Knowledo;e. 



With which is incorporated Hardwicke's Science Gossip, and the Illustrated Scientific News. 



A Monthly Record of Science. 



Conducted 1)\- Wilfred Mark W'ebb. F.L.S., and K. S. Grew, M.A. 

 DECEMBER, 1912. 



THE EVOLUTION OF THE CUCKOO. 



By G. \V. BULMAX, M.A.. B.Sc. 



Thosk who hold the theory of natural selection, 

 looking back in time, see the Cuckoo as a bird with 

 the normal instincts of its kind. Darwin was per- 

 suaded that even to-dav it still sometimes shows 

 traces of those ancestral instincts. " It has also 

 recently been ascertained," he wrote, "on sufficient 

 evidence, bv Adolf Miiller, that the Cuckoo occasion- 

 ally lays her eggs on the bare ground, sits on 

 them, and feeds her voung.'" How, then, in the 

 struggle for e.xistence did it attain its present strange 

 and unique position among the birds of our countr\- .' 

 That instinct which leads it to hand over the care of 

 its eggs and young to foster-parents is doubtless a 

 '^reat advantage from the Cuckoo's point of view. 

 There results for it a life of ease, no troublesome 

 nest-building, no trying work of brooding over eggs, 

 no voracious young to be fed at the expense of much 

 time and trouble. The Cuckoo is free to speed off for 

 Africa's sunn\- shores early in Jul\'. N\hilc other birds 

 are still toiling over their broods. For the individual 

 cuckoo the advantages are obvious ; its life becomes 

 a " primrose path of dalliance." The advantages to 

 the race, however, are not so obvious. It is not 

 clear that the Cuckoo leav^es a more numerous and 

 stronger progeny than it would if it reared its brood 

 in the usual way, or than it did while it was still 

 respectable. This, of course, is mere speculation, 

 but the fact remains that the Cuckoo is not specially 

 numerous in this countrv. It is less so than many 

 birds which rear their young in the ordinary way. 

 But. waiving this doubt as to the possible ultimate 

 advantage of the habit, let us go back to the common- 

 place respectable Cuckoo, or Cuckoo-like bird build- 

 ing its nest and hatching and rearing its own young. 

 Some abnormal twist in the brain of a certain 

 Cuckoo led it to pick up one of its eggs and place it 

 in the nest of a Titlark. We will suppose it thus 

 disposed of all its eggs,' though we are not quite sure 

 that the etiquette of natural selection would not 

 bind us down to one in the first instance. Let us 

 suppose that this brood is more successfully reared 

 than those treated in the ordinary way. This might 



arise from the fact that the Cuckoos were careless 

 nurses — though this is not a quality that could be 

 evolved by natural selection. Or an adult Cuckoo 

 may have'^ been unable to feed a full brood as well as 

 a Titlark could feed a single young Cuckoo. But, 

 however it ma\- have arisen, let us assume the 

 advantage to have been with the Cuckoo which got 

 its voung reared b)- the Titlark. The qualit>- or 

 instinct which led "the female Cuckoo to act thus 

 would probably appear in some of the descendants, the 

 rest inheriting normal instincts from paternal sources. 

 But in the descendants of those which did inherit 

 the new instinct, this would run a heavy risk of being 

 swamped by intercrossing with others of normal 

 instmcts. This, however, is a very common difficulty 

 in the case of an incipient new species, and we will 

 suppose the new instinct managed to survive the 

 flood. It might then be reinforced by the sporadic 

 appearance of the like in other individuals. Those 

 possessing the instinct would leave more numerous 

 and stronger progeny, and those which did not adopt 

 it would be finally weeded out by natural selection. 

 Thus the Cuckoo race sauntered down a curious b\- 

 path of evolution to the idle life. 



But if this be accepted as the general outline of 

 the evolution of the Cuckoo there are also certain 

 special points which call for attention. There are, 

 for example, the strange instincts and actions of the 

 voung Cuckoo in the nest. In the Cuckoo's respect- 

 able daj-s it cannot have been the little demon it 

 now is. ' It cannot have been in the habit of turning 

 its brothers and sisters and eggs out of the nest. So 

 it probably had not then the convenient hollow in its 

 back for holding the eggs. And yet these habits, 

 instincts and structure seem absolutely essential to 

 the well-being of the young Cuckoo. Only by turn- 

 ing everything else out of the nest can it obtain 

 sufficient nourishment for itself. And yet the first 

 Cuckoo hatched in a Titlark's nest cannot be 

 supposed to have had these characters. It \\ould 

 get no advantage in the strange nest, and would 

 probably be starved. 



