26o 



FLIGHT 



ing its food on the ground, and to breed in holes which it excavates 

 in clifts, the banks of streams, or in old walls ; but latterly, accord- 

 ing to Mr. Hudson {Argent. Ornithol. ii. p. 25), it has taken to make 

 its nest in trees, reverting presumably to the habit of its ancestors. 

 The fact rests on the best of evidence, but any inference is open to 

 criticism. Curiously enough, a very similar state of things is pre- 

 sented by an apparently cognate bird from South Africa, Geocolaptes 

 olivaceus or arator, which bears a strong superficial resemblance 

 and possibly (though this has not yet been ascertained) a deep- 

 seated affinity to the American forms. This Woodpecker, as Mr. 

 Layard remarks (B. S. Afr. p. 239), "never pecks wood, but bores 

 its way into the banks of rivers, sides of hills, or the walls of mud- 

 buildings, in search of its prey, and for a home for its young." 

 Mr. Buckley states {Ibis, 1874, p. 369) that in Natal he never 

 noticed it among trees ; but found it on the open hills and sitting 

 among stones. Considering how few Woodpeckers there are in the 

 Ethiopian Region, the occurrence at its southern extremity of this 

 simulacrum of a New- World type raises more than one question of 

 the deepest interest. 



FLIGHT.^ Birds have three chief modes of flight, each differ- 

 ing from the others in certain important particulars. These are — 



I. By gliding or skimming, supported on the extended wings, 

 which do not flap up and down. Most probably all birds that fly 

 can move in this manner. It requires a certain velocity of motion 

 of the bird through the air (relative velocity), which is acquired 

 (1) by previous strokes of the ■\vings, (2) by descending from a 

 higher to a lower level, or (3) by commencing flight in a Avind of 

 sufficient velocity. 



II. By active strokes of the tvings. The manner in Avhich this 

 mode of progression is carried out varies in detail in different birds, 

 and in the same bird at different times, but its main features appear 

 to be the same for all. 



III. By sailing or soaring with motionless extended Avings. This 

 appears to be only possible for certain birds, and is not described 

 as taking place except in a wind of a certain minimum velocity, 

 and differs from ordinary gliding in the fact that the bird 

 does not necessarily lose either in velocity or in vertical posi- 

 tion, as a result of the resistance of the air to the bird's passage 

 through it. 



^ I am indebted for this article to my colleague Prof. Eoy, wlio remarks 

 that, in it, he has "sought to avoid inaccuracy of fact or method of statement, 

 the main object being to put the matter in as simple a form as possible, so as not 

 to confuse the non-scientific reader. The references given to the most important 

 authorities on the subject will enable those who wish to pursue it further to do 

 so "—A. N. 



