Adaptation in Parasitic Cuckoos' Eggs. 387 



towards proving my second point in reference to the com- 

 parative size of Cuckoos' and fosterers^ egg^j foi* anything 

 more ridiculous than the sight of a Cuckoo's huge e^^ on 

 the top of a clutch of C. cisticola's it is difficult to conceive, 

 yet the Fantail accepts the egg as her own and cheerfully 

 undertakes the duties of incuhation and its following troubles. 

 At the same time I admit that there must be some reason 

 for the comparatively small size of the parasitic Cuckoos' 

 eggs generally, which would seem to point to the fact that 

 amongst fosterers are here and there birds who are frightened 

 by the formidable size of the intruder, and in consequence 

 refuse to act in toco par'entis. 



As regards coloration, an inspection of a big series shows 

 that the most common types of eggs of Cucidus canorus 

 bakeri range from an egg coloured precisely like that of a 

 Cisticola, i. e. white, sparsely speckled or spotted with 

 reddish, to a very deep dull reddish egg with deeper 

 mottlings or specks of the same colour. 



Now, of the eggs which the Fantail Warbler is a^ked to 

 incubate, the majority are either of the same type as its own 

 egg or else somewhat approaching it in coloration ; and jt 

 seems that in this case we have a slow process of elimination 

 going on which will eventually result in only that strain of 

 Cuckoo surviving which lays eggs agreeing in everything but 

 size with that of the little Cisticota. At the same time this 

 })rocess has not advanced very far, and a large number of 

 Cuckoos still continue to pLice eggs in the Cisticota's nest 

 w liich are very unlike those of the foster-parent. 



A few Cuckoos of this subspecies also deposit their eggs 

 in the nests of small Warblers, such as Cisticola tytteri, 

 C. volitans, Orthotomns, and Franklinia -, but it may well be 

 that in these cases the Cuckoo mistakes the nests for those 

 of Cisticola cursitans, which they closely resemble. It is, 

 however, only odd eggs at odd times which are deposited in 

 these nests, and I can trace no signs of any process of 

 adaptation being in progress. 



When, however, we come to the eggs placed in the nests 

 of birds of the genus Trihiira, there are good grounds for 



