54 U.S. NATIONAL MUSEUM BULLETIN 233 



rearing of a young cowbird by a pair of these flycatchers. Occasion- 

 ally the Acadian flycatcher may build a new floor over a cowbkd's egg, 

 especially if the parasitic egg is laid before any of the hosts' eggs are 

 laid. Bendire (1895, p. 304) mentions such a case, which was found 

 in Illinois by Loucks. AU the records relate to the nominate race of 

 the cowbird, M.a. ater. 



In Michigan it has been the experience of Walkinshaw and others 

 that, in nests of Empidonax {virescens, traillii, and minimus) which 

 produce fledghng cowbirds, none of the young flycatchers survive. 

 Walkinshaw (1961, p. 268) examined 67 Acadian flycatcher nests in 

 southern and western Michigan; of these, 16, or 24 percent, were 

 parasitized. 



Traill's Flycatcher 



Empidonax traillii (Audubon) 



Both races of this flycatcher {brewsteri and traillii) are known to be 

 victimized by the brown-headed cowbird, all three races of which are 

 involved in the total number of records. In all, I have learned of over 

 150 instances of cowbird parasitism, in addition to several loose state- 

 ments which implied still other cases. The records range from British 

 Columbia, Saskatchewan, and Ontario to southern Quebec in Canada; 

 and from New England to Ohio, Illinois, Michigan, Wisconsin, Minne- 

 sota, Iowa, Colorado, New Mexico, Washington, and California in 

 the United States. The largest number of records comes from southern 

 California (41 cases), but it is not known what percent of the total 

 number of nests found by the observers is represented by this figure. 

 This flycatcher also appears to be a somewhat commoner victim in 

 the Nipawin area of Saskatchewan than in many other areas, ac- 

 cording to the experience of M. G. Street (1943, p. 7), who states 

 that he has seldom found an unparasitized nest of this species. In 

 Ohio, Hicks (1934, p. 386) noted that, of 108 nests found, 23, or 21 

 percent, were parasitized; in Michigan, Berger and Parmelee (1952, 

 p. 37) found that the ratio of parasitized nests was 20.8 out of 48 

 nests observed; in the next eight summers, Berger observed 216 

 nests, of which 17, or 8 percent, were parasitized. Walkinshaw 

 (1961, p. 268) found that 4 out of 53 Michigan nests were parasitized. 

 At Buckeye Lake, Ohio, Trautman (1940, p. 296) found cowbu^d 

 eggs or young in 9 out of 16 nests, or over 50 percent. In Wisconsin, 

 McCabe (in litt.) found 418 nests of this flycatcher during a period 

 of 17 years. Of these, 29, or 6.9 percent, w^ere parasitized. The 

 incidence of parasitism varied from year to year, ranging from 3 to 

 10 percent of the nests exammed. If we add the results of these 6 

 studies (Hicks, Berger, Walkinshaw, Parmelee, Berger, Trautman, 

 and McCabe) made in Ohio, Michigan, and Wisconsin, we get a 

 total of 859 nests, of which 92, or almost 11 percent, were parasitized. 



