HOST RELATIONS OF PARASITIC COWBIRDS 119 



this limit was nearer to the lower than the higher figure. Applying 

 Schonwetter's formula (1924) to average measurements for the yellow- 

 throat and cowbird eggs, Hofslund calculated the average volume 

 of a yellowthroat egg to be close to 1.7 cc. and that of a cowbird 

 egg to be almost 3.1 cc. The volume of a normal 4-egg clutch of 

 the yellowthroat would come to about 6.8 cc, which is little more 

 than that of 2 cowbird eggs — 6.2 cc. If 2 eggs of the yellowthroat 

 were replaced by 2 of the cowbird, the resulting set would total 

 9.6 cc. or 1.4 times that of a normal 4-egg set of the host alone. Wliile 

 these figures are calculations and have not been tested in the field, 

 Hofslund's observations indicate that 1.3 times the normal clutch 

 volume "closely represents the limit of egg-volume that a Yellow- 

 throat \vill normally hatch, and ordinarily one can say that a nest 

 with more than one Cowbird egg in it is doomed to failure as far 

 as the Yellow-throat eggs are concerned." 



A surprising feature emerged from Hofslund's observations. Three- 

 quarters of the parasitized nests contamed more than a single cowbird 

 egg; the average number in 20 parasitized nests was 2. Hah* of 

 these eggs failed to develop to the fledgling stage; some were lost 

 before hatching by nest desertion, predation, etc.; and some nestlings 

 were lost to predation. The percent of cowbhd eggs hatched was 

 42.5; of yellow-throat eggs, 65.8. Only 9 of the 20 parasitized nests 

 produced cowbird fledglings — an average of 1.3 cowbirds per suc- 

 cessful nest but only 0.6 cowbird per parasitized nest. It can be 

 concluded that the cowbird is successful only moderately with the 

 yellowthroat as a host, nevertheless, the former is an unportant 

 check on the increase of the latter, with most of the damage to the 

 host occurring during the egg stage. 



The incidence of cowbird parasitism in several studies — reported 

 in some detail — ranged from 7 percent to 46 percent of all the yellow- 

 throat nests found. If we combine the totals of several surveys 

 done in Michigan (Hofslund, 1957, Stewart, 1953; Batts, 1953; and 

 Detroit Audubon Society, 1953-56) we find that, of a total of 90 

 observed nests, 35 nests, or 39 percent, were parasitized by the 

 brown -headed cowbird. 



Shaver (1918) studied the progress of a nest in which one young 

 cowbu"d and two young yellowthroats were hatched. A summary 

 of aU the feedings indicated that the young parasite received 55 

 percent of the total amount of food. On the last day m the nest, 

 the nestlings were visited by the adult warblers 348 times; the young 

 cowbird received all, or part, of the food brought on 190 of these 

 visits. 



OccasionaUy, the yellowthroat may bury the cowbhd eggs under 

 a new nest lining. Weed and Dearborn (1903, pp. 162-163) record 



630590—63 9 



