26 STUDIES IN BIRD-MIGRATION 



as a guide to those that have already made the passage, 

 when coast lines and rivers, mountains and plains lay 

 before them. But of what avail would they be when a 

 vast expanse of sea has to be traversed ? Here we 

 are told that during their voyages the migrants fly at 

 a great height/ sometimes at 20,000 feet, and from 

 such elevations the sea and land would be spread below 

 them like a map. To this explanation is added the 

 experience gained through parental guidance. 



Such hypotheses, however, are quite insufficient to 

 explain the mystery. In the first place, we know that 

 most birds travel by night, and thus one fails to realise 

 how siofht would enable them to steer a correct course 

 over hundreds of miles of ocean. Secondly, many 

 young birds migrate apart from their parents, and hence 

 parental guidance is not an essential factor. In support 

 of this latter contention, we have the remarkable evidence 

 afforded by the migrations performed by the young of 

 the Cuckoo. It is well known to naturalists that the 

 young Cuckoo does not leave our islands and its summer- 

 haunts elsewhere in the autumn, until some weeks after 

 the adult birds have left for their winter home ; hence 

 parental guidance in their case is not conceivable. 

 Almost everyone is aware that the parent Cuckoo 

 deposits its eggs in the nests of other birds, and that the 

 young are reared by foster parents at the expense of 

 their own offspring, which the infant Cuckoo soon ejects 

 from the nest. Now, these foster parents mainly belong 

 to species which do not quit our islands, or, if so, do 



1 It is possible, however, that a limit is imposed by temperature to the 

 height at which birds can travel during their migrations. We must 

 remember, too, that they wing their way through the air at a high rate of 

 speed, whereby the effect of cold is considerably intensified. (See Nature^ 

 vol. xix., p. 481.) 



