210 SPECIAL INSTINCTS. Chap. VII. 



It has been objected that I liave not noticed other related 

 instincts and adaptations in the cuckoo, which are falsely 

 spoken of as necessarily coordinated. But in all cases, specu- 

 lation on an instinct known only in a single species is useless, 

 for we have no facts to guide us. Until quite recently the in- 

 stincts of the European and of the non-parasitic American 

 cuckoo alone Avere known ; now, owing to Mr. E. Ramsaj^'s 

 observations, we know something about three Australian spe- 

 cies, which lay their eggs in other birds' nests. The chief 

 points are three : first, that the cuckoo, with rare exceptions, 

 lays only one egg in a nest, so that the large and voracious 

 young l)ird may receive ample food. Secondly, that the eggs 

 are of remarkably small size, not exceeding those of the sky- 

 lark — 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. Thirdl}"-, 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, it has been boldly maintained, is a 

 beneficent arrangement, in order that the young cuckoo may 

 get sufficient food, and that its foster-brothers may perish be- 

 fore they have 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 any 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 

 tliere is a relation between size and the period of 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. ^Ir. Ramsay remarks that two of the Australian 

 cuckoos, Avhen they lay their eggs in an open nest, manifest a 

 decided preference for nests containing eggs similar in color to 

 their own. Tlie Eurojiean sjiecies certainly manifests some 

 tendency toward a similar instinct, but not rarely departs from 

 it, as is shown by her laying her dull and pale-colored eggs in 

 the nest of the Hedge-Avarbler with bright greenish-blue eggs. 

 Had our cuckoo invariably displaj-ed the above instinct, it 



