16 U.S. NATIONAL MUSEUM BULLETIN 233 



wood thrush, and red-eyed vireo; 50 to 60 percent in the chimney 

 swift, bkiebird, chipping sparrow, brown thrasher, field sparrow, and 

 goldfincli; 40 to 50 percent in the mourning dove, black-capped 

 chickadee, cedar waxwing, and crested flycatcher; and less than 40 

 percent in the house sparrow. 



While no percentages based on such ample data are available for 

 the brown-headed cowbkd, it may be noted that Norris (1947) reported 

 on 237 nests of various passerine host species at Preston Firth, Penn- 

 sylvania. In these nests the hosts laid a total of 668 eggs, of which 

 383, or 57.3 percent, hatched; cowbirds laid 108 eggs, of which 46, or 

 42.6 percent, hatched. The fledging success was 37.7 percent for the 

 eggs of the various hosts, and 26.8 percent for those of the cowbird. 

 Of the host eggs that hatched, 64 percent produced fledglings; of the 

 cowbird eggs that hatched, 63 percent produced fledglings. All but 4 

 of the parasitized nests that produced fledglings produced at least 

 one host fledghng. The 35 successful non-parasitized nests produced 

 2.94 fledglings per nest; 19 successful parasitized nests produced 2.05 

 host fledglings per nest — from which it follows that each young cow- 

 bird was raised at the expense of approximately one host young. 

 These figures agree very closely with those given by Nice (1937, p. 

 200) for cowbird parasitism on the song sparrow (see p. 170). 



Another study, on a smaller scale, by Betts (1958, p. 143), of the 

 passerine birds on a farm in southern Michigan, revealed that, of 39 

 cowbird eggs laid in 34 nests of 12 species of open -nesting hosts, 20 

 hatched and 14 survived to leave the nest — a fledging success of about 

 36 percent. However, this is about three times the fledging success 

 of cowbirds in ovenbirds' nests, mentioned below (pp. 112-113), and 

 this, in turn, is better than the fledging success of cowbirds in nests of 

 yellow-throats studied by Hofslund (see below, and pp. 118-119). 



On the other hand, the rate of cowbkd success is higher in the 

 case of larger, less susceptible hosts. For example, Nice's data on 

 the song sparrow showed that 30.7 percent of the cowbird eggs sur- 

 vived to the fledging stage, as compared with 35.8 percent of the 

 song sparrow eggs. 



In other words, the general breeding success of the cowbu-d, like 

 that of so many other parasitic animals, is often much lower than 

 that of its chief victims. This is the saving factor in the picture. It 

 helps to allow for the smwival of the host population in sufficient 

 numbers for its continuing availability and usefulness to the parasite, 

 even though at times this seems to be made difficult by the very 

 degree to which the hosts are successful as fosterers. If the common 

 hosts were to suffer too drastically in a numerical sense from the 

 parasitism of the cowbird, there would result a scarcity of hosts, 

 which in turn would cause a diminution in the possible number of 



