their comparative Numbers and Distribution. 637 



of them migrate singly. The adults change the clothing 

 plumage, and also the tail, during their stay with us; but 

 retain the quills to bear them on their migratory journey. 

 The barred markings on the sides of the neck are exhibited 

 in the new feathers which the adult females put forth at the 

 close of the summer, or, rather, a little after midsummer ; and 

 appear, therefore, to be a permanent characteristic of the 

 sex ; whereas Mr. Selby states that " the female differs in no 

 respect from the male." The young of the year do not change 

 a feather till after they have left us. Unlike many other 

 species with which I am acquainted, the females often con- 

 tinue to lay while in deep moult. They deposit, on the 

 average, five eggs in the course of a season, rarely more ; 

 and these, judging from the result of numerous experiments 

 which I have instituted, are not unfrequently ejected by the 

 birds to whose fostering care they are entrusted: hence, I 

 believe, the disproportionate scarcity of their occurrence. A 

 considerable period intervenes between their successive de- 

 position ; during the lapse of which the female cuckoo in- 

 stinctively watches the proceedings of birds that are carrying 

 about building materials ; the time her egg requires to be- 

 come matured for laying being at least equal to that which 

 the majority of smaller birds consume in the process of nidifi- 

 cation. That she cannot, however, much protract the time 

 of disburdening herself, as has been suggested, when the egg 

 has been received into the oviduct, is sufficiently proved by 

 the fact of her sometimes laying into an unfinished nest. 

 There is no division of her eggs into separate lots, as in the 

 case of birds which incubate their own; a fact which might, 

 indeed, have been suspected a priori, seeing that no purpose 

 could be effected by such an arrangement. In point of fact, 

 each separate consecutive egg is analogous to a lot in other 

 species. The remarkable deviation of the cuckoo from the 

 general habit of the feathered race, in confiding its progeny 

 to the care of strangers, is dependent on a peculiarity of the 

 vascular system, and particularly on the minuteness of those 

 blood-vessels which supply the generative parts ; in conse- 

 quence of which the male organs, even at the full period of 

 sexual excitement in summer, are most disproportionately 

 small ; and each successive egg of the female requires so long 

 a time to become fit for deposition, that they could not be 

 incubated simultaneously : even when developed, they are, 

 from the same cause, remarkably small for the size of the 

 bird ; which, however, adapts them to that of the nests into 

 which they are intruded. I cannot suppose that this peculiar 

 conformation is intended merely for the purpose of retarding 



3 a 3 



