HOST RELATIONS OF PARASITIC COWBIRDS 13 



(1941, p. 220) ill his study of the ovenbird, wherein he found that 

 the cowbu'd usually laid but one egg in a nest "unless nests are scarce; 

 in that case she lays more. ..." 



As a matter of record, it may be noted that as manj^ as 9 cowbird 

 eggs have been reported from another nest of a wood thrush by 

 Hofslund (1950) in Minnesota, where apparently no DDT sprajnng 

 had been done. As many as 8 have been recorded from single nests 

 of the black-and-white warbler, the veery, the rufous-sided towhee 

 (several such instances, ranging from 1 to 5 of the hosts' eggs), and 

 the ovenbird. Seven cowbird eggs have been found in one nest of a 

 red-eyed vireo, in one of a scarlet tanager, and in one of a song sparrow. 

 Six cowbird eggs were recorded from individual nests of a yellow- 

 breasted chat and of a yellow-headed blackbird, and 5 from single nests 

 of the scarlet tanager, the indigo bunting, and the veery. Berger 

 (1955, p. 84) described a six-storied nest of the yeUow warbler with a 

 total of 1 1 cowbird eggs in its various levels. 



The number of parasitic eggs that may be accepted by a host 

 ordinarily appears to be limited to not more than its own maximum 

 clutch number. In the case of many of the smaller hosts, however, 

 such a number of cowbird eggs would overcrowd the nest to the point 

 of physical discomfort to the covering bird and might lead to desertion. 

 In these cases a smaller number seems to be the effective limit. What 

 we are concerned with at this point is merely the acceptance of the 

 egg situation, quite apart from the very different question of the 

 host's ability to hatch and rear this number of young cowbirds. 

 Current understanding and information suggest that what we have 

 here is not a matter of latent counting abihty in the hosts, enabling 

 them to sense a "correct" number of eggs in the nest, but more 

 probably a reaction to the visible proportion of the combined mass 

 of eggs to the available space in the nest, or even the amount of 

 surface stimulation the eggs of a given clutch size produce on the 

 brooding surfaces of the body of the host birds. 



Furthermore, as indicated by Tinbergen (1951, p. 45; 1954, pp. 

 246-247) it is conceivable that additional eggs or larger eggs, such 

 as often result in a parasitized nest, may actually enhance the 

 released incubatory response of some birds. For example, he mentions 

 that, when given the choice in experimental cases, oyster-catchers 

 seem to prefer clutches of one or two eggs above their usual clutch 

 number and to prefer eggs of double, or even more than double, 

 the size of their own. I am not aware that similar reactions have 

 been tested critically in small passerines, but if they were found 

 to behave in similar fashion, it might help to explain some of their 

 otherwise surprising acceptances of parasitism. 



