RUBY-THROATED HUMMINGBIRD 345 



demonstrated by means of motion pictures taken in 1936 by a new 

 application of photography. Dr. Harold E, Edgerton took advantage 

 of the intermittent flashes in a low-pressure tube in which the flashes 

 occur for 1/100,000 of a second with a period of darkness between 

 them lasting 1/500 of a second. He used a constantly moving film, 

 geared so that a new bit of film came opposite the lens of the camera 

 at each flash, and thus secured about 540 pictures a second. Pictures 

 of hummingbirds in flight taken in this way, when thrown on a screen, 

 apparently reduce the speed of the birds' wing beats to that of a 

 leisurely flying gull and make it possible to study the flight of the bird 

 in detail. 



Dr. Charles H. Blake examined with great care the films taken of 

 hummingbirds in flight and found that the birds beat their wings 55 

 times (completed strokes) a second when hovering, 61 a second when 

 backing, and as rapidly as 75 a second when progressing straightaway. 

 Probably this last figure would be found to increase as the bird 

 gained speed, if the camera could keep the bird in focus. Dr. Blake 

 calculated that, during hovering, the wing tips moved at the rate of 

 20 miles an hour, and he also learned that the bird is in flight before 

 it leaves its perch (the takeoff took 0.07 second) and pulls the perch 

 after it a little way, a phenomenon that Mrs. Webster had suspected 

 from feeling the birds leave her hand. 



Dr. Blake kindly explained to me the mechanism of backward fly- 

 ing thus: In backing away from a flower or feeding tube the hum- 

 mingbird stands almost vertically in the air with its tail pointing 

 downward and a little forward. In this pose its wings beat hori- 

 zontally, and what would be the downward half of each complete 

 wing stroke if the bird's long axis were parallel to the ground forces 

 the air forward, away from the bird's breast in its upright position, 

 and drives the hird hackward. Then, on the return half-stroke, the 

 whole wing is rotated at the shoulder joint so that its upper surface 

 strikes the air, and, driving it downward, balances the pull of gravity. 

 Dr. Blake also points out that the distribution of weight in the hum- 

 mingbird's wing is evidently favorable to a very low inertia upon 

 which the quick reversal of motion depends, the weight being concen- 

 trated close to the body by reason of the short, heavy humerus. 



The following quotation shows the high rate of speed the humming- 

 bird may attain by the lightninglike strokes of its wings. H. A. 

 Allard (1934), who was making a fast trip by auto out of Washing- 

 ton, D. C, says : "Not far out of Warrenton we had settled down to a 

 speed of fifty miles per hour on highway 211, when a Ruby-throated 

 Hummingbird (Archilochus colubris) suddenly paralleled our course 

 along the side of the roadway as if deliberately racing with us. It 

 actually passed us for a short distance keeping straight with our 



