107 [Vol. xlii. 



places where Crows are not available, the Koel will use the 

 nests of Magpies, but for these no further evolution is 

 required, and the only abnormal fosterer I have seen is one 

 Myna, and this bird always deserted her nest immediately 

 the Cuckoo's egg was therein. 



So much for evolution, which has eliminated all but one 

 permament type, which never varies more, if as much, as 

 most eggs vary. But there is another form of evolution 

 going on which is creating more than one type of egg for 

 the same species. Amongst the most notable of these are 

 Cucidus poliocephalus and Hierococcyx sparveroides (Boxes 

 Nos. 22 and 26). The first has evolved eggs either pure 

 white or bright terra-cotta in colour, whilst the second has its 

 eggs either olive-brown or pure pale blue, and in this latter 

 species also we have, what I have seen in no other Cuckoo's 

 oology, eggs of two sizes, the blue eggs averaging ever so 

 much bigger than the brown. 



When we study the reasons for this dual evolution, they 

 are at once self-evident. To take the case of Cuculus polio- 

 cephalus first (Box No. 22). In Japan the little Himalayan 

 Cuckoo deposits her eggs in the nest of Cettia, and the eggs 

 of the two agree beautifully in colour, both being terra- 

 cotta. In Assam birds of the genus Horornis are common, 

 and, as they lay chocolate-coloured eggs, the terra-cotta 

 ones do for them also. Phylloscopi and other suitable 

 fosterers laying white eggs are rare, so we have no white 

 Cuckoos' 6ggs. When, however, we get to the Western 

 Himalayas there is a change. Horornis still exists, and 

 another suitable but rare fosterer with red eggs is Oligura, 

 but for every nest of these birds there are dozens of 

 Phylloscopi^ which lay eggs either pure white or white with 

 a few specks and spots of reddish. The red type of Ouckoo's 

 egg has not ceased to exist, but the dominant type here is 

 the white egg, whilst in the extreme North-West the white 

 type alone is found. As might be expected, however, where 

 the two types are both found, we often find most startling 

 contrasts. The darkest egg in my collection (Box No. 22) 

 is one taken from the nest of .Phylloscopus by Mr. Whymper 



