NO. 4 AVIAN GENUS CLAMATOR — FRIEDMANN 33 



from its Mediterranean homelands, it seems likely that it was growing 

 population pressure, such as we have seen recently in the case of the 

 cattle egret, mentioned above. The latter bird had increased greatly 

 in numbers in Africa prior to its sudden geographic advance. Coin- 

 cident with a situation of overpopulation, it may be remembered that 

 the nature and the intensity of natural selection varies with different 

 degrees of abundance of a species. When a species is numerically 

 uncommon the selection pressure it experiences is exerted chiefly by 

 the environment, whereas when it is more abundant the selection is 

 often between members of its own species. It was selection of the 

 latter kind that seems to have been involved in the emigration of the 

 less adapted members of the glandarius population. 



The lack of any fine control in the intensity of parasitism, as 

 evinced by multiple-egg deposition and the resulting uncorrelated egg 

 complements in parasitized nests, in sub-Saharan glandarius is more 

 than a matter of an as-yet-unachieved adaptation. It is also an 

 indication that the cuckoo is a recent arrival and is increasing in 

 numbers, because at a time when the size of the population of a 

 species is growing, selection is usually relatively weak, and such 

 excesses as extreme multiple parasitism would be tolerated, whereas 

 in a stable, "climax" situation this would be less apt to succeed. 



Conversely, selection is apt to be stronger when the population 

 of a species is decreasing. This must have been the case in the 

 Mediterranean glandarius when part of the species emigrated south- 

 ward, thereby reducing the intraspecific competition and permitting 

 a more active environmental selection. This may actually have con- 

 tributed to the development of an even better controlled host-parasite 

 relationship there. As Carter (1954, p. 255) has stated, ". . . the 

 population that survives the decrease of numbers will be a selected, 

 and not a random, sample of that at the preceding maximum. Only 

 the better adapted are likely to survive ... It follows from this 

 that adaptive evolution will be accelerated at the time of decrease . . ." 



As the great-spotted cuckoo extended its range into sub-Saharan 

 Africa, where there were no magpies, it undoubtedly used at first 

 the nests of various species of crows for its egg laying, just as it had 

 already done in eastern Egypt and the Near East. However, while 

 it continued to use the arboreal nests of corvine hosts throughout 

 its new domain it also extended its host choice to include such very 

 different types of nest structures as those of an earth-tunneling 

 starling, Spreo hicolor. It is known that in some animals specific 

 types of nest structure may act as isolating mechanisms, preventing 



