no. 3633 ALLOXENIA — FRIEDMANN 9 



with blotches of slate and dark greenish olive brown, more numerous 

 at the large pole, but present elsewhere as well ; and very pale pink with 

 spots of mauve and russet, the spots slightly more concentrated at 

 the blunt end of the egg than elsewhere (Pitman, 1957, p. 139). The 

 first two of these were taken in the same area in Southern Rhodesia 

 by Neuby-Varty (1948, 1950), a fact that shows the nongeographical 

 nature of the variability. An uncertainly identified egg found in a 

 scrub robin's nest (Cercotrichas) in Northern Rhodesia and attributed 

 to this cuckoo by Lees (1938, p. 18) was said to have a coffee- 

 colored ground, darkening at the wide end. The identification of the 

 two eggs taken by Neuby-Varty is completely acceptable; the one 

 reported by Lees was identified "by elimination," i.e., it was thought 

 to be C. gularis because it did not fit either C. solitarius or C. clam- 

 osus. This record cannot be looked upon as more than a guess. 



In light of the limited data on the yellow-billed cuckoo, it is pre- 

 mature to generalize, but what little we know does not suggest any 

 marked development of host egg mimicry. The fact that eggs with a 

 pinkish-ground color, described above, were all found in nests of 

 drongos, whose own eggs have a pinkish ground, may suggest some 

 incipient tendency in this direction, but the number of cases is still 

 too small to show a real treDd. 



The absence of marked adaptive host egg resemblance in the 

 eggs of the three species of largely sympatric African Cuculus, all 

 of which show considerable variation in coloration, makes one wonder 

 if alloxenia may have the effect of reducing the need for such a devel- 

 opment and if this differential host selection, therefore, may be 

 reflected in the fact that little or no adaptive egg evolution has taken 

 place. It seems hardly likely that this in itself could have had that 

 result although natural selection is by no means restricted to items 

 of large value: small changes, provided they are advantageous, are 

 equally apt to be acted upon by natural selection over a sufficient 

 period of time. If the three species of Cuculus were homoxenic, adap- 

 tive host egg resemblance might be more critically important because 

 of the competition for the same hosts by the several parasites. Even 

 in a state of relatively good, if not total, alloxenia, such adaptations 

 would seem to be of sufficient value to have occasioned more evolu- 

 tionary change than appears to have transpired. This is what has 

 happened with the completely allopatric European cuckoo and has 

 resulted in that case in the formation of host-specific gentes with 

 well-marked egg mimicry. The situation in the European cuckoo's 

 African congeners is, therefore, puzzling, but it has the features 

 outlined in this paper. The gentes in the European cuckoo differ 

 from each other only in their egg shell coloration and in their host 

 choice. It is only in their egg shells that the gentes show any change 



