THE CUCKOO 117 



early because she has no young brood to detain her 

 in the north ? Has she a motive independent of her 

 young for retreating in July ? We cannot tell. 



It has been supposed that the parasitic egg-laying 

 of the Cuckoo depends upon the circumstance that 

 the eggs instead of being laid daily, mature in suc- 

 cession, with intervals of two or three days. If all 

 the eggs were hatched in the same nest, the operation 

 would be protracted, and inconvenience would result 

 from the existence of eggs, young nestlings and older 

 nestlings in the same nest. This actually happens 

 in the case of the American Cuckoo which is non- 

 parasitic. Such a negative exception is not a refutation, 

 but we have no proof that the rate of formation of the 

 eggs is a fixed and unalterable condition, capable of 

 dictating the mode of incubation. 



It is interesting to note that the habits of the 

 Cuckoo are not absolutely determined by obvious 

 facts of structure, and also that other Birds exhibit 

 the beginnings of a possible parasitic instinct. Some 

 species of Cuckoo build their own nests, hatch their 

 own eggs, and feed their own young. The common 

 American Cuckoo is one of these. Our common 

 Cuckoo has been said to lay her eggs on the bare 

 ground, to hatch them, and to feed the young. 1 There 

 are several truly parasitic Cuckoos besides our 

 familiar species. One of these is European, three are 

 Australian. 2 This affords the possibility of deciding 



1 Mr. Harting suggests that there may be an error of obser- 

 vation here, and that the Nightjar has been taken for a 

 Cuckoo. 



2 Ramsay, quoted in Darwin's Origin of Species, Chap. VII. 



