HOST RELATIONS OF PARASITIC COWBIRDS 33 



to determine for the simple reason that usually no birds other than 

 the parents come to the nest and have any contact with the young 

 birds. However, Hinde (1961, pp. 171-172) showed that the feeding 

 behavior of altricial nestlings is originally responsive to quite diverse 

 objects. "Once the eyes are open, young passerines will gape to a 

 wide variety of objects — forceps, spatula, fingers and so on — as well 

 as to the parent. . . . The stimuli evoking anxiety responses from 

 young bullfinches are equally generalized . . . and the precocious 

 adult behaviour of young birds ... is often directed towards func- 

 tionally inappropriate objects. . . ." Hinde uses the term imprinting 

 simply for the learning of parental (for parasitic birds: the foster- 

 parental) characteristics by young birds. 



In the case of some brood parasites such as the European cuckoo, 

 it is assumed generally that the host-specific egg-laying bird deposits 

 its eggs in the nests of the same species of fosterer by which it origi- 

 nally was reared. In the absence of any more testable explanation, 

 it has been assumed that each cuckoo becomes imprinted on its own 

 foster-parent species. ^Miether or not this is true, the assumption 

 hardly applies to the cowbu'ds; in these birds, the evidence for such 

 specific host fixation is miore in the nature of occasional rather than 

 general occurrence. Even in cases of apparent host specificity, 

 there is no corroborative evidence to make the possibility of imprint- 

 ing anything more than an inference. 



Reactions of Host to Parasitism 



On the whole, the majority of American species of passerine birds 

 do not act as if they recognize an enemy in the cowbird. At least, 

 they do not attack or mob cowbirds on sight as some Palearctic birds 

 attack the Em'opean cuckoo. Many wi'iters have described the 

 active hostility with which some of these potential victims react to 

 the mere sight of a European cuckoo; other writers have recorded 

 how certain African species resort to "mobbing" attacks on some of 

 the small African glossy cuckoos or how the former drive oflp honey- 

 guides. These accounts give the impression that the parasites are 

 recognized as unwelcome intruders on sight, but this does not apply 

 to most Am.erican birds when confronted with one or more cowbirds. 

 There are exceptions, such as the tendency of red winged blackbirds 

 to repel cowbirds from their nesting areas (see pp. 128-129), but this 

 antagonism holds only during the breeding season, whereas in some 

 of the Old World birds, the hostility appears to be a fixed year-round 

 pattern. Redwings and cowbirds certainly flock and roost together 

 amicably in the autumn and winter months. Hann (1937, p. 201) 

 studied the ovenbird, a very frequent victim of the brown-headed 

 cowbird, and found no evidence of any innate hostility between them. 

 The same lack of antagonism is the case in South America with the 



