1918.] by Birds nf K(jf)s unlike their own. 151 



without arousing suspicion. This may actually be so if the 

 theory lias been based on observation, but I am impressed 

 by the fact that our African Drongos are more intolerant of 

 the approach of another bird of their own species^ not merely 

 to their nest, but even within their '^ beat," than any other 

 bird I know — and this is saying much. 



Once the Cuckoo's egg has been placed in the nest it has 

 to trust to the host's lack of discrimination, or, alternately, to 

 its resemblance to the host's eggs — and to this last its small 

 size is generally acknowledged to contribute. It remains 

 large enough, however, to be likely to give so small a host 

 as a warbler much difficulty in removing it, as I saw in my 

 experiments on Cistlcola, and one might even amuse oneself 

 by supposing that the optiinuni size might be one not large 

 enough to fill the victim with complete conviction that it was 

 a fraud, yet just sufficiently large to dissuade it, after an 

 attempt or two, from trying to eject it ! One of my Grass- 

 Warblers, again C. natalensis, accepted and continued to incu- 

 bate a Layard Bulbul's egg after what appeared to have been 

 an initial attempt to eject it : here the fraud was obvious from 

 the wrong coloration, but the bird had not the enterprise to 

 remove its own eggs on failing with the Bulbul's. It is just 

 conceivable, again — the point could be tested experimentally, 

 — that the thick shell of a Cuckoo's egg, explicable, I believe, 

 as the result of a reduction in size without a corresponding 

 reduction in the amount of lime used, and useful as enabling 

 it to be carried about, may also protect it from being pierced 

 by such weak birds as Warblers — as the Bar-throated 

 Warbler of my experiments pierced thinner-shelled eggs ; 

 and that this, with the difficulty of handling it otherwise 

 which must be experienced by such small birds, may account 

 for the Cuckoo's egg being so often left deserted in the nests 

 of Wrens, Willow-Warblers, &c. At the same time, even 

 should it be so strong — which probably it is not, — this would 

 be of no use to it unless its own parent then removed it to 

 another nest. If such intervention is the rule in relation to 

 the eo-ff, it seems hard to understand how Walter could have 

 found as many as 150 deserted Cuckoos' eggs in Wrens' nests 



