PARASITIC WEAVERBIRDS 33 



compare the various groups of parasitic birds. Brood parasitism is 

 newer, relatively more recently acquired in the weavers than in the 

 cuckoos and the honey-guides, and possibly even newer than in the 

 cowbirds. The one known parasitic duck is still too imperfectly 

 studied to enable us to estimate the antiquity of its brood parasitism 

 here, although what is known of it gives the impression of relative 

 newness. 



The absence of any structures or habits directly inimical to the 

 young of the host, such as we find in the cuckoos and hone3''-guides, 

 is an indication of the relative recency of the parasitic mode of breed- 

 ing in the weavers. Furthermore, in captivity one of the most 

 primitive of the parasitic viduines, the black-winged combassou, may 

 occasionally exhibit considerable "attentive behavior" (caring for 

 young) although at other times it shows the usual parasitical behavior, 

 which alone has been recorded for it in the wild state. To a lesser extent, 

 bits of similar attentive behavior have also been recorded for individ- 

 uals of two of the more advanced species, the pin-tailed and the para- 

 dise widow birds, living in similar conditions of captivity. These 

 data suggest a relative ease of regression into a forsaken ancestral 

 behavior pattern, which, in turn, indicates relative newness of the 

 parasitic mode. Furthermore, that the nestlings of weavers grow up 

 together with, but not at the expense of, the young of the host species 

 suggests less adaptation for a parasitic existence than in the cowbu-ds. 

 The cowbirds have a slightly shorter incubation period and a somewhat 

 faster growth rate than many of their foster nest mates, a large per- 

 centage of which do not survive the nestling stage. This relatively 

 less stringently "adapted" parasitism of the weavers presupposes a 

 shorter time for its evolution than the more host-aggressive features of 

 the cowbirds' reproductive pattern seems to presuppose. 



In arriving at this estimate of the recency of brood parasitism in 

 the weavers, I am discounting the argument advanced by R. Neunzig 

 (1929b), who assumed that the similarity in the nestling plumage and 

 especially the mouth markings of the nestling viduines and of their 

 chief estrUdine hosts was an adaptive resemblance developed in the 

 evolution of the parasitic habit. This concept was accepted by South- 

 ern (1954, p. 229), who considered the parasitic weavers as the most 

 specialized of all avian brood parasites, "for in these each parasitic 

 species has its own host species to which it is permanently attached 

 and a very complicated form of mimicry has arisen involving the 

 recognition pattern of the chick's mouth markings." If this view were 

 accepted it would presuppose a much greater antiquity for the habit, 

 but, as was indicated in the previous section, these similarities are far 

 more probably due to community of descent than to subsequent 

 adaptive convergence. Furthermore, we now know that the extreme 



