98 U.S. NATIONAL MUSEUM BULLETIN 2 33 



parasitized, a percentage of 42. Berger (1951a, p. 29), in Michigan, 

 recorded 18 parasitized nests out of 44 nests, or 40.9 percent. Terrill 

 (1961, p. 6) found that 42 out of 307 yellow warbler nests in southern 

 Quebec were parasitized, an incidence of parasitism of 13.6 percent. 

 The data assembled in the Detroit area in 1954 (Detroit Audubon 

 Soc, 1956, p. 90) reveal that, of 208 nests of the yeUow warbler 

 reported in 1954, 74, or 35.6 percent, were parasitized. In the pre- 

 ceeding year the percentage of parasitism had been 31.2 percent. 

 The percentage varies locally within the "Detroit area," which 

 includes eight counties of southeastern Michigan and adjacent 

 Ontario. For example, in the Cranbrook Area in 1953, 21 of 49 

 nests, or 42.8 percent, were parasitized, but in 1954, 29 of 49 nests, 

 or 59 percent, were affected; at Rondeau Park in 1953, 6 of 45 

 nests, or 13.3 percent, had cowbird eggs or young, whereas in 1954, 

 27 of 84, or 32.1 percent, were so recorded; at Otter Lake in 1953, 

 15 of 42 nests, or 35.7 percent, were parasitized, but in 1954, 7 of 

 23, or 30.4 percent, were affected; at Pontiac Lake in 1953, 7 nests 

 were found, not one of which had been molested, but in 1954, 1 nest 

 out of 18, or 5 percent, was parasitized. The overall average fre- 

 quency of parasitism for five years in the "Detroit area" was 37.1 

 percent of the nests which were found. 



The situation at Pontiac Lake is of interest. The apparent freedom 

 from cowbird attention which the yellow warblers seem to enjoy 

 there appears to be explained by the fact that nearly all the nests 

 which were found were situated in, or close to, colonies of redwinged 

 blackbirds. The latter, with their aggressive dispositions, act as a 

 deterrent to intruding cowbirds, and, furthermore, cowbirds are less 

 liable to use nests in marshy areas, where the redwings nest. Sutton 

 (1928, p. 163) found at Pymatuning Swamp, Pennsylvania, that 

 redwings refused to tolerate cowbirds in their breeding area. He 

 saw "a flock of Red-wings once pursue a female Cowbird until she 

 was utterly exhausted and plunged into the water to escape. Her 

 pursuers chased her to the edge of the Swamp then headed her off 

 and forced her to the opposite bank." 



Although the yellow warbler is, in many cases, a tolerant host, 

 accepting the parasitic eggs and rearing the emergent young, it 

 often does eliminate the foreign eggs by building a new lining or a 

 new nest floor over them and leaving them buried in the structure. 



Perhaps an extreme instance of this tendency is a case reported by 

 Schrantz (1943), who studied 41 nests of this warbler in two successive 

 summers at West Okoboji Lake, Iowa. Of the 41 nests, 12 were 

 parasitized, and 11 of the 12 had cowbird eggs buried under the nest 

 lining. In some of these nests the foreign eggs were so deeply buried 



