Hurroy.—On the Flight of the Black-backed Gull. 143 
Pettigrew to the extraordinary opinion that the forward movement of a bird 
is derived from a stroke delivered downward and forward. 
Dr. Pettigrew, and many other authors, hold the opinion that the wing 
feathers of a bird open and close during the wp and down strokes respectively. 
But however this may be with birds that only flap their wings slowly, it is, I 
think, almost impossible that such rapid changes should take place in the 
wings of a bird like the sparrow, which, according to Professor Marey, makes 
thirty-three vibrations per second. Dr. Pettigrew’s experiments, also, upon 
the sparrow, with alternate feathers taken out of the wing, show that an 
opening and shutting movement is not necessary for flight ; and we may safely 
assume on the principle of greatest economy of force, a principle always acted 
upon throughout nature, that what is not necessary is not used. 
The falconers of olden days were well aware that rapidity of flight depended 
on the primary feathers of the wing, and they called these the “ flight 
feathers,” while the secondaries they called the “sail feathers,” and it will be 
found that the swiftness of a bird’s flight depends on the length of the 
primaries in proportion to the size of the bird, and on the number of strokes 
it makes per second. Thus the swift, which has proportionately longer 
primary feathers than any other bird, is probably the fastest flier, while the 
partridge, which has broad wings but short primaries, flies heavily, and has to 
make very rapid strokes. The wild-duck has less area of wing in proportion 
to its weight than a partridge, but its primaries are longer, and consequently 
it flies much faster. The landrail also is another example of a slow-flying’ 
bird with considerable expanse of wing for its weight, but with short pri- 
maries. The heron also furnishes another instance of the same kind, and it is 
well known that the long winged falcons are far superior fliers to the round 
winged buzzards, vultures, and eagles, although in the latter the area of wing 
surface is probably greater than in the former. 
The way in which birds turn in the air has also been much misunderstood. 
Professor Owen (Comp. Anat. of Vert. IT., 115) advances the extraordinary 
theory that when a bird wishes to turn it beats the air more rapidly with one 
pinion than with the other, which however originated with Borelli in his 
“ De Motu Animalium.” 
Van der Hoven (Handbook of Zoology, IT., 371) also reiterates the same 
opinion, while Macgillivray (/.c. I., p. 420) says that turns are effected by the 
contraction of one wing and the extension of the other, aided by the tail. 
The real method of turning, however, is very simple, and was, I believe, 
first pointed out by me in the Zdis for July, 1865, p. 297. It must be remem- 
bered that when a bird is flying the reaction of its. wings against the air is 
not only forward but also upward, the latter being necessary to counteract the 
force of gravity. If now a bird lowers its right side, so that the axis from 
