8 PROCEEDINGS OF THE NATIONAL MUSEUM VOL. 125 
comforting to find reliable and rigid key characters by which to 
separate narokensis from small meliphilus, but this is not always 
feasible. 
A similar state of affairs occurs, however, with such generally 
accepted species as extlis (pachyrhynchus) and minor. Small individuals 
of the latter often are difficult to distinguish from examples of the 
former, but this does not cause serious doubts as to their specific 
distinctness. In these birds, not only is there a serial gradation in 
size among sympatric species, but also the very limits of the species 
are at times difficult to distinguish. The resulting situation makes 
one wish for more observational data regarding the critical isolating 
mechanisms involved. In the case of pumilio and exilis (pachyrhyn- 
chus), I was able to find skeletal characters that corroborated their 
external specific distinctness, but these differences are not necessarily 
isolating mechanisms (1963). Benson and Irwin (1964, p. 110) write 
that meliphilus has a thinner, softer skin than ezilis and other local 
species of Indicator. They even suggest that, in this respect, meli- 
philus is more like Prodotiscus than like other species of Indicator. 
So far, however, no pertinent observations on the feeding habits of 
meliphilus have been brought forth that indicate whether or not it 
is more or less given to feeding on small insects generally than are 
other species of Jndicator. It is known that it does feed on bee comb 
like its congeners and unlike Prodotiscus. 
Sympatry and Parallelism in Sibling Species 
Considering the fact that sibling species, or even species of only 
approximately similar external characters, usually are related fairly 
closely, and considering the further fact that the origin and establish- 
ment of the discontinuities between such species ordinarily involve 
allopatry of some duration, it is noteworthy how quickly and how 
extensively some of these small Jndicator species once more have 
become sympatric. This fact argues for effective, but as yet unrecog- 
nized, isolating mechanisms. The number of instances of two or more 
of these morphologically similar honey-guides occurring together is 
very impressive—Kabompo District, Zambia: eatlis (pachrhynchus) 
and meliphilus (Benson and Irwin, 1965, p. 4); Tshibati, Kivu District, 
Congo: pumilio and exilis (pachyrhynchus) (Chapin, 1958, p. 48); 
Doinyo Narok, Kenya: meliphilus (specimen in American Museum of 
Natural History, collected by Jackson) and narokensis (Jackson, 
1906, p. 20); Kibale Forest, west Uganda: exilis, willcocksi, conirostris; 
Impenetrable Forest, west Uganda: willeocksi, exilis pumilio; Sigor, 
West Pokot, western Kenya: narokensis, meliphilus; Mt. Moroto: 
minor, narokensis; Victoria, British Cameroons: minor (controstris), 
exilis, willcocksi (Serle, 1965, p. 77). 
