260 PHENOMENA OF FLIGHT IN THE ANIMAL KINGDOM. 



to me that it is executed in general under the following special 

 conditions: Along the cliffs of the coast of Normandy I have seen the 

 gulls and sea-mews performing their evolutions without moving their 

 wings. I have seen the daws and rooks flying in the same manner 

 around old cathedrals. But the same birds, when they left these 

 special stations, have always appeared to me to .use the rowing method 

 of flight; that is to say, making regular strokes of their wings, vSome- 

 times interrupted in the daws by swoops of short duration. I then 

 sought to determine the direction of the wind, and this is what seemed to 

 me to occur : When a bird finds itself in the neighborhood of a cliff, 

 where the air is calm or agitated by eddies iu a contrary direction to 

 the prevailing wind, it can pass successively from the calm to the agitated 

 air, and conversely. A sea-mew surrendering itself to the force of the 

 wind, receives an impulse which carries it with a certain rapidity, and 

 if, by simply turning, the bird enters a region of calm air, it can utilize 

 the impulse which the wind has given it in returning to the height 

 which it had left. Plungingagain into the zone of agitated air, it recom- 

 mences the evolution whicli 1 liave just described, without moving its 

 wings, except to give them different inclinations. The daws and rooks 

 ai)pear to me to find the same conditions around the cathedral towers. 

 The authors who have reported the most curious cases of sailing flight 

 have observed them in mountainous regions. It is a condor in the 

 Cordilleras, or an eagle iu the Pyrenees. The sailing flight has often 

 been described of certain birds of prey, who, in the middle of a i)laiu, 

 rise and turn without moving their wings. I myself have often seen 

 harriers fly in this manner, but I have always determined, also, that in 

 this case the si>iral which they describe is altered by the wind, and 

 that the birds are definitely carried to leeward with a more or less rapid 

 motion. 



Even when reduced to these limits the influence of the wind on the 

 flight of birds is very difficult to explain. It is complicated by very 

 diflerent conditions in which the motion acquired by the bird, opposed 

 from various directions by the force of the Avind, gives rise to the most 

 varied combinations of motion. It is also known that in the upper 

 regions of the air various currents exist, sometimes even iu a contrary 

 direction to those which obtain near the surface of the earth, so that the 

 bird, passing from one to another, find forces which carry it in opposite 

 directions.* 



Finally, the question of sailing flight seems to me one of the most 

 difficult to solve. It would be temeritous to absolutely condemn the 

 opinion of observers upon such vague theories and ideas as we possess 

 upon the subject. 



One of the most interesting points in the conformation of birds con- 

 sists in the determination of the relations of the extent of the alar sur- 

 faces to the weight of the animal. Is there a constant relation between 

 the weight and these surfaces"? This question has been the cause of 

 numerous controversies. It has been already shown that if birds of 

 very diflerent kinds, yet of the same w^eight, be compared, the wings of 

 some species are foAnd to have four or five times the extent of others. 

 The birds which have large wings are usually those which have been 

 called "sailors," while those which have the wing short and narrow are 

 generall}^ classed as "rowers.' But if we compare two "rowing" birds 

 with two "sailing" birds; if, for still closer comparison, we take them 



* The late Mr. Espy suggested that the phenomcuou of sailing in the flight of birds 

 is due to upward currents of air which talie place in warm weather, or beneath clouds, 

 and esiiecially up the side of a mountain against which the wind is blowing. — J. H. 



