PARASITIC WEAVE RBIRDS 23 



outside the mouth. The colored marks inside the mouth are assumed 

 to function, as in other bu'ds with comparable structures, as directive 

 indices to guide the food-laden adult to place the food directly in 

 the mouth of the nestling and not to one side or the other. The 

 tubercles have a similar usefulness, as the birds involved are all 

 species that make or use more or less spherical, domed-over nests with 

 only submarginal interior Ulumination. The tubercles glow with 

 reflected light in this semidarkness and indicate by their presence 

 the limits of the mouth of each nestling. They may help to illuminate 

 the pattern of the mouth itself, but this possibihty has not yet been 

 established. Thus we have a pair of directive structures — one func- 

 tioning best iu good light and the other in poor light. 



When one considers that many birds manage adequately without 

 such aids, one wonders at the apparently prodigal equipment here. 

 The estrildines supposedly have become so attuned to these recognition 

 devices that they cannot carry on as well without them. It would be 

 worthwhile to insert very young nestlings of species not so equipped 

 into nests of some of these bu^ds to see if they would receive enough 

 food and care for normal growth, even though the experiment would 

 not explain the evolution of these structures. 



Swynnerton (1916, pp. 279, 284) did attempt such experiments. He 

 placed a nestling Ploceus ocularius in a nest of a flycatcher, Chloropeta 

 natalensis,^^ a species whose young have a very differently colored 

 mouth lining. He recorded that the "parent flycatchers seemed to 

 experience no inconvenience whatever from the different mouth- 

 colour and the absence of twin spots, or even from the rapid vibration 

 of the head." He also placed a young Ploceus ocularius in a nest of 

 a rock thrush, Monticola angolensis,^"^ and the nestling was adopted 

 "in spite of its different external appearance, its very different mouth, 

 its extraordinary manner, and its different call-note." 



When Neunzig first described the very close approximation of the 

 buccal pattern of the viduines and of selected species of estrildines, 

 he assumed that each parasitic species restricted its attentions to a 

 single kind of host, or at least tended to do so. We now know that 

 this assumption is false. The best known of the viduines, the pin- 

 tailed widow bird, Vidua macroura, is known to parasitize at least 1 8 

 species, not all of them estrildines, and other species of Vidua are 

 similarly free of rigid, obligate host specificity. True, the hosts most 

 frequently selected are few in number, but they are stiU too numerous 

 to account for the degree of parasite-host resemblance postulated by 

 Neunzig, as the various species of estrildines differ from each other 



3« Chloropeta natalentis A. Smith, Illustrations of the zoology of South Africa . . ., vol. 2, Aves, 1847, pi. 

 112, flg. 2 (Durban), 



" Monticola angolensis Sousa. Jorn. Sci. Math. Fyslcas e Naturacs, Lisbon, vol. 12, 1S8S, pp. 225, 233 

 (Caoonda). 



