260 TING AA: 
ing its food on the ground, and to breed in holes which it excavates 
in cliffs, 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, Geocoluptes 
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. 8. 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 ([bis, 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 wings, (2) by descending from a 
higher to a lower level, or (3) by commencing flight in a wind of 
sufficient velocity. 
II. By active strokes of the wings. The manner in which 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 wings. 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. . 
1] am indebted for this article to my colleague Prof. Roy, who 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. 
