78 U.S. NATIONAL MUSEUM BULLETIN 2 65. 



a "fair" degree to others; cupreus, with a systematically very diverse 

 group of hosts, gives evidence of probable egg adaptation toward 

 two hosts, a bulbul, Pycnonotus barbatus, and an olive sunbird, 

 Chalcomitra olivacea, the very disparity of which makes one wonder if 

 the present "probability" is not merely a matter of coincidence in 

 fairly small samples and not really a statistically significant adapta- 

 tion. Finally in caprius there is some evidence for host-egg similarity 

 for two of its frequent fosterers, the red bishop Euplectes orix and 

 the Cape sparrow, Passer melanurus, some, but not constant, for its 

 other most frequent fosterer, the masked weaver, Ploceus velatus, 

 and no convincing adaptation toward its numerous other, less-used 

 hosts. Interestingly, the caprius eggs seemingly adapted to the Cape 

 sparrow, Passer melanurus, have been reported so far only from 

 southern Africa, in the range of that host species. This makes it 

 appear like a real adaptation; if it occurred as well in more equatorial 

 regions where this particular host is absent, it might seem that it 

 was a widespread phenotype that happened to be suitable in southern 

 Africa to the Cape sparrow. 



Egg adaptation occurs to a well-developed degree in the Australian 

 species, osculans, to a lesser degree in basalis, in the two Indian 

 species, maculatus and xanthorhynchus — but only with some of their 

 host species, and to a variable degree in three African species, caprius 

 (well-developed with two hosts, not with others), capreus (again only 

 for two of its hosts and not others), and to only a "fair" degree in 

 Haas with a few of its frequent hosts. The most perfect adaptations 

 are between O. osculans and its chief host Chthonicola sagittata, 

 between C. maculatus and Arachnothera longirostris , and between 

 C. caprius and Euplectes orix and Passer melanurus. 



There is, then, no weU-defined, progressive path of adaptation; 

 it is more of a sporadic type of development in different sections of 

 the genus Chrysococcyx. In some instances it is possible to "explain" 

 the lack of adaptive evolution, as in many instances of the parasitism 

 of C. caprius on Ploceus velatus, which host lays such a wide range of 

 egg phenotypes that it would be difficult for the parasite to come 

 under any steady, selective influence. Another instance is the frequency 

 with which C. lucidus plagosus parasitizes birds that buUd domed 

 or covered nests in which the poor illumination tends to minimize the 

 efl[ect of visual differences between the eggs of the parasite and those 

 of the host. 



In the case of the European cuckoo, Cuculus canorus, it is well 

 known that the egg is extremely small for the size of the bu'd, a little 

 over 3 percent of the body weight, while the egg of most self-breeding 

 birds is usually about 10 percent of the body weight. Whether the 

 small egg of C. canorus is the result of a progressive, adaptive reduc- 



