CHAP. VIII.] INSTINCTS OF THE CUCKOO. 199 



case of reversion to the long-lost, aboriginal instinct of nidi- 

 fication. 



It has been objected that I have not noticed other related 

 instincts and adaptations of structure in the cuckoo, which are 

 spoken of as necessarily co-ordinated. But in all cases, speculation 

 on an instinct known to us only in a single species, is useless, for 

 we have hitherto had no facts to guide us. Until recently the 

 instincts of the European and of the non-parasitic American 

 cuckoo alone were known ; now, owing to Mr. Ramsay's observa^ 

 tions, we have learnt something about three Australian species, 

 which lay their eggs in other birds' nests. The chief points to be 

 referred to are three: first, that the common cuckoo, \vith rare 

 exceptions, lays only one egg in a nest, so that the large and vora- 

 cious young bird receives ample food. Secondly, that the eggs 

 are remarkably small, not exceeding those of the skylark, a bird 

 about one-fourth as large as the cuckoo. That the small size of 

 the egg is a real case of adaptation we may infer from the fact of 

 the non-parasitic American cuckoo laying full-sized eggs. Thirdly, 

 that the young cuckoo, soon after birth, has the instinct, the 

 strength, and a properly shaped back for ejecting its foster- 

 brothers, which then perish from cold and hunger. This has been 

 boldly called a beneficent arrangement, in order that the young 

 cuckoo may get sufficient food, and that its foster-brothers may 

 perish before they had acquired much feeling ! 



Turning now to the Australian species; though these birds 

 generally lay only one egg in a nest, it is not rare to find two and 

 even three eggs in the same nest. In the Bronze cuckoo the eggs 

 vary greatly in size, from eight to ten lines in length. Now if it 

 had been of an advantage to this species to have laid eggs even 

 smaller than those now laid, so as to have deceived certain foster- 

 parents, or, as is more probable, to have been hatched within a 

 shorter period (for it is asserted that there is a relation between 

 the size of eggs and the period of their incubation), then there is 

 no difficulty in believing that a race or species might have been 

 formed which would have laid smaller and smaller eggs ; for these 

 would have been more safely hatched and reared. Mr. Ramsay 

 remarks that two of the Australian cuckoos, when they lay their 

 eggs in an open nest, manifest a decided preference for nests 

 containing eggs similar in colour to their own. The European 

 species apparently manifests some tendency towards a similar 

 instinct, but not rarely departs from it, as is shown by her laying 

 her dull and pale-coloured eggs in the nest of the Hedge-warbler 

 with bright greenish-blue eggs. Had our cuckoo invariably dis- 

 played the above instinct, it would assuredly have been added to 

 those which it is assumed must all have been acquired together. 

 The eggs of the Australian Bronze cuckoo vary, according to Mr. 



