HOST RELATIONS OF PARASITIC COWBIRDS 3 



that, while the cowbhds do have host species which are most frequently 

 used by them, the parasites show little tendency to ignore other 

 potential victims. On the other hand, a small but appreciable 

 number of hosts responds adversely to the intrusions of the parasite, 

 although not to a degree that has become critical m an evolutionary 

 sense. Even here, the adverse responses (which constitute desertion 

 of the nest, covering over the parasitic egg with a new nest floor, 

 or actually throwing out the intruder) are not behavior patterns that 

 appear to have been developed as defenses against parasitism. These 

 responses are not specifically "anti-cowbird" in their organization 

 but rather are generalized types of reaction to something foreign 

 entering the nest. As far as I know, no bird has actually developed 

 a special defense against parasitism. In fact, it is difficult to imagine 

 a clearly defined defense against an unspecialized parasite. In most 

 cases, the normal fecundity of the host species enables it to survive 

 the inroads of the parasite. 



There is no evidence which suggests the existence in any of the 

 cowbirds of what have been called gentes in some species of parasitic 

 cuckoos — infraspecific units mtermediate in nature between true 

 polymorphic types and the more usual, geogi'aphically delimited, 

 subspecies. The chief, indeed the only visible, characters of these 

 gentes are the color and pattern of their egg shells and the corre- 

 sponding degi'ee to which they resemble the eggs of their usual hosts. 

 As Southern (1954, p. 220) has rightly concluded, if this egg mimicry 

 has evolved from an originally wide range of variations under the 

 operation of natural selection, the most probable selective agent 

 must have been, and still is, the discrimination shown by the fosterers. 

 It follows that such adaptive evolution could only have taken place 

 with parasites with a marked tendency to individual host specificity, 

 and, for the existence of such traits in some of the species of Cuculus, 

 there is good supporting evidence. In the brown-headed and in 

 the shiny cowbu'ds, however, the evidence (see pp. 14-15) is sporadic 

 in nature and gives the general unpression that individual host 

 specificity is the exception rather than the rule, although such cases 

 may be expected to increase in number as field studies become more 

 critical and more intensive. 



Furthermore, if we tabulate the frequency of nest desertion after 

 parasitism by the cowbirds, and then compare the dissimilarity 

 (to human eyes) of the eggs of the deserting hosts and those of the 

 parasites, we find no correlation. The most frequent deserters among 

 North American victims of the brown -headed cowbird, such as the 

 yellow-breasted chat and the cardmal, lay eggs quite similar to 

 those of the parasite. Desertion seems due more to "nervousness" 

 about alien interference with the nest than to any obvious incon- 



