3IO THE BIOLOGY OF BIRDS 



size and age It gets the nest all to itself, and it is attended 

 with fatuous diligence by its foster-parents, who may con- 

 tinue their attentions even after it leaves their nest. The 

 adult cuckoos leave Britain some six weeks before the young 

 ones, so that the latter must make the southward journey 

 without any help from their own kindred. 



The riddle of the cuckoo's behaviour has not been wholly 

 read, but there are three considerations that make it less 

 mysterious than it seems at first sight, (i) It is not a unique 

 phenomenon ; it occurs in many other species of cuckoo 

 and in the quite unrelated Cow-Birds ; at least one species 

 of Oriental cuckoo is parasitic in one part of the country 

 and nests in another ; many different kinds of birds utilise 

 the nests of their neighbours either as a practice, as in one 

 of the ducks {Metopiana peposaca), or as an occasional 

 aberration. 



Hilzheimer notes that the Great Spotted Cuckoo 

 (Coccystes glandarius) lays several eggs in the nest of a crow 

 or some related bird, and that the young cuckoos do not try 

 to oust the rightful tenants ; that Scythrops novce hollandice 

 keeps to the nest of one of the " Piping Crows " (Strepera 

 tibicen) ; that some other cuckoos make a joint nest ; and 

 that in some other forms both parents brood normally. It 

 is important not to confine attention to the highly evolved 

 case of the European Cuckoo [Cuculus canorus). 



(2) The mother cuckoo is a bird of many peculiarities. 

 She is polyandrous, and the parasitism is congruent with 

 that ; she is very prolific, often laying ten or a dozen eggs ; 

 her egg-laying is strangely interrupted, which would not fit 

 in well with personal incubation. Moreover, cuckoos feed 

 very largely on hairy caterpillars which become scarce after 

 midsummer — an economic reason for the early migration 

 and for leaving the care of the young to others. 



(3) Most light, however, is to be got from Professor 

 F. H. Herrick's suggestion that the loss of the nesting- 

 instinct is due to an irregularity in the rhythm of the life- 

 cycle — a formula which covers many a variation among 

 animals. Just as one kind of bird may build supernumerary 



