191 8.] by Birds of Eggs unlike tlieir own, 153 



throughout, were, of course, quite iudispensable to the per- 

 formance of the part of the operation just described. The 

 wings, as Mr. Craig describes, were used largely for steadying 

 the victim on the Cuckoo's back and were very sensitive and 

 useful. The stimulus to the commeucemeut of the opei'a- 

 tion seemed always to be movement on the part of the 

 fellow-nestling. 



I have referred above to the coloration of the young Koel. 

 If the explanation given for it in Mr. Pycraft's useful 

 little book, ' The Story of Bird-life/ be the correct one— 

 namely, that it resembles its male parent instead of, as is 

 usual, the mother, because, were it not black, the foster, 

 parents with black young "would promptly kill it on 

 detecting the fraud," — then it is obvious that some foster- 

 parents continue to discriminate after the e^^ is hatched. 

 This may be so, and it will be very interesting if it is, but 

 the theory is one that ought to be tested carefully in the 

 field by substitution of wrongly coloured nestlings, or eggs 

 that will produce them, for those of the Crows, &c., that are 

 the Koel's hosts. Possibly it has been so tested. 



Finally, we come to the adult Cuckoo, with, in many- 

 cases, a close resemblance to some unrelated bird. I have 

 referred to one explanation of these resemblances — that they 

 enable the bird the more easily to insert its egg in its victim^Si 

 nest, An alternative — or additional — possibility must not, 

 however, be overlooked. It is that, as in most cases of 

 mimicry, the resemblances will be useful in relation to, 

 enemies. The Drongo is likely to be a particularly useful 

 model, not merely for its aggressive qualities, but for ita 

 nauseating effect on the eater, tested by me so far, however^ 

 only on mammals, not hawks. " Nauseousness " seems to 

 me likely to be the modePs qualification in nearly all cases 

 of mimicry in birds that have been suggested as models, 

 though " fighting-weight," such as hawks possess, will doubt- 

 less also tell. An argument against mimicry generally has 

 been drawn from the extraordinary closeness of the resem- 

 blance oiHierococcyx variiis to Asiur badius and from the 

 fact that the resemblance extends to the immature plumage. 



