MIGRATION. 179 



noisy when building. They make a sound re- 

 sembling the croaking of wood-frogs, and their 

 combined clamour can be heard at a distance 

 of 4 or 5 miles when the atmospheric conditions 

 are favourable. 



There are one or two notable facts which are 

 well worth mentioning in this connection. Thus, 

 it seems to be well-established that in many 

 species the males arrive sometimes days, or even 

 weeks, before the females. In the autumn the 

 adults often leave first, leaving the young to 

 follow later — and generally by a different route. 

 The cuckoo is an instance of this. How these 

 young find their way is a mystery. An innate 

 sense of direction has been suggested as a possible 

 explanation, but this is not very convincing. 



The routes followed by some of our more 

 familiar friends will doubtless prove of interest. 

 Concerning this subject comparatively little is 

 really known. Facts however are being slowly 

 accumulated. In Great Britain, much valuable 

 and lasting work has been, and is being done, by 

 Mr Eagle Clarke. Much more has been done by 

 the careful collection of birds' skins from all parts 

 of the world as may be found at the British 

 Museum. From such collections, for instance, we 

 gather that our common swallow winters on the 

 west coast of Africa. 



The " birds-of -passage " to which we referred 

 at the beginning of this chapter are those on 

 their way to and from their breeding-grounds, 

 outside the limits of these islands. They make 

 but a brief sojourn — possibly for rest and food — 

 and then disappear. 



