Nest-building, Incubation, and Migration. 241 



to be the usual rule, the old birds are the first to migrate, 

 leaving their young to follow as best they may. 



The hosts selected if indeed the word is appropriate 

 by the cuckoo are numerous, and of different sizes, from 

 that little scrap of a fellow, the blue tit, on the one hand, 

 to the magpie, the jay, and the lesser grebe on the other 

 hand. No less than forty-three several hosts are recorded 

 for England alone, and if continental records be included, 

 the list includes eighty-seven species belonging to eleven 

 different families the warblers and finches being the most 

 abundant,* and the hedge-sparrow, one of the first birds 

 to build a nest easily discovered either by boy or cuckoo, 

 among the commonest of all. It is said by some ornith- 

 ologists that the egg is dropped by the parent bird in nests 

 where its colour will assimilate with that of the eggs proper 

 to the nest ; and it has been suggested by some that this 

 may be due to intelligent selection on the part of the 

 cuckoo, and by others that cuckoos act instinctively ; that 

 is to say, there is said to be an instinctive tendency in 

 cuckoos which lay eggs of a given type to drop them into- 

 appropriate nests. This is the view taken by Dr. Key, 

 of Leipzig, who contends that cuckoos instinctively deposit 

 their eggs, the colour of which for any particular bird is, 

 he believes, constant, in the nests of the species by which 

 they were reared. Each species of host thus rears a 

 particular kind of cuckoo. He says that the eggs deposited 

 among the variable eggs of the red-backed shrike are 

 themselves variable ; while those deposited in the nest of 

 the wren, among her uniform eggs, have great uniformity 

 of colour. On the other hand, Dr. J. A. Norton, of Bristol, 

 who has paid much attention to the matter, which is one 

 of direct observation, and who has kindly furnished me 

 with some interesting notes on the whole question of cuckoo 



* See Mr. Edward Bidwell, Norf. and Nor. Nat. Hist. Soc., vol. iii. p. 536, 



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