell and Storer (1924) describe its note as "a high-pitched 

 twitter, not so shrill or long-continued as that of the White-throated 

 Swift." 



Field Tnarks. — ^The black swift can be recognized by its size and 

 coloration. It is the largest of the three western swifts. The white- 

 throated swift, which most nearly approaches it in size, differs from 

 it in having a well-marked and conspicuous pattern of black and 

 white on the under parts, as well as the white rump patches, which 

 show from above. The black swift appears wholly dark, except 

 under certain conditions of reflected sunlight ; the white markings on 

 the immature bird are not conspicuous, except at close range. Vaux's 

 swift is the smallest of the three and does not show in flight the 

 slightly forked and fan-shaped tail of the black swift. Mr. Hoffmann 

 (1927) suggests that "the beginner may take the much commoner 

 Purple Martin for a Black Swift; the deeply notched tail, never 

 spread like a fan, the habit of perching on stubs, the loud, musical 

 notes and the difference in the sexes should readily identify the 

 Martin." Grinnell and Storer (1924) state that, in the black swift, 

 "the fore margin of the two wings as viewed from below is a double 

 convex, and not a single continuous arc as in the Wliite-throated 

 Swift; moreover, the movements of the wings are more deliberate 

 than in that species." 



Fall. — Mr. Rathbun says in his notes: "The autumnal migration 

 straggles over a more or less extended period. In the Puget Sound 

 region it begins to take place soon after September 1, and it lasts 

 three weeks or slightly more. I have observed this for many years, 

 and the swifts do not pass by in the large groups that one will see in 

 spring. Day after day in fall they straggle by in small numbers. 

 Very often we have noticed that these groups were in multiples of 

 three, which in our opinion would represent the parent birds with 

 their single young. Invariably associated with the black swifts were 

 the Vaux's swift, and if either was seen you could expect to see the 

 other. And quite often, in company with both species of swifts, 

 would be violet-green and barn swallows, all straggling past at odd 

 times throughout the day." 



In the Stikine region Mr. Swarth (1922) observed: "At Sergief 

 Island, August 17 to September 7, black swifts were abundant, though 

 seen only in cloudy or rainy weather. Then large flocks appeared, as 

 many as seventy-five or a hundred being in sight at once flying over 

 the marshes, the individuals moving about in wide circles, and the 

 flock as a whole moving in a definite path. The birds sometimes flew 

 very low, occasionally skimming along just over the tall grass. A 

 flock would appear, circle about overhead awhile, and then vanish. 

 About fifteen or twenty minutes later, others, or perhaps the same 

 flock, would come in sight again." 



