ii8 ROUND THE YEAR 



which facts of structure and life-history are and which 

 are not necessary to the parasitic mode of incubation. 

 It would seem probable that the small size of the 

 egg is directly connected with parasitism. Whether 

 early migration or even migration at all is an essen- 

 tial condition I do not know. By comparison of 

 various species of parasitic Birds it appears that 

 they are prone to lay their eggs in the nests of 

 Birds whose eggs are somewhat similar in size and 

 colour. This tendency perhaps exists in our com- 

 mon Cuckoo, though the contrast between her eggs 

 and those of the Hedge-sparrow is notorious. There 

 is some reason for supposing that the colour of the 

 eggs laid by every female is peculiar and constant 

 Each Cuckoo returns, it is believed, year after year to 

 the same place, and lays her eggs in the nests of one 

 particular species only. Dr. Rey supposes that each 

 Cuckoo keeps to the nests of that species by which 

 she was herself reared. 1 Certain Birds which are 

 not Cuckoos at all regularly lay their eggs in the 

 nests of other Birds. Among these are more than 

 one species of Icteridae, some of which are named 

 Orioles, though not belonging to the family of true 

 Orioles. Various Birds of the most diverse kinds 

 have been known to practise the same trick casually. 

 The Starling's egg for instance, has been found in a 

 Woodpecker's nest. Much work remains to be done 

 in the way of collecting, authenticating, and com- 



1 Altes und Neucs aus dcni Haushalte des Kuckitcks An in- 

 teresting discussion of the question is to be found in Harting's 

 Summer Migrants, pp. 224-8. 



