The Cowbird: A Parasite 187 



Not knowing the complete history of the case, both assumptions are equally 

 probable. Two other similar nests were found late in July. 



One other observation proves conclusively that only Cowbird eggs may be 

 found in a nest without having been deposited originally in an empty nest. 

 On June 18 I found the nest of a Warbler containing three eggs of the Warbler 

 and two of the Cowbird. On my next visit to the nest, June 21, 1 found only 

 the Cowbird eggs remaining in the nest. The others had been destroyed, and 

 the shell of at least one of them was lying on the ground near the nest. This, 

 in addition to giving light on the subject in question, is circumstantial evidence, 

 at least, that the Cowbird sometimes destroys the eggs of the rightful owner 

 of the nest. Unless the Warbler eggs in this case were destroyed by the Cow- 

 bird, why were the Cowbird eggs spared? The same two eggs were still in the 

 nest on June 26, when I last visited it, but they were cold, and numerous ants 

 were crawling about the nest and upon the eggs. Evidently the destruction 

 of the eggs caused the desertion of the nest. 



I do not contend that the Cowbird always lays its eggs in nests containing 

 other eggs. There is evidence that they frequently do otherwise. According 

 to Barrows of the Michigan Agricultural College, "The Yellow Warbler is 

 constantly victimized by the Cowbird, and in places where this parasite is 

 abundant many deserted nests are found containing from one to four eggs of 

 the Cowbird, with or without some of the Warbler. ... As is well known, 

 this Warbler not infrequently covers a Cowbird egg with a new layer of ma- 

 terial in the bottom of the nest, raising the rim of the nest correspondingly, 

 and instances have been known where this has been done a second time, making 

 a three-storied nest." I have in my possession one two-storied nest of this 

 species, found on July 6 in a raspberry bush just north of Indiana University 

 Biological Station. The Cowbird egg is almost completely buried in the bottom, 

 and the two stories of the nest are very distinct. Undoubtedly a brood of 

 birds was raised in the upper story while the Cowbird egg remained in the base- 

 ment unincubated. 



Of three other nests of the Yellow Warbler found in use, one contained a 

 Cowbird egg. Owing to the fact that after the birds hatched the nest was 

 robbed, I did not observe the struggle for existence between the Cowbird and 

 the Warblers of this nest. One Kentucky Warbler's nest, found June 22, also 

 contained a Cowbird egg, but this nest was robbed before any of the eggs 

 hatched. Out of a total of six Warbler nests found, whose history was certain, 

 four had been contaminated, indicating a very high percentage of parasitism 

 by the Cowbird among the Warblers. 



On June 17 I found the nest of a Scarlet Tanager containing one bird and 

 two eggs. The eggs never hatched and on June 22 when I last saw the young 

 bird, it was evident that it was a Cowbird. According to Amos W. Butler in 

 his 'Birds of Indiana,' the nest of the Scarlet Tanager is a frequent receptacle 

 for the eggs of the Cowbird. 



