24 U.S. NATIONAL MUSEUM BULLETIN 233 



number of eggs it laid to the opportunities presented in the favored 

 hosts' nests which were in the proper stage of construction and 

 receptivity. In commenting on Waikinshaw's data, Nice (1949, 

 p. 233) pointed out that, since the particular field sparrows involved 

 usually deserted their nests when parasitized, the continued attentions 

 of the cowbird unwittingly may have caused the sparrows to build 

 more nests — which, in turn, tended to increase the egg production 

 of the parasite in a way quite comparable with what Chance achieved 

 by manipulation of the available nest supply in the case of the Euro- 

 pean cuckoo. From a consideration of this cause-and-effect situation, 

 Walldnshaw's data are not merely 20 more instances of cowbird 

 parasitism on the field sparrov*^, each case no more significant than 

 other randojn records, but they are a reflection of a local intensifi- 

 cation of the host-parasite relations. Since there is no sure way 

 of knowing how often and in how many host species similar situations 

 may arise, i-t is impossible to allow for such situations with reasonable 

 accuracy when quantitatively interpreting the total data for each 

 species of common victim. It would follow that a host species, 

 known to desert easily when parasitized, might thus inadvertently 

 tend to increase the egg production of the parasite, provided that 

 the latter persevered in using the same host species. Conversely, the 

 effect of continued cowbird parasitism might serve thus to increase 

 the nesting activities of the host. A comparable case (p. 171) is 

 Berger's data on the song sparrow as a cowbird host. 



This is actually and essentially the case in the numerous instances 

 of multiple-storied nests of the yellow warbler, the lower stories of 

 which each held buried cowbu'd eggs. 



On the other hand, it must be kept in mind that, in the case of 

 many of the single-brooded species of hosts, these birds may succeed 

 in raising young of theu* own by renesting after the desertion or the 

 destruction of the fu'st parasitized nests. Parasitism may thus 

 cause an extension of the hosts' breeding season. It follows that a 

 mere calculation of the percentage of parasitized nests of these species 

 gives only a partial picture of the situation. The critical point, 

 namely the effect of parasitism on the total fledging success of the 

 host species, is not accurately described by such percentage figures. 



Cases of repetitive visits to the same host nest may be due to two 

 factors. They may involve multiple hens of the parasite or tlsey may 

 be due to successive visitations of the same individual. The data 

 essential for determinmg which may be the case are almost always 

 left unrecorded, but the general question of continued attention to 

 a host nest by an individual cowbird presents points worth consid- 

 ering, especially in connection with the problem of egg removal by 

 the parasite. 



