32 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. I46 



where these birds are not found, or are rare, it launches out into 

 the use of all kinds of nests which bear some resemblance to those 

 they usually cuckold . . ." 



The timespan involved in the southward spread of Clamator 

 glandarius to sub-Saharan Africa need not have been great. The case 

 may well have been similar to the recent rapid, almost "explosive," 

 spread of the cattle egret, Bubulcus ibis. In both instances the advanc- 

 ing birds filled vacant ecological niches. The cattle egret had no com- 

 petition from other herons because it was a dry land bird and lived 

 largely on insects, not an aquatic feeder on fishes, tadpoles, etc. The 

 great-spotted cuckoo was parasitic on corvids, a group until then un- 

 molested by any parasitic birds in Africa. The spectacular spread of 

 the collared turtle dove, Streptopelia decaocto, in Europe during the 

 past 50 years is a parallel example. 



It seems that the relatively recent, but very extensive, geographical 

 expansion of C glandarius originally was motivated by the bird rather 

 than by its environment. This statement may require a little elabora- 

 tion to make its meaning clear. Evolutionary changes are often the 

 result of a double process of selection ; selection by the environment 

 of the most advantageous, best adapted structural, functional, or 

 behavioral organization in the organism, and also selection by the 

 animal of the most comfortable, the most nearly optimal environ- 

 ment. The capacity for making a choice among available environments 

 is inherent in all animals that are able to move about freely. In effect, 

 this results in a process of sorting out the members of a species 

 environmentally instead of selectively eliminating the less fit in the 

 original ecological situation. 



Implied in the phrase "sorting out" is what appears to have been 

 behind the great move to sub-Saharan Africa. The part of the original 

 circum-Mediterranean population of C. glandarius that was relatively 

 less completely "fit" was the part that moved on to new territory — 

 in this case, to equatorial and southern Africa. That it was less 

 delicately, or less nicely, adapted to its original hosts than was the 

 part that stayed in the Mediterranean area is still evidenced by its 

 lack of adjustment in its tgg deposition to the size of the total re- 

 sulting clutches in the nests of its victims. This significant difference 

 in the two geographic segments of C. glandarius is discussed in detail 

 in our account of the intensity of parasitism (see pp. 38-47) , but a little 

 additional comment seems called for here. 



While it is obviously impossible to state precisely what factor, or 

 factors, motivated the dispersal of part of the glandarius population 



