174 BULLETIN 20 3, UNITED STATES NATIONAL MUSEUM 



Enemies. — The arch-enemy of the yellow warbler is undoubtedly 

 the cowbird. This warbler is one of the very commonest victims of 

 this parasite, and comparatively few of its nests are not visited at 

 least once by a cowbird in regions where the latter is very common. 

 Dr. Friedmann (1929) has about 500 records of such imposition on 

 the eastern yellow warbler. Everyone who has examined nests of 

 this warbler in any number has found one or more eggs of the cow- 

 bird in some of the nests. This parasitic habit has cost this species 

 of warbler many extra hours of unexpected labor and the loss of many 

 eggs and young. But the most interesting fact about it is that the 

 warbler has found a way to combat the evil and, in many cases, to 

 defeat the plans of the cowbird, by either deserting the nest in which 

 the strange ^gg is deposited or by building a second floor over it and 

 leaving the alien egg to cool off in the "cellar." 



The yellow warbler is not the only bird that has learned to do this 

 occasionally, but it is the only one that does it regularly and per- 

 sistently in spite of repeated contributions from the cowbird. Even 

 if the warbler has one of its own eggs in tlie nest when the cowbird's 

 Qgg is deposited it may bury both the eggs by building a story above 

 them, but if there are two or three warbler's eggs in the nest before 

 the alien Q,gig appears, the warbler may feel obliged to incubate and 

 hatch out the stranger, with the usual results of her own young being 

 crowded out and lost. Two or more cowbird's eggs are almost sure 

 to be deserted or buried. But the cowbird is very persistent and 

 keeps on laying, as successive stories are added to the nest by the 

 energetic and persevering warblers. Two-story nests are very com- 

 mon, and as many as three, four, five, and six stories have been re- 

 corded. Mr. Forbush (1929) was told by Dr. H. F. Perkins "of one 

 case where a six-storied nest was built, with a cowbird's egg in every 

 one." Mr. Du Bois tells me of a new nest he found in a low bush, 

 with another nest, about half completed and only about a foot below 

 it, containing a fresh, cold cowbird's egg. Out of 43 nests found by 

 Dr. George M. Sutton (1928) in Pymatuning Swamp, Pa., "a Cow- 

 bird egg was found in only one nest. This is most unusual, but is 

 due, as elsewhere stated, to the protection against these parasites 

 afforded by the Red-winged Blackbirds which would not tolerate a 

 Cowbird anywhere about the marshes." 



Snakes sometimes destroy the young, as related above; squirrels, 

 blue jays, and other predatory mammals and birds rob the nests; and 

 the adults must always be on the alert to escape the many enemies 

 that prey on all small birds. 



Harold S. Peters (1936) records only one louse, Philopterus sub- 

 favescens (Geof.), as an external parasite on the eastern yellow 

 warbler. 



