88 U.S. NATIONAL MUSEUM BULLETIN 233 



In recent years, data of higher quahty, especially in their significantly 

 quantitative aspects, have been put on record by observers such as 

 Southern, who made an extensive study (1958) of this vireo in the 

 Douglas Lake region of Michigan. Out of 104 nests which he found, 

 no fewer than 75, or 72.17 percent, were parasitized. Of the 69 

 nests which contained cowbird eggs when the nests first were found, 

 32, or 46.38 percent, contained more than a single cowbird egg apiece. 

 In several nests, the cowbird eggs were similar enough in size and 

 coloration to suggest that they were laid by one female, but in cases 

 where there were as many as 3 parasitic eggs, two cowbirds were 

 involved. 



Southern found that the incidence of survival of the nestling vireos 

 in parasitized nests was greater than earlier observational data had 

 indicated. There were 24 parasitized nests which produced fledglings 

 and, in these, the success of the cowbird was 87.08 percent. "Of the 

 nests fledging no vireo young, two produced three cowbirds; and four 

 produced two cowbu'ds. On the other hand, six fledged one vireo and 

 two cowbirds ; nine nests fledged one cowbird and no vireos ; and three 

 nests fledged one of each species. These figures indicate that the cow- 

 bird is highly successful after hatching occurs; the young usually 

 survive whereas those of the host do not." Still, nine nests did pro- 

 duce fledglings of both species — which is more than might have been 

 expected. Southern states that "the cowbu'ds were probably to 

 blame for the loss of many vireo eggs and young as well as causing the 

 actual desertion of vkeo nests by laying too many eggs in them. Of 

 the 19 vireo nests deserted, 17 contained cowbird eggs; four contained 

 eggs of the host and cowbird in equal numbers; 11 contained from 

 one to seven eggs of the parasite only; and two nests contained fewer 

 eggs of the cowbird than of the vireo. These figures substantiate 

 my contention that an excessive number of cowbird eggs caused the 

 desertion of many vireo nests." The nesting success of the vireo, 

 based on the number of eggs laid in the 32 nests producing vireo 

 young, was 87.49 percent; the nesting success of the vu-eo, with 

 reference to the 48 nests that fledged young vireos and/or young 

 cowbirds, was 66.66 percent. The total nestmg success of the vireo 

 in 78 nests, observed tlirough fledging, destruction, or desertion, was 

 41.03 percent. 



We should compare these figures with those published by Lawrence 

 (1953) from a study m central Ontario, a forested area where, because 

 the cowbii'd is locally absent, the vii'eo is unmolested. He found that 

 the nesting success of 35 nests was 63 percent and that the hatching 

 success of eggs in 30 nests was 60 percent. 



In his summary of cowbird parasitism in Ohio, Hicks (1934) noted 

 that, out of 231 observed nests of the red-eyed vireo, 84, or 36 percent, 



