PARASITIC WEAVERBIRDS 25 



however, at least five species of grassfinclies have lines but no spots, 

 and so even this character is not a rigidly unvarying one. 



The reflection globules or tubercles at the corners of the mouth, 

 which are found only in viduines and estrildines, seem to be more 

 directly useful and hence seem more apt to have selective value. 

 Even here it is simpler and far more probable to look upon this char- 

 acter as a common ancestral one and not as an adaptive one. 

 The success of an adaptation is a subsequent matter, and not a forma- 

 tive factor. Thus, Hoesch rightly concluded that the presence of 

 these structures may be looked upon as one of the presupposed con- 

 ditions under which brood parasitism in these bu'ds can succeed. 



In a semidark nest interior, young birds without any beak papillae, 

 or as they have been termed "light bulbs," might conceivably be over- 

 looked by the adults returning to the nest with food, especially if the 

 latter are accustomed to placing the food only between such directive 

 markings. Hoesch pointed out that the nestlings exhibit reflex actions, 

 which ma^'" be correlated with these organs or at least may supplement 

 them in getting the attention of the food-laden adults. Thus, he 

 observed that nestling grenadiers (Oranatina) and pj^tilias when about 

 to be fed engage in very definite twisting motions of the neck and head, 

 but do not move their wings. Other estrildines bend their necks with 

 a downward and lateral slanting motion. The young of Estrilda 

 melanotis tend to crouch down and sway the head and bod}'' in rapid 

 fashion from side to side (Moreau in Sclater and Aloreau, 1933, p. 415), 

 while nestlings of E. subjlava move one or sometimes both wings for- 

 ward in a jerking, spasmodic gesture. 



As Hoesch suggested, these peculiar food-begging movements may 

 well have more of a selective value for the parents than do the mouth 

 or bill markings. Viduine hosts that are not estrildines and that do 

 not possess buccal marks or light reflectors act as if the presence or 

 absence of these structures plays little part in their tending to para- 

 sites and their own nestlings, 



Hoesch went so far as to say that he considered impossible the evo- 

 lution "de novo" of such complex structures as the reflection globules 

 through chance occurrence and the subsequent fixation through natu- 

 ral selection. Very likely the presence of these structures is the result 

 of common phylogeny with the host species. Since viduines are largely 

 parasitic on estrildines and since the two groups are closely related, 

 most probably these characteristics are ancestral to both. 



There is no need to discuss fiu'ther whether parasite-host buccal 

 similarities are due to adaptive convergence or to community of 

 descent; I shall now briefly describe these buccal patterns. In the 

 Estrildinae Delacour (1943, pp. 72-73) found that the waxbills have 



