BIRDS OF PREY. 261 



celled and perhaps only equalled among such sea-birds as the petrels. Many of them 

 walk well, but slowly, when on the ground, while a few are able even to run ; but 

 these are exceptions, the great majority being only able to progress with much diffi- 

 culty on the ground, and these always by hopping instead of walking. There is little 

 evidence that any of them possess unusual keenness of scent, the general impression 

 to the contrary notwithstanding. But we shall recur to this again. 



In eating they often swallow much which is indigestible, such as bones, scales, hair, 

 feathers, etc., and these substances are afterwards ejected from the mouth in large 

 balls or rolls, technically known as castings. Their nesting habits vary much in 

 the different groups, but the birds are always monogamous and are believed frequently 

 to remain paired for life. The eggs are commonly few, rarely exceeding six, often 

 only one. The period of incubation is longer than in most other aerial birds, and 

 the young at first are covered with down, are quite helpless, and for an unusually long 

 time entirely dependent on the parents. 



In size the Accipitres vary from the tiny finch-falcon (Microhierax) of the East 

 Indies, less than six inches in length and weighing only a few ounces, to the Lammer- 

 geyer and griffon-vulture of the Alps and Pyrenees, with an expanse of ten feet or 

 more and a weight of eighteen or twenty pounds. 



It may not be out of place at this point to call attention to the fact that it is 

 among the largest birds of the order that we find the most remarkable power of flight, 

 that is, the longest sustained and that which is apparently accomplished with the least 

 effort. The fact that a crane, an albatross, a vulture, or an eagle can rise from the sur- 

 face after a slight impetus is obtained, and then ascend in 'circles' without any 

 perceptible motion of the wings, until actually lost to sight in the clear sky, is so well 

 known as hardly to need mention, but the "way of an eagle in the air," the real 

 manner in which this is accomplished, is often spoken of as an unsolved mystery. 

 That it is not so, any person with fair opportunities of observing the phenomena, and 

 a moderate amount of patience and common sense, may easily satisfy himself. The 

 points he will notice, though probably not in the particular order here mentioned, will 

 be about as follows : 



First, the bird must in some manner get a fair start, either by running a short 

 distance, by flapping the wings, by spreading them against the breeze, or, if the 

 ground be uneven, by gliding slightly downward from an elevation. Then it will be 

 noticed that if there is actually no breeze at all (which will rarely happen unless in a 

 small and closely hill-girt valley) the circling bird will be utterly unable to rise with- 

 out flapping ; each complete turn will bring him back to a point close to his starting- 

 place, or at least not higher. 



"When the bird is able to rise without flapping, it will be found that, while gaining 

 in height at every turn, he is also drifting off before the wind, so that the successive 

 rings of his spiral are never vertically over each other. Should the observer ever be 

 fortunate enough to stand on a mountain side and see an eagle rise past him in this 

 way from the valley below to the open air above, he will not only be convinced of 

 this, but of much more ; for he will see that, throughout one half or more of each coil of 

 the spiral, the bird not only does not rise, but actually sacrifices some elevation for the 

 sake of gaining speed, and this is in that part of the circuit during which he is gliding. 

 with the wind and across it ; the instant he once more turns to meet it he begins to 

 rise, soon converting most of his momentum into elevation, or, in other words, gaining 

 height at the expense of speed. When his headway is almost gone, he slowly turns 



