KNOW Li:i)r,i:. 



DKCKMIiliR, 1012. 



I ]TW i',i^},'s of ilu' Cuckoo have suggested another 

 evolutionary prolilom. More than a luindred years 

 ago, the naturahst Salerne gave currency to tlie idea 

 tliat the egg of tlie Cuckoo resembles those beside 

 w hicli it is placed. He was himself hardly a believer 

 in the assertion, but gave it on the authority of an 

 inhabitant of Sologne. In 1S5J, Dr. Baldamus 

 brought forward the same idea, and supfiorted it by 

 a series of eggs in his cabinet. But linglish ornitho- 

 logists were slow to accept this view. Thev saw, 

 for example, that in the case of the Hedge Sparrow, 

 in whose nest the Cuckoo so frequently places its 

 eggs, there was no resemblance at all. On the basis 

 of the collection of Cuckoos' eggs in the Natural 

 History Museum Sir \Vm. Flower gives the following 

 summing u|) of the question : — 



" We have now a fine series of Cuckoos' eggs, with those of 

 the birds in whose nests they were laid, showing in many cases 

 a great resemblance in colour, in others none at all. In some 

 Hedge Sparrows' nests the Cucl<oos' eggs are as blue as the 

 others ; but in some they are of the more usual specl;led-brown. 

 It has been doubted whether the blue eggs were really those 

 of the Cuckoo, but Mr. Seebohm set the question at rest by 

 taking an undoubted young Cuckoo (with its very different feet 

 from the Sparrow's) from one of them. The Cuckoos' eggs 

 vary much in colour, and, generally speaking (though with 

 many exceptions), show some conformity to the eggs of the 

 bird in whose nest they are laid." 



Professor Newton was inclined to accept the theory 

 of the resemblance with the reservation that there 

 are " numerous instances in which not the least 

 similarity can be traced." Granted the resemblance, 

 then, it is an obvious suggestion that its object is the 

 more successful!}' to deceive the foster-parents. .\nd 

 this being so it admits of an explanation on the 

 lines of natural selection, since those Cuckoos which 

 most successfully palmed off their eggs on their 

 dupes would succeed best in the struggle for exis- 

 tence. Thus the Cuckoo which laid a robin-like 

 egg and placed it in a Robin's nest would succeed 

 better than one whose egg had no resemblance to 

 those of its host. And having once confided its egg 

 to a Robin, a Cuckoo would be likely to continue 

 doing so, and the daughters would be likely to 

 inherit the habit. Thus there would be a tendency 

 to produce se[)arate races of Cuckoos, one laying blue 

 eggs in Hedge Sparrows' nests, another blue brown- 

 speckled eggs in Robins' nests and so on. It is easy 

 to suggest objections to this explanation. In the 

 first place it is not obvious that any dece[)tion 

 is necessarj-. As a rule bii'ds seem ready to sit on 

 any sort of eggs. Our domestic hen will hatch 

 ducks, turkeys or pheasants as readily as her own 

 chicks; in Sir John Sinclair's classical attempt to 

 introduce the nightingale into Scotland the eggs of 

 this bird were hatched by Robins ; the Hon. Daines 

 Harrington reared linnets under Skylarks, Woodlarks 

 and Titlarks: Starlings have been known to sit on 

 and hatch bantams' eggs. Professor Newton, how- 

 ever, suggested that while some species of birds are 

 thus easil\' deceived there may be others w hich are 

 not. And it would be only in the nests of the latter 

 that we should expect to find the Cuckoos' eggs 

 approximating closely to the others. This would 



exi)lain the numerous exceptions. But to test the 

 view thoroughly we should require to have figures 

 showing the relative frequency of the resemblance 

 in the case of a number of different species of small 

 birds. There is some evidence that the Reed 

 Warbler is one of the objectors, but in the one 

 case in which we have seen — in a museum — a 

 Cuckoo's egg in the nest of this bird it was very 

 different in size and markings. We suggest, how- 

 ever, that those Cuckoos which habitually placed 

 their eggs in the nests of the easily deceived Robin 

 and Hedge Sparrow would succeed so much better 

 in the struggle for life than those which went with 

 their not \'et perfectly matching eggs to the nests of 

 the more fastidious birds, that the latter would, 

 according to the principles of natural selection, be 

 weeded out. 



Let us suppose, however, that the Cuckoo species 

 is divided into races laying eggs of different colours, 

 blue, dark grey speckled, blue speckled with brown, 

 and so on. Should we not expect this race segre- 

 gation as regards eggs would be accompanied b\- 

 some differences in plumage and other characters ? 

 In the case of the domestic fowl we know that 

 differences in the eggs are associated with variations 

 in other characters. No case of two varieties laying 

 different eggs without variation in plumage, and so 

 on, can be brought forward. But no such racial 

 differences can be pointed out in the Cuckoo. 



.\nd then we must remember that the male 

 Cuckoo has his part to pla\' in the matter. Is there 

 any evidence that a male hatched in a Wagtail's 

 nest, for example, usuallj' seeks for its mate a female 

 la\ing Wagtail-like eggs, or reasons why it should ? 

 What marks are there by which he could recognise 

 the right female, supposing his tastes were orthodox ? 

 .■\nd if he did not choose the right partner would not 

 the variation in the direction of laying Wagtail- 

 Cuckoo eggs be swamped? Nay, further, might it 

 not happen that if a Hedge-Sparrow-Cuckoo mated 

 with a male hatched from a Robin-Cuckoo egg. the 

 blue-egg layers among the offspring would inherit 

 the instinct of placing their eggs in Robins" nests, and 

 those which laid eggs like the Robin's the instinct 

 of choosing the Hedge Sparrow as foster-parent ? 



.■\nd then must we not also consider the question 

 of the evolution of what we may call the receptivity 

 in the foster-parent ? Professor Newton points out 

 that this varies in different species, and thus it 

 becomes a quality subject to the action of natural 

 selection. In the beginning, again, it must have been 

 variable among individuals of the same species. 

 Some would receive the Cuckoos' eggs, and some 

 would reject thein. The latter would succeed best in 

 rearing their own offspring, while those which reared 

 young Cuckoos would leave no inheritors of their — 

 from the Cuckoo's point of view — virtues. Thus the 

 quality of receptivit\' could never be evolved on the 

 lines of natural selection : those possessing it would 

 be weeded out. 



The evolution of the Cuckoo by natural selection, 

 in fact, bristles w ith difficulties. 



