PARASITIC WEAVERBIRDS 37 



disturbed at not being able to find them. The absence of "striving" 

 can hardly be expected to leave etliological "footprints" for the pres- 

 ent day investigator to uncover. All that we may reasonably hope for 

 and that we have now found are the antecedent variations that could 

 conceivably produce a situation conducive to the development of 

 brood parasitism, and some glimpses of atavistic behavior in the species 

 that have become parasitic. Wliile there is great need for fuller in- 

 formation on every one of the parasitic species of weaverbirds, it 

 seems unlikely that a more complete, more detailed picture will jaeld 

 presently unsuspected clues or indices to the origin of their mode of 

 breeding. 



The lack of intermediate stages of parasitism may be explained by 

 the simple fact that such intermediates would not be able to survive 

 as they would be unable to reproduce effectively. In a behavioral way 

 this observation corresponds to the frequently noted, but less readily 

 explained, discontinuities in structural evolution. As Nissen (1951, p. 

 347) remarked, the evolutionary evidence often is exasperatingiy frag- 

 mentary at the critical points of transition from one entity to another. 

 Yet these discontinuities do not mvalidate the concepts involved. 

 As has already been intimated in the foregoing discussion, the evidence 

 enables us to conclude onlj^ that in the weavers the parasitic habit 

 arose among stocks which already had the habit of breeding in old 

 nests of other birds, and among which occasional loss in incubation 

 broodiness occurred. 



A further indication that loss of nest building and occasional loss of 

 incubation broodiness occiu"red in the phylogenetic source stock of the 

 viduines may be sensed from the fact that the courtship patterns of 

 these birds have, as yet, revealed no suggestion of ritualized derivatives 

 of nest-building actions. Morris (1954, a,b; 1958) found such 

 derivatives in a large number of estrildine species, especially grass- 

 finches and mannikins. He found that a considerable number of 

 their actions involved in sexual displays appeared to derive from nest- 

 building motions, such as straw holding, bomng with a straw in the 

 bill, looping straw, and movements suggestive of ca%nty scooping. 

 The lack of any such behavior in the viduines is suggestive of an 

 absence of nest-building activities in their direct ancestry. 



The preceding remarks outline a superficial solution of the problem 

 of brood parasitism in the weaver birds. The deeper, biological 

 explanation is one that cannot be made with present information. 

 Endocrine imbalance or change, may have brought about the loss of 

 broodiness and ushered in the parasitic mode of reproduction. Pos- 

 sibly the endocrine change was brought about genetically rather than 

 physiologically, but this possibility can only be determined by investi- 

 gating the latter possibility. The postulated endocrine change need 



52652»— 60 i 



