170 U.S. NATIONAL MUSEUM BULLETIN 233 



223 nests (43.9 percent) contained eggs or young of the cowbird 

 (the annual percentage varied from 24.6 to 77.7 percent). Sixty-six 

 unparasitized nests raised an average of 3.4 song sparrows whereas 

 28 successful but parasitized broods averaged only 2.4 song sparrows, 

 indicating that each cowbird was reared at the expense of one song 

 sparrow. In one instance Nice (1930) found that a pair of song 

 sparrows raised a young cowbird together with five of then* own 

 young. Apparently here no loss of sparrows was involved. In 

 another paper, Nice (1936) noted that, in all the song sparrow nests 

 which she had watched during a period of five years, adult cowbirds 

 removed 5.7 percent of the song sparrow eggs and nestling cowbirds 

 crushed or starved 3.5 percent of the young sparrows. The cowbird 

 eggs did not succeed as well as those of the host; only 30.7 percent of 

 the former, but 35.8 percent of the latter, reached the fledging stage. 

 In 1930-31 there was one female cowbird to about 11.5 pairs of suitable 

 hosts, but in 1934-35 there was one to 8.6 pairs of suitable victims. 



Of all song sparrow nests parasitized, Nice reported that 70 percent 

 held a single cowbird egg each, 27 percent held 2 each, and 3 per- 

 cent held 3 each. In the area of study — -near Columbus, Ohio^ — ^the 

 song sparrow was the most important host of the paeasite. Norris 

 (1947, p. 90) noted that 11 out of 27 nests (40.7 percent) in Pennsyl- 

 vania were parasitized, and Berger (1951a, p. 30) recorded 37 out 

 of 59 nests found in Michigan (62.7 percent). In the Detroit area, 

 as reported by the Detroit Audubon Society (1956, p. 90), the average 

 frequency of parasitism of the song sparrow was 40.1 percent of all 

 the nests found: in 1950, 41 nests were found, of which 20, or 49 

 percent, were parasitized; in 1951, 18 nests were located, of which 

 8, or 44 percent, were parasitized; in 1954, 39 nests were found, 

 of which 14, or 35.9 percent, contained eggs or young of the brown- 

 headed cowbird. These figures are considerably below Berger's 

 figures, which included the Ann Arbor section. 



One is drawn toward attempting an over-all estimate of the fre- 

 quency with which the song sparrow is victimized, but to do so 

 with any feehng of accuracy is difficult because the incidence of 

 parasitism appears to vary geographically (or, at least, the frequency 

 with which it is reported varies). From this it follows that the 

 over-all percentage depends on how many geographically different 

 areal data are used in the estimation. For example, if we put 

 together, as a geographical unit, a group of studies made in Penn- 

 sylvania (Norris, 1947), Ohio (Hicks, 1934; Nice, 1937), and Michigan 

 (Berger, 1951; plus Detroit Audubon Survey Kecords), we come up 

 with a total of 323 parasitized nests out of 804 nests observed, or 

 a httle over 40 percent. On the other hand, in southern Quebec 

 (TerriH, 1961, p. 11), out of 486 nests observed, only 62, or 12.7 



