80 U.S. NATIONAL MUSEUM BULLETIN 2 65 



observable, released incubatory response of the birds. In his experi- 

 ments the largest models used were too big for the bird to cover 

 with its body, but even this did not act as a deterrant. Within much 

 more reasonable dimensional discrepancies, the same effect seems to 

 apply to many hosts of brood parasites. In general, the eggs of brood 

 parasites are usually larger than those of their hosts, but this is not 

 always the case, as has already been mentioned in Chrysococcyx 

 basalis and its Malurus hosts. 



In proportion to the body size of their producers, the eggs of parasitic 

 cuckoos are relatively small compared with the same ratio of egg to 

 body size in the host species. Heinroth (1922) found this ratio in 

 self-breeding passerine birds to vary from 8 to 14 percent with an 

 average of 11 percent. In seven species of parasitic cuckoos Schon- 

 wetter (1964, p. 535) found the ratio to range from a minimum of 

 3.2 percent in Cuculus canorus to a maximum of 7.9 percent in Chry- 

 sococcyx lucidus plagosus, with an average of 5.4 percent. 



The genus Chrysococcyx represents one extreme development of 

 the family Cuculidae. Its included species are the smallest of all the 

 cuckoos and are parasitic wholly on fairly small species of hosts. 

 The glossy cuckoos do not have this "area" of hosts to themselves, 

 however, as some of the larger cuckoos (Cuculus, Urodynamis, Caco- 

 mantis, and others) also make use of these small fosterers. The need for 

 reduction in egg size in the species of Chrysococcyx has been less than 

 in the larger forms that share with them the nests of small passerine 

 victims. 



The evolutionary significance of dissimilarities in different stages 

 of the life cycle of organisms is well known and is a matter of particu- 

 lar importance in brood parasites. As was pointed out in my study of 

 the crested cuckoos of the genus Clamator (1964, p. 29), brood para- 

 sites are subject to the impact of natural selection on two levels. 

 "Whereas in the case of self -breeding birds the entire biology of the 

 species is a closely coordinated unit ... on which selection may 

 operate, in the case of brood parasites there is cleavage resulting in 

 two fairly separate parts. The evolutionary climate ambient to the 

 egg and nestling stages is that of the host species and has relatively 

 minor connections with, and repercussions upon, the selective factors 

 surrounding the life of the adult parasite." From this point of view 

 it is of interest to note that the coloration of the eggs of the sympatric 

 and partly homoxenic plagosus and basalis is markedly dift"erent, 

 plain olive-bronze in the former and white spotted with pink in the 

 latter. The two species reveal far greater differences in their egg shells 

 than in their plumages. 



Inasmuch as the available observational data fails to reveal any 

 reliable, measurable difference in the adaptive success or failure of 



