THE HONEY-GUIDES 17 



evolved, through a Melignomon-\\ke stage, from an Indicator-like 

 ancestral stock. The duration of the nestling stage in Prodotiscus 

 is one of the many facts sorely needed to round out a study of the 

 honey-guides. It would be highl}^ interesting to know if this tendency 

 to parasitize small passerines such as white-eyes and bush warblers 

 is accompanied by an accelerated nestling growth rate. 



The fact that in their ontogeny the species of Indicator are birds of 

 slow development forms a point of striking difference from all other 

 groups of parasitic birds — cuckoos, cowbirds, etc. In the literature 

 it is often assumed that rapid nestling growth has a definite selective 

 value for a parasite having to compete with a variety of nest mates. 

 The honey-guides show that this is not always true. 



The eggs of all the honey-guides yet known (and probably those 

 still to be described) are plain, unmarked white, like those of barbets, 

 woodpeckers, and other picarian birds. Inasmuch as the majority 

 of the host species (except for those of Prodotiscus insignis) are also 

 of picarian or coraciiform affinities and lay unmarked, whitish eggs, 

 it follows that in all these cases there is a close agreement in coloration 

 between the eggs of the parasite and of the victim. That such agree- 

 ment is not essential, however, is indicated by the fact that the honey- 

 guides have equal success with starlings, swallows, chats, wood- 

 hoopoes, and other birds whose eggs are quite dissimilar to those of 

 the parasite. It is possible that the darkness prevailing in the nesting 

 holes of these hosts may reduce the possible selective value of egg 

 resemblance. 



Another point of departure from what we find in many other para- 

 sitic birds is that the known eggs of the honey-guides are not generally 

 larger than those of their common hosts; in fact they are frequently 

 considerably smaller. Thus, the black-coUared barbet, Lybius tor- 

 giiatus, the most frequent victim of the lesser honey-guide, /. minor, 

 lays eggs that measure 23-25.5 by 17-18.5 mm. as against 20.3-22.6 

 by 14-17.5 mm. for those of the parasite. The eggs of the honey- 

 guides are not small for the size of the bird, as in some cuckoos of the 

 genus Cuculus. Thej'- do not appear to have shorter incubation 

 periods than those of the common host species, but on this point 

 reliable data are ver}^ scanty. 



Inasmuch as aU the honey-guides of whose breeding habits we have 

 any knowledge are parasitic, they afford little in the way of clues as 

 to the mode of origin of this habit. Similarly the data, admittedly 

 poor, on the various species of barbets, their supposedly closest 

 relatives, give no hint as to any incipient trends towards parasitic 

 reproduction. The barbets (or, more correctly, a few species of 

 barbets that have been studied) are birds in which both sexes take 

 part in excavating the nesting hole, and share alike the tasks of 



