34 TJ.S. NATIONAL MUSEUM BULLETIN 22 3 



host specificity assumed by Neunzig and accepted by Southern is not 

 correct. 



While the lack of specialized adaptive features makes the parasitic 

 habit appear less highly developed here than in other avian groups, 

 it may be questioned whether any further adaptation would add to 

 the success of the parasitic species. In a conversational discussion 

 on parasitism in social insects, the late W. M. Wheeler once suggested 

 that in one respect the most perfect parasites are those that do not 

 kill off their fosterers, as in this way they practice parasitism without 

 decreasing the supply of their normal hosts. They live successfully 

 in the present without in the least mortgaging the future. To put 

 the matter in a different way, we may say that in contrast to the other 

 groups of avian brood parasites, the parasitic weavers possess no 

 known special structures, habits, or functional gradients that give 

 them particular advantages over their nest mates, but this condition 

 does not mean that they are not adequately equipped for competing 

 with them on equal terms. The parasitic habit here often involves 

 no discomfort to the hosts other than the need for gathering more 

 food to nom"ish their enlarged broods of nestlings. 



The viduines are parasitic chiefly on not too distantly related 

 estrildine species, of essentially similar body and egg size and food 

 requirements, and of supposedly comparable incubation and nestling 

 periods. In this respect they resemble the most primitive of the 

 parasitic cowbirds, Molothrus rufoaxillaris, which is parasitic chiefly 

 on its nonparasitic ancestral relative Molothrus badius (Friedmann, 

 1929). The cuckoo finch, Anomalospiza imberbis, which is parasitic 

 chiefly on grass warblers of the genus Cisticola, has, it is true, gone 

 farther afield taxonomicallj^ in its host fixation, but without encoun- 

 tering any essential differences in adaptive requirements, as far as may 

 be learned from present data and understanding. 



Contributive Factors 



Precise causes of brood parasitism are not definable in the sense of 

 pin-pointing critical spots of antecedent ethological variations or 

 weaknesses; however, the problem itself requires no great stretch of 

 the imagination for a superficial solution. By superficial solution, I 

 mean a logical, historical statement couched in terms of the observable 

 habits of the parasites and of their relatives. 



That the cuckoo finch is more nearly allied to the ploceine weavers 

 and that the viduines are related to the estrildines clearly indicate that we 

 have two separate, independent but parallel developments of brood 

 parasitism. Thus, brood parasitism has happened twice in the 

 history of the weavers. Apparently, antecedent variations in breed- 



