HOST RELATIONS OF PARASITIC COWBIRDS 109 



to 4 cowbird eggs were found in individual nests; the average number 

 of cowbird eggs was 1.67 — with 1.89 fewer warbler eggs in the para- 

 sitized nests than in unmolested ones. Hence, it follows that 1.13 

 warbler eggs were removed for each cowbird egg laid. "The loss of 

 warbler eggs in parasitized nests was 41 percent of the eggs laid; the 

 loss in all nests, 55 percent of them parasitized, was 25 percent of 

 warbler eggs laid — these losses from egg removal alone." 



Mayfield's calculations showed that it is not only in the matter of 

 host egg removal that the parasite exerts a heavy toll, but also in the 

 fact that this continues at all stages of the breeding process. "The 

 probability that eggs present at hatching time will hatch is 85 percent 

 among warbler eggs alone, but 75 percent with cowbird eggs present; 

 the rate is lower in nests ^vith several cowbird eggs than in nests with 

 only one. The presence of young cowbirds in the nests reduces by 

 .55 the probability that warblers will be fledged. The presence of 

 two or more cowbirds hatched ahead of the warblers is lethal to the 

 warbler nestlings." Mayfield's figures suggest that Kirtland's war- 

 bler, as a total species, would produce annually about 60 percent 

 more fledglings of its own kind if there were no interference from 

 the parasite. 



Mayfield summarized (p. 176) the losses suffered by Kirtland's 

 warbler through cowbird parasitism as follows: 41 percent of the total 

 warbler eggs laid were removed by the cowbird; 10 percent of the 

 warbler eggs present at hatching time failed to hatch as a result of 

 the cowbird eggs present (as estimated from the excess over hatching 

 failures in nonparasitized nests), which, if calculated as percentage 

 of warbler eggs laid, is 6 percent; 59 percent of the warblers hatched 

 are not fledged because of cowbird nesthngs present (again, as esti- 

 mated from excess over nestling loss in nonparasitized nests), which 

 figure is 31 percent of the original total warbler eggs laid. In other 

 words, 41 percent+6 percent+31 percent, or 78 percent, of all warbler 

 eggs laid in nests which were parasitized by the cowbird failed to 

 produce fledghngs. Since 55 percent of all Ku'tland's warbler nests 

 were parasitized, the cowbird was responsible for the loss of about 

 43 percent of all Ivirtland's warbler eggs in nests not abandoned or 

 destroyed. If this is added to the other perils which the warbler has 

 to face, such as accidents to one or both of the adult birds, flooding of 

 the nests, nest predators, aU of which, according to Mayfield's data, 

 cause the loss of two-thirds of Kirtland's warbler nests, one can see 

 that cowbird parasitism is an insupportable affliction. 



When we consider the limited population of Ku'tland's warbler in 

 its entirety, estimated in 1951 as comprising only one thousand adult 

 birds, and its strict dependence on a specialized and decreasingly 

 available type of nesting habitat, the added impact of the brown- 



