40 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. I46 



in the British Museum. In these the combinations of eggs of the 

 parasite and of the host were as follows: 1 cuckoo egg in each of 10 

 nests with from 1 to 6 eggs of the magpie; 2 cuckoo eggs in each of 

 12 nests with from 2 to 8 of the magpie ; 3 cuckoo eggs in each of 

 3 nests with from 1 to 3 of the magpie ; and 4 cuckoo eggs in each of 

 3 nests with from to 6 of the magpie. 



Data are at hand on nine parasitized nests of the Spanish blue- 

 winged magpie {Cyanopica cyanus cooki) from the Iberian Penin- 

 sula, ex Rey (1872, p. 143) and others, plus three sets in the British 

 Museum ; all of these had only single eggs of the cuckoo, with from 

 one to five of the host. It is not possible, however, to say whether 

 there is a significant difference in the intensity with which the 

 two species of magpies are parasitized, although the available data 

 appear to suggest that there may be. In both sets of data (Pica pica 

 and Cyanopica cyanus), few of the sets approach the maximum size 

 recorded for complete, unparasitized clutches — up to eight or nine 

 eggs of either of the magpies. It can only be conjectured if this may 

 have been due to elimination of host eggs by the parasite. 



However, in the total count of instances of all host species, the 

 recorded numbers of eggs of the fosterers in the individual nests do 

 not consistently follow any variation directly proportional to the 

 number of parasitic eggs found with them. We may recall that in 

 his discussion of the great-spotted cuckoo as a parasite on species of 

 magpies and of crows, birds larger than itself, Lack (1947, p. 323) 

 reasoned that it might ". . . be anticipated that the host could raise 

 more young than a single Cuckoo, and in fact, the young Clamator 

 does not eject the members of the host brood, which are raised with it. 

 However, the argument of this paper is that the full clutch of the 

 Corvid host is determined by the average maximum number of young 

 which the parents can successfully raise, hence even one additional 

 nestling should upset the balance. It is therefore interesting that, 

 according to Baker (1942) and Jourdain {in Witherby, et al, 1938- 

 41 ) the parent Clamator removes one egg of the host species. Jourdain 

 states further that a Clamator sometimes lays more than one egg in 

 the same nest, in which case it is thought to remove one host egg 

 for each egg of its own." 



Jourdain (1925, p. 657) did expressly state that the female great- 

 spotted cuckoo usually removes an egg of the fosterer when laying 

 one of her own, but in a later paper ( 1936, p. 739) he further wrote 

 that "... in some cases the eggs of the Magpies are removed by the 

 Cuckoos, for on one occasion I met with a Magpie's nest containing 



