The direction of the wind must necessarily be either along the line of flight of the bird, against it, or at an angle with it. In the first of these instances the speed of the bird over the ground is determined merely by adding the velocity of the wind to, and in the second by subtracting it from the air speed of the bird, in the same way as a swimmer’s speed is increased or reduced by the speed of the current. The third case is more complicated, as in this calculation allowance must be made for “drift,” _i.e._ the tendency of a bird under such circumstances to deviate from its desired course. It is, however, unnecessary to say anything further as to this third case, as the comparison of speeds of various birds can only be made satisfactorily by ascertaining their speeds under identical conditions in horizontal flight. 2. _The path of flight must be horizontal._ In the words[6] of Captain C. F. A. Portal, D.S.O.: “If any one has seen a peregrine stooping from 1000 feet at between 150 and 200 miles per hour at a partridge, and has later seen the same peregrine chase the same partridge from a standing start, he will appreciate the importance of considering only level flight. In the first instance, the hawk is nearly 100 miles per hour faster than the quarry, in the second, he can only just overtake it at all. There is no conceivable way of measuring the speed of these downward flights accurately, but no one who has done any hawking will deny that 120 miles per hour is within the power of a great many species. When we come to consider level flight, there is a very different story.” [Illustration: THE PEREGRINE FALCON. By V. R. BALFOUR-BROWNE.] 3. _There must be some evidence to show whether the bird is flying at its maximum speed or not._ As was recently pointed out in an interesting article[7] by Colonel R. Meinertzhagen, D.S.O.: “Birds have two speeds: a normal rate, which is used for everyday purposes and also for migration, and an accelerated speed, which is used for protection, or pursuit, and which in some cases nearly doubles the rate of their normal speed; some of the heavier birds can probably only accelerate to a slight extent. In this conclusion I am naturally excepting courtship flight, which is usually of an accelerated nature.” To quote the words of Major C. R. E. Radclyffe:[8] “The only possible test we can accept is where two birds are matched one against the other, and we are certain they are both trying their hardest. No better test than this is the case of a hawk pursuing its quarry, when it means to one of them its food and to the other its life.”