Host Relations 

 of the 

 Parasitic Cowbirds 



A Comment on Cowbird Parasitism 



IT IS OBVIOUS that the host-parasite relation is an essential aspect 

 in the appraisal and understanding of any given example of a 

 parasitical mode of life. What is less obvious is the equal importance 

 of the delimitation of the requirements involved in these relations. 

 It is necessary to determine the degree and the frequency with which 

 individual host species are parasitized and to ascertain not merely 

 how amenable they may seem to be as victims but how successful 

 they are as fosterers. The results of such a survey should make it 

 possible to deduce which factors tend to make certain species suc- 

 cessful hosts from the standpoint of the parasite and others unsuc- 

 cessful — or, to view it from the opposite side, which factors tend to 

 protect certain potential hosts, and not others, from the attentions of 

 the parasite. 



Inasmuch as the cowbirds are altricial birds — hatched naked, blind, 

 and helpless — they could not succeed with precocial birds as hosts — 

 which are hatched down-covered, seeing, and active — as I stressed 

 in my first (1929, pp. 189-190) description of the situation. At that 

 time I listed three other requu'ements in addition to the need for an 

 altricial host: the host species should lay eggs that are not much 

 larger, if at all, than those of the cowbird; its manner of feeding its 

 young should not depart greatly from the normal passerine method; 

 and it should feed its young on more or less typical passerine food — 

 insects, worms, soft seeds. 



The cowbirds are not specialized for a parasitic existence in the 

 sense that some cuckoos and honey-guides are. They have no adap- 

 tive structures, functional gradients, or innate reactions that may be 

 looked upon as oriented especially toward survival at the expense of 

 their nest-mates. While more often than not the nestling cowbird is 

 larger than its nest-mates, which gives it an advantage in competing 



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