Hutton. — Our Migratory Birds. 261 



north. But the case of the bronze cuckoos is different. 

 These mix together in New Guinea in the winter, but separate 

 to breed, and have, through this, become differentiated into 

 different species. This is another illustration of the effects of 

 isolation in forming species by preserving variations, for we 

 •cannot suppose that the slight differences in colour between 

 the species are special adaptations to their surroundings ; and 

 they cannot be recognition marks, as the birds separate long 

 before they begin to pair. 



As a rule the migratory birds in the Northern Hemisphere 

 follow the land, either the shore-line or the interior, according 

 to their habits. But in their movements some have to cross 

 the Mediterranean Sea, which blocks the way. This they do 

 in three streams — one crosses the Straits of Gibraltar, another 

 passes through Sicily and Malta to Tripoli, and the third 

 goes by the islands of the iEgean Sea to Egypt. There are 

 also other cases of trans-oceanic migrations. Skylarks, field- 

 fares, and redwings pass backwards and forwards between 

 Norway and Britain. Other birds cross the North Sea ; others 

 the English Channel ; and, of course, many shore-birds must 

 pass from island to island in the Malay Archipelago. But the 

 boldest flight of all is to New Zealand and the Chatham 

 Islands, probably from New Caledonia, a distance of a thou- 

 sand miles or more. Why should they do this ? How do 

 they know that they will find land after so long and weary a 

 flight ? They are not blown out to sea, but go voluntarily, 

 and they must know that there is land ahead of them. How 

 did they acquire this knowledge ? 



Stragglers which have lost their way from a migrating 

 flock and gone to some other country do not start new lines 

 of migration. Birds never fly to sea at random and, having 

 discovered new land, come back and tell their comrades of it. 

 Xf this were the case we should find that oceanic islands, like 

 the Azores and Madeira, were visited by migratory birds. 

 These wanderers have no tradition behind them of the new 

 route, and they cannot travel it again. Stragglers either 

 perish or, if sufficiently numerous, establish themselves in the 

 new country as residents. 



We have examples of this in the swallow, which is resident 

 in Tonga, and the spotted plover, which has established itself 

 in New Caledonia and in Tonga. In New Zealand, also, the 

 spotted plover has been known to breed, and, if undisturbed, 

 might perhaps establish itself with us. Heteractitis in- 

 camts migrates annually from Alaska to Mexico, and has sent 

 stragglers to Hawaii and southern Polynesia, and has become 

 a, resident in Fiji. 



In all these cases the stragglers do not return. They 

 ^either die out or establish themselves in the new locality. 



