76 U.S. NATIONAL MUSEUM BULLETIN 2 65 



that in the parasitized nests (12, or 10 percent of the total) the didric 

 eggs more often resembled one of these color morphs, less often those 

 of two other types. 



Since the egg pattern of the masked weaver is so variable as to 

 permit the designation of 10 recognizable types, it is obvious that 

 it would be very difficult, if not quite improbable, for a parasite's 

 egg to have evolved any great degree of resemblance. Yet, it seems 

 that some adaptive evolution has transpired and is going on, but in so 

 unclear and complicated a situation that it is not possible to measure 

 or to define it. The colored illustrations given by Ottow and Duve 

 (1965, pi. 1, figs, a and b) of two very different sets of eggs of Ploceus 

 velatus, each with an egg of caprius, show a marked resemblance 

 between parasite and host. These are extreme instances of positive 

 egg similarity with this favorite, but ovopolymorphic host. In other 

 cases the eggs of the two show no such close agreement. Duve's own 

 records reveal that four of the egg morphs of caprius were found in 

 nests of Eupledes orix, a host that lays unmarked bluish eggs only, 

 with which only two of the parasitic egg types correspond. Yet, so 

 experienced an oologist as Pitman (MS., 1965) considered that the 

 didric cuckoo's eggs have become adapted to at least half a dozen 

 species of hosts. The evidence for this conclusion is unknown to me; 

 when Pitman's work is published this point will be of much interest. 

 Of the large number of species of hosts (67) recorded in this report 

 for the didric, with a total of 426 instances of parasitism, the three 

 most frequent fosterers are the red bishop, Euplectes orix, with 77 cases, 

 the masked weaver, Ploceus velatus, with 105 instances, and the Cape 

 sparrow. Passer melanurus, with 43 records. It is, perhaps, due to the 

 polymorphic nature of the masked weaver's eggs that only individual, 

 but not invariable, egg resemblance is to be found with them, while 

 with the other two fosterers the egg adaptation of the didric is quite 

 readily discernible, even if not highly perfected with regard to the 

 Cape sparrow. 



There is also some evidence for adaptive resemblance of egg size 

 in the didric. Pitman (MS., 1965) reported that seven didric eggs taken 

 from nests of Ploceus cucullatus and Ploceus nigerrimus measured 22.2 

 X 15 (20.4-23.3 X 14-16 mm.), whUe nine others from nests of smaller 

 species of Ploceus with correspondingly smaller eggs, Ploceus inter- 

 medins and Ploceus pelzelni, measured 19.9 X 13.7 (18-21.4 X 13- 

 14.7 mm.). For ready comparison the egg sizes of the hosts involved 

 may be stated: 



Ploceus cucullatus: 21.7-25.7 X 14.9-16.5 mm. 



Ploceus nigerrimus: 22-26.2 X 15.4-17 mm. 



Ploceus intermedius: 20-23.6 X 13.8-15.4 mm. 



Ploceus pelzelni: 18-19.5 X 13-19 mm. (all, ex Chapin 1954, 

 pp. 316, 338, 358-363, 367). 



