94 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. I46 



breeding range. In the course of its rapid spread over this vast 

 P/ca-allopatric area, it has not altered its eggshell pattern although it 

 has broadened greatly its range of acceptance of host nest types, 

 enabling it to parasitize such divergent fosterers as arboreal, open- 

 nesting corvids and earth-tunnel nesting sturnids. As is pointed out 

 in the present paper, the fact that glandarius was parasitic largely on 

 birds of greater size, capable of rearing multiple parasites as well as 

 some of their own young, gave the cuckoo an immunity from selective 

 pressure, but as it increases its use of smaller, sturnid hosts the 

 parasite may find itself affected by this pressure, from which it is 

 relatively protected as yet. 



A factor that appears to have been of considerable importance in 

 the advent of the geographic spread that consummated in the emigra- 

 tion to sub-Saharan Africa of the less perfectly adapted portion of 

 the original circum-Mediterranean population of glandarius was the 

 shift in the main stress of selective pressures when that original 

 population became numerically high. Until that demographic satura- 

 tion had been reached, and especially while the species was developing 

 through its adaptive evolution with regard to its primary host, the 

 chief focus of natural selection was between glandarius and its en- 

 vironment (including in the latter, its magpie fixation). Once glan- 

 darius had become successful and numerous, the primary selective 

 pressure was between members of its own kind, and it is this change 

 that seems to have been involved in its geographic "shedding" of those 

 segments that were less able to stand the new orientation of natural 

 selection. This left only the better adapted individuals in the original 

 homeland, which is the reason for the difference still apparent be- 

 tween them and their sub-Saharan emigres. 



Another evolutionary trend found in some other groups of brood 

 parasites, a gradual shortening of the incubation period, is absent in 

 Clamator. In fact, the meager data available suggest just the opposite, 

 although the more advanced species of the genus, with longer in- 

 cubation periods, tend to make use of hosts with still longer ones. 



To summarize, brood parasitism in Clamator has achieved a high 

 degree of adaptive excellence by virtue of a restriction of host choice 

 to birds of generally similar eggs (northern races of jacobinus, and 

 levaillantii, coroinandus, and glandarius), and only relatively recently 

 has this smoothly functioning correlation been upset by a portion of 

 the membership of the most advanced, most "perfectly" adapted 

 species, glandarius. Traces of incipient tendencies toward tgg mor- 

 phism may be detected in levaillantii, but they have not developed 

 very far. 



