HOST RELATIONS OF PARASITIC COWBIRDS 25 



Duration of Parasite's Interest in Host Nests 



In his detailed study of the relations of Kirtland's warbler with the 

 brown-headed cowbird, Mayfield (1960, pp. 144-181; 1961, p. 166) 

 brought together evidence that the cowbirds involved had a con- 

 tinuing, almost proprietary, interest in the nests they parasitized. 

 Individual females tended to return to the same nests, to remove eggs 

 from them or to lay eggs in them, and did not molest other unpara- 

 sitized nests indiscrimmately. Mayfield found that the parasites did 

 not remove eggs on the pre-dawn visits when they laid their own but 

 did so several hours later, a day before, or, less often, a day later. The 

 cowbhd would not merely lay an egg and take one out; it would lay 

 several and remove one; it would lay one and take several; but if 

 there was only a single egg in the nest, the parasite would not remove 

 it. 



The interest in individual nests begins when the cowbird intently 

 watches the nest construction by the warblers. This interest con- 

 tinues as the parasite inspects and looks into the nests at intervals. 

 Later, as it repeatedly visits the nests to remove eggs from them, it 

 shows its definitely localized concentration by the fact that it removes 

 eggs only from nests it is using or is about to use, or that it removes 

 only eggs of the host and not its own, or that it destroys several 

 eggs at one visit to a crowded nest. In Mayfield's studies, the cow- 

 birds were not banded or otherwise individualized, but his observa- 

 tions were close and careful, and they may be looked upon as an 

 accurate indication of the actual state of affairs. He concluded that 

 the cowbird has an impulse to lay an egg and an impulse to steal or 

 remove an egg, and that it has a continuing and proprietary mterest 

 in the nests it uses. 



Although evidence for similar contmuing interest in host nests 

 has not been recorded or documented from other portions of the 

 brown-headed cowbird's range, one must admit that this may be a 

 reflection of inadequate observation; the absence of such evidence 

 does not prove the nonexistence of such continued attention. Some- 

 thing akin to Mayfield's experience seems to have been present in 

 Hann's work (1941) with the cowbu-d and the ovenbird in Michigan. 

 He noted that the female cowbird made regular trips of inspection 

 to the nests dm'ing the absence of the owners, between the time she 

 first discovered the nests and the time she laid eggs in them, and that 

 she seemed to be aware in advance of ovulation where the egg was 

 to be laid. Parasitized nests had one or more eggs of the ovenbird 

 removed by the female parasite. The eggs were removed, not at 

 the time of ovulation, but during the forenoon of the previous day, 

 or the day of laying, or, more rarely, the following day. The cow- 



