12 U.S. NATIONAL MUSEUM BULLETIN 233 



One cannot avoid the general conclusion that the brown-headed 

 cowbird, biologically a very successful species, both in terms of its 

 great population numbers and of its wide geographic range, is a 

 success by virtue of its lack of specialization. This success may be 

 due to the fact that throughout its range there are no discontinuities 

 of any significance; consequently, no opportunities are present for 

 locally restricted and inbred groups to express themselves in differ- 

 ential host preferences. In terms of host relationships the brown- 

 headed cowbird seems to bear out Mayr's (1954, p. 178) statement 

 on morphological changes: "Successful species are usually widespread 

 and rich in genetic variability, but they tend to be rather conservative 

 from the evolutionary point of view. . . ." 



Intensity of Parasitism of Frequent Hosts 



Of aU the parasitized nests recorded with full data as to the number 

 of eggs present, approximately 60 percent contained but a single 

 cowbird egg apiece; in many of the others, however, 2, and less 

 frequently 3, or even more, of the parasitic eggs were present. 

 These figures are composites, based on aU available records, but if 

 we restrict our considerations to the more recent, more carefully made 

 studies, the data from which were recorded in greater detail, we find 

 actuall}'- the opposite phenomenon — a slightly greater number of 

 nests with multiple cowbird eggs than with single ones. Including 

 even the old "scattering" records made largely by unsystematic egg 

 collectors and not by careful students of the birds concerned, we 

 find, for example, in the case of one host, the Louisiana waterthrush, 

 that multiple cowbird eggs were discovered more frequently than 

 single ones. Of 55 parasitized nests, 25 held 1 cowbird egg apiece, 

 20 had 2, 7 had 3, and 3 had 4. 



For an example of results from newer, more fully investigated and 

 more completely documented source materials, we may note that in 

 Hofslund's study of the yellowthroat (discussed on pp. 118-119 of the 

 present report) about three-quarters of the parasitized nests contained 

 more than one cowbird egg apiece. While this condition is true in 

 the area he studied, it is not certain that a similar degree of parasitism 

 is general throughout the range of the yellowthroat. 



In 1960 Ussher found an unusual situation in the nest of a wood 

 thrush in Rondeau Provincial Park, Ontario; this nest contained a 

 single egg of the thrush and 12 of the brown-headed cowbu'd. I was 

 informed by W. P. Nickell that the park area had been sprayed earlier 

 with DDT, which greatly decreased the population of nesting birds, 

 thereby reducing the number of nests available for the cowbirds. 

 This may have been a cause behind the excessive use by the cowbirds 

 of this particular nest. A similar conclusion was expressed by Hann 



