HOST RELATIONS OF PARASITIC COWBIRDS 99 



that it seemed they probably were deposited before the completion 

 of the original nest lining. 



Other species of victims have been known to do this also, but none 

 to the extent of the yellow warbler. The situation may be described 

 as follows. 



If a cowbird's egg is deposited in an empty nest before the warbler 

 has laid any of its own, the strange egg frequently is buried under a 

 new floor, but not infrequently the nest may be deserted and a com- 

 pletely new one built. If, however, the warbler has laid even 1 egg 

 prior to the cowbird's egg being introduced into the nest, the host 

 usually accepts and incubates the strange egg along with its own. 

 Sometimes it buries the eggs, its own included, in what seems an 

 effort to get rid of the foreign egg. The bird also may desert the nest. 



It sometimes happens that, after building a new floor to the nest, 

 the warbler again is parasitized. A great deal of variation arises in 

 the reactions of the victim to such conditions. Usually, if there are 

 several eggs of its own in the nest, the warbler will tolerate the cowbird 

 egg and will incubate. If the cowbird should lay first, the warbler 

 either may desert or build a new nest floor. There are numerous 

 records of two-, three-, four-, and even five-storied nests of the yellow 

 warbler, each of the lower stories containing eggs of the cowbird and, 

 in some, eggs of the warbler as well. Berger (1955, p. 84) has reported 

 what must be a record case: a six-storied nest with a total of 11 cow- 

 bird eggs buried in the various layers. As many as 4 eggs of the 

 warbler, a complete set, have been found buried with 1 of the cowbird, 

 but such extreme cases are rare. It is very uncommon to find more 

 than 1 or 2 eggs of the warbler covered over in this manner. 



Near Pontiac, Michigan, the McGeens (in litt.) found 40 parasitized 

 nests of the yellow warbler. In 13 of these, the warblers had covered 

 over the alien eggs with new floor lining; in 13 others, the nests were 

 deserted after being parasitized; of the other 14, 6 met with failure 

 because of predation and 9 carried through to fledging success. 

 Recently, Berger (1961, p. 273) reported that, in the McGeens' study, 

 the synchronization of the cowbird eggs with those of the yellow 

 warbler (i.e., the time of deposition in the nests) was important; 

 it was found that 86.5 percent of non-synchronized cowbird eggs 

 were covered over or deserted, but only 22.5 percent of well-syn- 

 chronized ones were treated thusly. 



One of the older records of cowbird parasitism on the yellow warbler 

 merits some mention. Savage (1895, p. 13) reported one of these 

 warblers feeding a fledged cowbird in Iowa on October 2, 1893. If 

 this is correct, which is highly doubtful, it is an unusually late date 

 for both the parasite and the host. Bent (1953, p. 182) gives no egg 

 dates for the warbler later than June 8 in the central United States. 



