PARASITIC WEAVERBIRDS 63 



from subsequent examination of specimens, as the species involved 

 differ only in the breeding plumage of the adult males. Even in this 

 respect they differ only in the degree and hue of sheen, the degree of 

 darlcness of the remiges, and the color of the bill and feet. Also the 

 bill and foot color is quicldy lost if not recorded on the label by the 

 collector, so that the diagnostic usefulness of this color is frequently 

 quite limited, if not absent. 



The obvious similarity m appearance and the equal similarity in 

 habits of the species of combassous makes me wonder how much com- 

 petition exists between them. WliUe there is as yet not enough in- 

 formation on this point, possibly the combassous are less in conflict 

 interspecifically than mere geographic occmTence records indicate. I 

 was informed by M. P. Stuart Irwin (in litt.) that in his area in South- 

 ern Rhodesia, the species Vidua amauropteryx and V. funerea are 

 ecologically fairly well separated, the former livmg on the outsldrts 

 of human habitations, gardens, and roads, while the latter is largely 

 restricted to the wilder areas. This separation is defuiite enough to 

 impress the observer in the field as a weU marked difference. Ir^vin 

 found that plottmg the positions of smging adult males in the breeding 

 season on a large scale local ecological chart bears out the field im- 

 pression completely. During the nonbreeding season, these com- 

 bassous occur in flocks of as many as 30 or 40 individuals, but diuing 

 these months it is impossible to say whether or not the two species 

 remain apart. When the birds begin to come into breeding condition, 

 the flocks do not seem to "split up" but rather to "fan out" over a 

 suitable area, where the males establish their singing posts. This 

 result is true of both species. Since the local population of both in 

 Irwin's area is rather smaU for the total amount of space available, 

 in some instances the two species may not come in contact and hence 

 have little or no cause or opportunity to compete. 



There apparently is, however, some variation in this matter as 

 Irwin informed me that once when he shot a male Vidua amauropteryx 

 from its singing tree, its place was taken quickly by a V. funerea, 

 which, in turn, after being collected was replaced rapidly by another 

 of the former species. In Mozambique, Lamm (in litt.) saw the two 

 species in mixed flocks together with pin-tailed widow birds, V. 

 macroura. 



Ross's recent (1957) study of natural coexistence of very similar 

 species of leaf-hoppers is of comparative interest here. His data sug- 

 gest that even closely similar species differ slightly physiologically, and 

 hence any two species will have slightly different ecological optunum 

 needs. He also pointed out that every ecological niche is not con- 

 tinuously uniform, but is subject to changing degrees of local and 

 annual variation around ecological means. As a result, each such 



