PARASITIC WEAVERBIRDS 31 



The viduines and the estrildines have, on the whole, largely mono- 

 chromatic, unadorned juvenal plumages, predominantly of shades of 

 brown and gray — other colors being chiefly confined to the rump 

 and lower back, where they are less noticeable in a nest crowded 

 with young birds reaching up toward the feeding adult. There is 

 little evidence that such plumage similarity could elicit effective 

 selective responses from the hosts, especially in the semidarkness of 

 these domed nests. Furthermore, as Hoesch pointed out about the 

 paradise widow bird, Steganura paradisaea, and its common hosts of 

 the genus Pytilia, the similarity in the nestling plumage is greatest on 

 the underside of the body, which is largely hidden from view in the 

 nest. In other words, the similarity is most pronounced on those parts 

 of the body that are of least significance for adaptation. 



I note that the juvenal plumage of Pytilia is more like that of 

 Steganura than it is like the adult feathering of its own species. 

 This similarity suggests that the juvenal plumage may be a phylo- 

 genetic character lost in the more highly developed adult plumage of 

 the host — a further indication of relationship between the estrildine 

 and the viduine stocks. 



NoNAGGRESsivE BEHAVIOR OF NESTLINGS: Unlike most Other avian 

 brood parasites, such as cowbu'ds, cuckoos, and honey-guides, young 

 viduine parasites exhibit no hostile or aggressive behavior toward 

 their nest mates. They grow up together with no more jostling or 

 individual struggling than would normally take place within a brood 

 of the host's young. In his general discussion of aggressive behavior 

 in vertebrate animals, Collias (1944, pp. 114-115) pointed out that 

 in birds, the young usually remain dependent on their parents for a 

 longer time than in reptiles, amphibians, or fishes, and that this 

 dependence is correlated with the decrease in precocity of the young 

 with respect to the first appearance of aggressive behavior. While 

 this picture is not true of most brood parasites, it is true of viduines. 



It is also true that egg removal by the laying parasite sometimes 

 eases the ensuing crowding and competition, but only to the extent 

 of obviating what would otherwise be excessive crowding. In this 

 connection, we cannot assume such a "goal" as the original cause 

 of the development of the egg removing habit, which merely seems 

 to have been favored by natural selection by virtue of the result. 

 The fact that egg removal operates in this manner bears on the 

 general problems of clutch size and brood size. 



According to Lack, in his recent (1954) book on the natural regula- 

 tion of animal numbers, parasitic birds support his contention that in 

 birds generally clutch size is limited by the amount of food available 

 for the young. He wrote that those parasites that are much larger 

 than their usual victims ordinarily either remove an egg of the latter 



