MIGRATION 1 9 



scarcely any writers on the subject have had sufficient data to 

 form an hypothesis, so that it is not suprising that hardly any 

 two agree in theory. 1 In other parts of the world there is still 

 less ground for theorising, though in North America many 

 valuable observations have been made ; and these, in conjunction 

 with those carried on in Europe, will no doubt in due time lead 

 to satisfactory results as regards the Northern Hemisphere. 

 Concerning the Southern our ignorance is almost complete. 



Of the way in which Migration is performed there is still 

 much to learn but one thing is certain, all Birds do not migrate 

 in the same manner. Some gather in flocks, great or small, others 

 seem to accomplish their northward journey in pairs, or at any- 

 rate arrive at their breeding-quarters already paired. Some 

 undoubtedly voyage by night, others may be seen to travel by 

 day. Of the Birds which in spring arrive unpaired, it is now 

 incontestable that the males outstrip or precede the females. 

 There is, moreover, equal diversity in the southward movements 

 towards the close of summer and all through the autumn. Of 

 some species the earlier broods disappear without attracting 

 attention, and the later broods as well as the parents slip away 

 almost as imperceptibly. In one remarkable case, that of the 

 Cuckoo, the adults leave this country long before the young are fit 

 to follow ; but, in by far the greater number, the young start first, 

 and are followed, often at an interval of some weeks, by their 

 parents. 2 It is contended by many that of actual Migration we 

 see very little, since it is constantly carried on at a height where 

 the Birds are beyond our ordinary observation, and as regards 

 some species this seems to be true. Moreover, it would seem 

 that the longest flights are performed by night, and when the 

 sky is clear, so that only in thick weather do the Birds come near 

 enough to the earth to be heard seeing them being of course 

 impossible in the dark, though in a few cases they have been 

 telescopically observed passing across the face of the moon. It 

 is certain that many of the smaller land-birds gradually press 



1 For the best collection of facts, see the various reports of the Migration Com- 

 mittee of the British Association, 1880-1888 ; and especially that for 1896, con- 

 taining the Digest of the observations (made at Lighthouses and Lightships) by 

 Mr. W. Eagle Clarke. Also Mr. Harrington's work on Migration in Ireland (1900). 



2 It has been suggested that these flocks of young birds are led by older members 

 of their own species which, though for some cause not breeding, have yet had 

 experience of migration ; but of this there is no evidence whatever. 



