HOST RELATIONS OF PARASITIC COWBIRDS 171 



percent, were parasitized. If we put all these studies together, 

 we get a total of 382 out of 1,285 nests victimized, or 29 percent. 

 This figure becomes yet smaller when we attempt to include data 

 from other parts of the continent. 



An interesting case, reported by Berger (1951b), involves 10 

 consecutive nests of a single color-banded song sparrow over a period 

 of three seasons, from 1948 through 1950. In seven of these nests, 

 18 cowbird eggs were laid, plus at least 27 sparrow eggs. One cowbird 

 and six sparrows were fledged from all 10 nests. In the summer 

 of 1949, the sparrow had no fewer than five consecutive nests; and 

 in 1950, four. It would seem that, if none of these nests had been 

 interfered with, there would not have been sufficient time for four 

 or five in one season. This, therefore, must be kept in mind when 

 attempting to evaluate the damage to the host which is done by 

 the cowbird. It appears that one of the effects of parasitism may 

 be to increase the "nesting potential" of the host. (See also the 

 discussion of Walkinshaw's field sparrow data, p. 164.) 



As many as 7 cowbird eggs have been found in a single nest of 

 this sparrow; there are numerous records of 3, 4, and 5 parasitic 

 eggs to a nest. Occasionally, but not often, song sparrows may 

 partly bury cowbird eggs by building a new nest lining over them — 

 if the ahen egg is laid before any eggs of the host. 



Salmon (1933, p. 100) has reported seeing a song sparrow feeding 

 three fledghng cowbirds; no young sparrows were mentioned. Lees 

 (1939, p. 121) recorded that near Wetaskiwin, Alberta, he watched a 

 song sparrow feeding no less than five young cowbirds. This must 

 be a record of fledgling success for any host species. 



McCown's Longspur 



Rhynchophanes mccownii (Lawrence) 



McCown's longspur is not well known as a host of the brown- 

 headed cowbird since relatively few observers have studied it. Raine 

 (1894, p. 120) listed it as a cowbird victim (race M.a. artemisiae) as 

 did Bendu-e also, the following year, possibly on the basis of Raine's 

 statement. Many years later, about 1921, Alfred Eastgate informed 

 me that he had found this longspur to be a victim in North Dakota. 

 Later still, the late L. B. Bishop \vrote me that, out of three nests 

 of this species found near Cando, North Dakota, two contained eggs 

 of the cowbird m addition to those of the longspur. S. J. Darcus 

 sent me data on one record from Saskatchewan — a parasitized nest 

 he found at Cypress Hill on June 7, 1920. No other records have 

 come to my attention. In his study of McCown's longspur, Mickey 

 (1943) found no evidence of cowbird parasitism near Laramie, Wyo- 

 ming, nor did DuBois^(1935, 1937) in Montana. 



