Natural History 425 



young cuckoo sitting on a post, while the much smaller pipit duti- 

 fully feeding him must needs stand on his shoulder, so to speak, 

 for the purpose ! The whole story is one of effective adaptation on 

 the part of the Cuckoo and of the weakness of blind instinct on the 

 part of the foster-parent. 



The most interesting theoretical point about the Cuckoo has 

 to do with the colour of the eggs, which is very variable, but tends 

 to be like that of the eggs of the chosen foster-mother. That one 

 hen cuckoo always lays the same type of egg seems to be 

 thoroughly established, but it is still a matter of speculation 

 whether the character is hereditary and, if so, in what manner. 



The Cuckoo victimises a large number of different species as 

 foster-parents for its young, but all the usual ones are small in- 

 sectivorous birds. The degree to which the Cuckoo's egg re- 

 sembles the others varies greatly; sometimes there is almost a 

 perfect match, at least in colour, but in other cases the similarity 

 is slight or even non-existent. 



10 

 MIGRATION 



The scientfiic investigation of migration is greatly compli- 

 cated by the difficulties of making observations. It is not now 

 believed that the greater part of migration takes place at immense 

 altitudes, and at an accelerated rate of flight which makes enor- 

 mous journeys possible for birds in a single night. Nevertheless 

 it remains true that a great deal of migration is nocturnal, and 

 that, for other reasons also, it is difficult to observe. At certain 

 times and places, however, much migratory flight can be actually 

 observed. We have, for example, this recent description of the 

 passage of swallows on Heligoland: "All through the forenoon, 

 as we sat in the autumn sunshine near the narrow northern apex 

 of the island, the swallows came in over the sea from the north- 

 east in the teeth of a southerly gale. No large flocks were seen ; 



