92 U.S. NATIONAL MUSEUM BULLETIN 233 



subspecies of the cowbird. The degree to which the warbler is im- 

 posed upon in certain areas is demonstrated by the fact that, in the 

 J. P. Norris collection alone, 18 out of 70 sets of prothonotary eggs 

 contain 1 or more eggs of the cowbird and that all of these 18 victim- 

 ized sets were collected within two weeks at Burlington, Iowa, 

 where a total of some 35 sets of eggs were taken. In other words, 

 the incidence of parasitism at Burlington during that period was a 

 Uttle over 50 percent. 



In his detailed study of the prothonotary warbler, Loucks (1894, 

 pp. 31-32) wrote that to find 1 or more eggs of the cowbird in nests 

 of this bird was not unusal. As many as 4 eggs of the parasite, 

 together with 4 of the host, have been reported from a single nest; 

 Bent (1953, p. 27) mentions a nest containing 7 cowbird and no 

 warbler eggs. Several instances of double-storied nests of this bird, 

 with a cowbird egg buried in the lower part, are on record; however, 

 the warbler usually accepts and incubates the strange eggs. I am 

 not aware of a definite record of this bird actually rearing one of the 

 parasitic young, but we can assume that it not only can but does do so. 



Swainson's Warbler 

 Limnothlypis swainsonii (Audubon) 



Swainson's warbler is loiown to be parasitized by the cowbird in 

 Copan County, Oklahoma. A. J. Kirn (1918, pp. 97-98) reported 

 that, "during the season of 1917, six different nests were found in a 

 strip of woods a mile long and a little over a quarter of a mile in width, 

 built by at least four different pairs of birds; two held eggs, one with 

 Cowbird's, two were deserted, one held a punctured egg of the owner, 

 the other a Cowbird's and a punctured Warbler's egg and a broken 

 Warbler's egg on the ground beneath. Tlu-ee or four eggs are laid or 

 if Cowbirds are present, and they usually are, sometimes three eggs 

 are laid." This statement, quoted by me in an earlier publication 

 (1929, p. 239), is still the only pertinent observation on record. The 

 fact that in more than 40 years since the record was made no one has 

 added to it, although not a few nests of the warbler have been found, 

 suggests that, at best Swainson's warbler is a very local host. Since 

 most of its breeding range lies outside that of the parasite, v/e are safe 

 in concluding that neither bird plays an important role in the economy 

 of the other. 



Worm -eating Warbler 



Helmitheros vermivorus (Gmelin) 



This is an uncommonly reported host. When I fii-st (1929, p. 239) 

 compiled the available data on the worm-eating warbler as a cowbu'd 

 host, I knew of 21 definite records. In more than 30 years since then 

 I have been able to add only 16 more. In some places, however, such 



